Jan 21 - Jan 25, Xiaopeng Li
Jan 25
1. arXiv:1301.5854 [pdf, other]
Dynamical critical phenomena in driven-dissipative systems
L. M. Sieberer, S. D. Huber, E. Altman, S. Diehl
We explore the nature of the Bose condensation transition in driven open quantum systems, such as exciton-polariton condensates. Using a functional renormalization group approach formulated in the Keldysh framework, we characterize the dynamical critical behavior that governs decoherence and an effective thermalization of the low frequency dynamics. We identify a critical exponent special to the driven system, showing that it defines a new dynamical universality class. Hence critical points in driven systems lie beyond the standard dynamical classification of equilibrium phase transitions. We show how the new critical exponent can be probed in experiments with driven cold atomic systems and exciton-polariton condensates.
2.arXiv:1301.5825 [pdf, other]
An ultracold analogue to star formation: Spontaneous concentration of energy in trapped quantum gases
M. P. Strzys, J. R. Anglin
Stars form when cold cosmic nebulae spontaneously develop hot spots that steadily intensify until they reach fusion temperatures. Without this process, the universe would be dark and dead. Yet the spontaneous concentration of heat is exactly what the Second Law of Thermodynamics is in most cases supposed to forbid. The formation of protostars has been much discussed, for its consistency with the Second Law depends on a thermodynamical property that is common in systems whose strongest force is their own gravity, but otherwise very rare: negative specific heat. Negative specific heat turns the world upside down, thermodynamically; it implies that entropy increases when energy flows from lower to higher energy subsystems, opposite to the usual direction. Recent experiments have reported negative specific heat in melting atomic clusters and fragmenting nuclei, but these arguably represent transient phenomena outside the proper scope of thermodynamics. Here we show that the counter-intuitive thermodynamics of spontaneous energy concentration can be studied experimentally with trapped quantum gases, by using optical lattice potentials to realize weakly coupled arrays of simple dynamical subsystems that share the peculiar property of self-gravitating protostars, of having negative micro-canonical specific heat. Numerical solution of real-time evolution equations confirms the spontaneous concentration of energy in such arrays, with initially dispersed energy condensing quickly into dense 'droplets'. We therefore propose laboratory studies of negative specific heat as an elusive but fundamentally important aspect of thermodynamics, which may shed fresh light on the general problem of how thermodynamics emerges from mechanics.
3. arXiv:1301.5822 [pdf, ps, other]
Fractional Fermions with Non-Abelian Statistics
Jelena Klinovaja, Daniel Loss
We introduce a novel class of low-dimensional topological tight-binding models that allow for bound states that are fractionally charged fermions and exhibit non-Abelian braiding statistics. The proposed model consists of a double (single) ladder of spinless (spinful) fermions in the presence of magnetic fields. We study the system analytically in the continuum limit as well as numerically in the tight-binding representation. We find a topological phase transition with a topological gap that closes and reopens as a function of system parameters and chemical potential. The topological phase is of the type BDI and carries two degenerate mid-gap bound states that are localized at opposite ends of the ladders. We show numerically that these bound states are robust against a wide class of perturbations.
4. arXiv:1301.5639 [pdf, other]
Z2 topological zero-energy modes in commensurate Aubry-André-Harper models
Sriram Ganeshan, Kai Sun, S. Das Sarma
Aubry-Andr\'e (AA) model has been the subject of extensive theoretical research in the context of quantum localization. Recently, it is shown that one-dimensional quasicrystals described by the incommensurate Aubry-Andr\'e model has non-trivial topology. In this paper, we show that the commensurate off-diagonal Aubry-Andr\'e model is topologically nontrivial in the gapless regime and supports zero-energy edge modes with Z2 index. Unlike the incommensurate case, the nontrivial topology in the off-diagonal Aubry-Andr\'e model is attributed to the topological properties of the one-dimensional Majorana chain. We discuss the feasibility of experimental observability of our predicted Z2 topological phase.
5.arXiv:1301.5646 (cross-list from quant-ph) [pdf, other]
Area laws for thermal free fermions
H. Bernigau, M. J. Kastoryano, J. Eisert
We provide a rigorous and asymptotically exact expression of the mutual information of translationally invariant free fermionic lattice systems in a Gibbs state. In order to arrive at this result, we introduce a novel framework for computing determinants of Toeplitz operators with smooth symbols, and for treating Toeplitz matrices with system size dependent entries. The asymptotically exact mutual information for a partition of the one-dimensional lattice satisfies an area law, with a prefactor which we compute explicitly. As examples, we discuss the fermionic XX model in one dimension and free fermionic models on the torus in higher dimensions in detail. Special emphasis is put onto the discussion of the temperature dependence of the mutual information, scaling like the logarithm of the inverse temperature, hence confirming an expression suggested by conformal field theory. We also comment on the applicability of the formalism to treat open systems driven by quantum noise. In the appendix, we derive useful bounds to the mutual information in terms of purities. Finally, we provide a detailed error analysis for finite system sizes. This analysis is valuable in its own right for the abstract theory of Toeplitz determinants.
Jan 24
1. arXiv:1301.5556 [pdf, other]
Unconventional Superconductivity from Local Spin Fluctuations in the Kondo Lattice
Oliver Bodensiek, Rok Zitko, Matthias Vojta, Mark Jarrell, Thomas Pruschke
The explanation of heavy-fermion superconductivity is a long-standing challenge to theory. It is commonly thought to be connected to non-local fluctuations of either spin or charge degrees of freedom and therefore of unconventional type. Here we present results for the Kondo-lattice model, a paradigmatic model to describe heavy-fermion compounds, obtained from dynamical mean-field theory which captures local correlation effects only. Unexpectedly, we find robust s-wave superconductivity in the heavy-fermion state. We argue that this novel type of pairing is tightly connected to the formation of heavy quasiparticle bands and the presence of strong local spin fluctuations.
2. arXiv:1301.5427 [pdf, other]
Mie scattering analogue in graphene: lensing, particle confinement, and depletion of Klein tunneling
R. L. Heinisch, F. X. Bronold, H. Fehske
Guided by the analogy to Mie scattering of light on small particles we show that the propagation of a Dirac-electron wave in graphene can be manipulated by a circular gated region acting as a quatum dot. Large dots enable electron lensing, while for smaller dots resonant scattering entails electron confinement in quasibound states. Forward scattering and Klein tunneling can be almost switched off for small dots by a Fano resonance arising from the interference between resonant scattering and the background partition.
3.arXiv:1301.5403 [pdf, ps, other]Unconventional states of bosons with synthetic spin-orbit coupling
Xiangfa Zhou, Yi Li, Zi Cai, Congjun Wu
Spin-orbit coupling with bosons gives rise to novel properties that are absent in usual bosonic systems. Under very general conditions, the conventional ground state wavefunctions of bosons are constrained by the "no-node" theorem to be positive-definite. In contrast, the linear-dependence of spin-orbit coupling leads to complex-valued condensate wavefunctions beyond this theorem. In this article, we review the study of this class of unconventional Bose-Einstein condensations focusing on their topological properties. Both the 2D Rashba and 3D \sigma \cdot p type Weyl spin-orbit couplings give rise to Landau-level-like quantization of single-particle levels in the harmonic trap. The interacting condensates develop the half-quantum vortex structure spontaneously breaking time-reversal symmetry and exhibit topological spin textures of the skyrmion type. In particular, the 3D Weyl coupling generates topological defects in the quaternionic phase space as an SU(2) generalization of the usual U(1) vortices. Rotating spin-orbit coupled condensates exhibit rich vortex structures due to the interplay between vorticity and spin texture. In the Mott-insulating states in optical lattices, quantum magnetism is characterized by the Dzyaloshinskii-Moriya type exchange interactions.
4. arXiv:1301.5365 [pdf, ps, other]
Possible quantum phase-manipulation of a two-leg ladder in mixed-dimensional fermionic cold atoms
Wen-Min Huang, Kyle Irwin, Shan-Wen Tsai
The recent realization of mixed-dimensional systems of cold atoms has attracted much attention from both experimentalists and theorists. Different effective interactions and novel correlated quantum many-body phases may be engineered in these systems, with the different phases being tunable via external parameters. In this article we investigate a two-species Fermi atom mixture: one species of atom exists in two hyperfine states and is confined to move in a two-leg ladder, interacting with an on-site interaction, and the other moves freely in a two dimensional square lattice that contains the two-leg ladder. The two species of atoms interact via an on-site interaction on the ladder. In the limit of weak inter-species interactions, the two-dimensional gas can be integrated out, leading to an effective long-range mediated interaction in the ladder, generated by to the on-site inter-species interaction. We show that the form of the mediated interaction can be controlled by the density of the two-dimensional gas and that it enhances the charge density wave instability in the two-leg ladder after the renormalization group transformation. Parameterizing the phase diagram with various experimentally controllable quantities, we discuss the possible tuning of the macroscopic quantum many-body phases of the two-leg ladder in this mixed-dimensional fermionic cold atom system.
5. arXiv:1301.5329 [pdf, other]
Expansion dynamics of interacting bosons in homogeneous lattices in one and two dimensions
Jens Philipp Ronzheimer, Michael Schreiber, Simon Braun, Sean S. Hodgman, Stephan Langer, Ian P. McCulloch, Fabian Heidrich-Meisner, Immanuel Bloch, Ulrich Schneider
We experimentally and numerically investigate the expansion of initially localized ultracold bosons in homogeneous one- and two-dimensional optical lattices. We find that both dimensionality and interaction strength crucially influence these non-equilibrium dynamics. While the atoms expand ballistically in all integrable limits, deviations from these limits dramatically suppress the expansion and lead to the appearance of almost bimodal cloud shapes, indicating diffusive dynamics in the center surrounded by ballistic wings. For strongly interacting bosons, we observe a dimensional crossover of the dynamics from ballistic in the one-dimensional hard-core case to diffusive in two dimensions, as well as a similar crossover when higher occupancies are introduced into the system.
Jan 23
1. arXiv:1301.5305 [pdf, other]
The plateau-insulator transition in the Integer Quantum Hall Effect: When simulation meets experiment
Juntao Song, Emil Prodan
The transport coefficients for the Hofstadter model of non-interacting electrons in the presence of a magnetic field and strong on-site disorder are computed using an efficient numerical implementation of the non-commutative Kubo formula. The conductivity $\sigma$ and resistivity $\rho$ tensors are mapped as functions of Fermi energy $E_F$ and temperature $T$, and an asymptotic analysis in the limit of low temperatures is performed. In particular, the critical behaviors of the model at the plateau-insulator transition is simulated. The results reproduce all the key experimental findings on the plateau-insulator transition in the Integer Quantum Hall Effect: 1) The graphs of $\rho_{xx}$ as function of $E_F$ at different temperatures intersect each other at a single critical point; 2) All these graphs collapse into a single curve after a one-parameter re-scaling; 3) The scaling exponents are in good agreement with the existing theoretical predictions; 4) The flow of $\sigma$ with the temperature, plotted in the $(\sigma_{xy},\sigma_{xx})$ plane, obeys the semi-circle law; 5) At the critical point, $\sigma_{xx}=\sigma_{xy}=1/2\frac{e^2}{h}$; 6) The Quantized Hall Insulator phase, characterized by $\sigma_{xx}=\sigma_{xy}=0$ and $\rho_{xy}=h/e^2$, is observed at low temperatures.
Jan 22
1. arXiv:1301.4872 [pdf, other]
Density Matrix Topological Insulators
A. Rivas, O. Viyuela, M. A. Martin-Delgado
Thermal noise can destroy topological insulators (TI). However we demonstrate how TIs can be made stable in dissipative systems. To that aim, we introduce the notion of a band Liouvillian as the dissipative counterpart of a band Hamiltonian, and show a method to evaluate the topological order of its steady state. This is based on a generalization of the Chern number valid for general mixed states (referred as density matrix Chern value), which witnesses topological order in a system coupled to external noise. Additionally, we study its relation with the electrical conductivity at finite temperature, which is not topologically invariant. Nonetheless, the density matrix Chern value represents the part of the conductivity which is topological due to the presence of quantum mixed edge states at finite temperature. To make our formalism concrete, we apply these concepts to the two-dimensional Haldane model in the presence of thermal dissipation, but our results hold for arbitrary dimensions and density matrices.
2. arXiv:1301.4776 [pdf, other]
Laser cooling to quantum degeneracy
Simon Stellmer, Benjamin Pasquiou, Rudolf Grimm, Florian Schreck
We report on Bose-Einstein condensation (BEC) in a gas of strontium atoms, using laser cooling as the only cooling mechanism. The condensate is formed within a sample that is continuously Doppler cooled to below 1\muK on a narrow-linewidth transition. The critical phase-space density for BEC is reached in a central region of the sample, in which atoms are rendered transparent for laser cooling photons. The density in this region is enhanced by an additional dipole trap potential. Thermal equilibrium between the gas in this central region and the surrounding laser cooled part of the cloud is established by elastic collisions. Condensates of up to 10^5 atoms can be repeatedly formed on a timescale of 100ms, with prospects for the generation of a continuous atom laser.
3.arXiv:1301.4494 [pdf, other]DiracQ: A Quantum Many-Body Physics Package
John G. Wright, B. Sriram Shastry
We present a software package DiracQ, for use in quantum many-body Physics. It is designed for helping with typical algebraic manipulations that arise in quantum Condensed Matter Physics and Nuclear Physics problems, and also in some subareas of Chemistry. DiracQ is invoked within a Mathematica session, and extends the symbolic capabilities of Mathematica by building in standard commutation and anticommutation rules for several objects relevant in many-body Physics. It enables the user to carry out computations such as evaluating the commutators of arbitrary combinations of spin, Bose and Fermi operators defined on a discrete lattice, or the position and momentum operators in the continuum. Some examples from popular systems, such as the Hubbard model, are provided to illustrate the capabilities of the package.
Jan 21
1. arXiv:1301.4492 [pdf, other]
Constructing gapless spin liquid state for the spin-1/2 J1-J2 Heisenberg model on a square lattice
Ling Wang, Didier Poilblanc, Zheng-Cheng Gu, Xiao-Gang Wen, Frank Verstraete
We construct a class of projected entangled pair states (PEPS) which is exactly the resonating valence bond (RVB) wavefunctions endowed with both short range and long range valence bonds. With an energetically preferred RVB pattern, the wavefunction is simplified to live in a one parameter variational space. We tune this variational parameter to minimize the energy for the frustrated spin 1/2 J1-J2 antiferromagnetic Heisenberg model on the square lattice. Taking a cylindrical geometry, we are able to construct four topological sectors with even or odd number of fluxes penetrating the cylinder and even or odd number of spinons on the boundary. The energy splitting in different topological sectors is exponentially small with the cylinder perimeter. We find a power law decay of the dimer correlation function on a torus, and a lnL correction to the entanglement entropy, indicating a gapless spin liquid phase at the optimum parameter.
2. arXiv:1301.4473 [pdf, ps, other]
Super Efimov effect of resonantly interacting fermions in two dimensions
Sergej Moroz, Yusuke Nishida, Dam Thanh Son
We study a system of spinless fermions in two dimensions with a short-range interaction fine-tuned to a p-wave resonance. We show that three such fermions form an infinite tower of bound states of orbital angular momentum |l|=1 and their binding energies obey a universal doubly exponential scaling, E_3 exp(-2e^{3\pi n/4+\theta}), at large n. This "super Efimov effect" is found by a renormalization group analysis and confirmed by solving the bound state problem. We also provide an indication that there are |l|=2 four-body resonances associated with every three-body bound state at E_4 exp(-2e^{3\pi n/4+\theta-0.188}). These universal few-body states can be observed in ultracold atom experiments and should be taken into account in future many-body studies of the system.
3. arXiv:1301.4426 [pdf, ps, other]Engineering quantum anomalous Hall phases with orbital and spin degrees of freedom
Hongbin Zhang, Frank Freimuth, Gustav Bihlmayer, Marjana Ležaić, Stefan Blügel, Yuriy Mokrousov
Combining tight-binding models and first principles calculations, we investigate the quantum anomalous Hall (QAH) effect induced by intrinsic spin-orbit coupling (SOC) in buckled honeycomb lattice with sp orbitals in an external exchange field. Detailed analysis reveals that nontrivial topological properties can arise utilizing not only spin but also orbital degrees of freedom in the strong SOC limit, when the bands acquire non-zero Chern numbers upon undergoing the so-called orbital purification. As a prototype of a buckled honeycomb lattice with strong SOC we choose the Bi(111) bilayer, analyzing its topological properties in detail. In particular, we show the emergence of several QAH phases upon spin exchange of the Chern numbers as a function of SOC strength and magnitude of the exchange field. Interestingly, we observe that in one of such phases, namely, in the quantum spin Chern insulator phase, the quantized charge and spin Hall conductivities co-exist. We consider the possibility of tuning the SOC strength in Bi bilayer via alloying with isoelectronic Sb, and speculate that exotic properties could be expected in such an alloyed system owing to the competition of the topological properties of its constituents. Finally, we demonstrate that 3d dopants can be used to induce a sizeable exchange field in Bi(111) bilayer, resulting in non-trivial Chern insulator properties.
Jan 14 - Jan 18, Saubhik Sarkar
Jan 18
1. arXiv:1301.4156 [pdf, ps, other]
Self-organized exotic lattices with ultracold gases
O. Dutta, A. Przysiezna, M. Lewenstein
We study an ultracold Fermi-Fermi mixture of strongly attractive atoms trapped in a square lattice. We show that the interaction-induced multi-band nature of such system can change the inherent structure of the original lattice. The dynamically generate lattice resembles geometrically a Lieb lattice with interaction-driven topological insulator phases.
2.arXiv:1301.4119 [pdf, ps, other]
Fermi edge polaritons in a highly degenerate 2D electron gas: a diagrammatic theory
Maarten Baeten, Michiel Wouters
We present a theoretical study on polaritons in highly doped semiconductor microcavities. In particular, we focus on a cavity mode that is resonant with the absorption threshold (`Fermi edge'). In agreement with experimental results, the strong light-matter coupling is maintained under very high doping within our ladder diagram approximation. While the lower polariton is qualitatively unaltered, it acquires a finite lifetime due to relaxation of the valence band hole if the electron density exceeds a certain critical value. On the other hand the upper polariton has a finite lifetime for all densities, because it lies in the electron-hole continuum where no bound state exists. Our calculations show that a narrow upper polariton quasiparticle still exists as a result from the interplay between light-matter coupling and final state Coulomb interaction.
Jan 17
1. arXiv:1301.3642 [pdf, ps, other]
First-Order Quantum Phase Transition in Bose Gases
Nguyen Thanh Phuc, Yuki Kawaguchi, Masahito Ueda
The Bogoliubov theory has proven to be an accurate and versatile tool in the study of weakly- interacting dilute Bose gases at low temperatures, yet there is one exception where it goes qualita- tively wrong, i.e., near the first-order quantum phase transitions. By examining a phase transition in spinor Bose-Einstein condensates (BECs), we find that the energy spectrum given by the Bogoli- ubov theory is inconsistent with the fact that the phase transition is first order. We resolve this problem by calculating the spectrum based on the spinor version of the Beliaev theory. We also discuss the ground-state phase diagram of spin-2 BECs which is modified by quantum fluctuations and the possibility of macroscopic quantum tunneling near the cyclic-nematic phase boundary.
2. arXiv:1301.3566 [pdf, ps, other]
Deviation from Universality in Collisions of Ultracold $^6$Li$_2$ Molecules
Tout T. Wang, Myoung-Sun Heo, Timur M. Rvachov, Dylan A. Cotta, Wolfgang Ketterle
Collisions of $^6$Li$_2$ molecules reveal a striking deviation from universal predictions based on long-range van der Waals interactions. Li$_2$ closed-channel molecules are formed in the highest vibrational state near a narrow Feshbach resonance, and decay via two-body collisions with Li$_2$, Li, and Na. For Li$_2$+Li$_2$ and Li$_2$+Na, the decay rates agree with the universal predictions of the quantum Langevin model. In contrast, the rate for Li$_2$+Li is exceptionally small, with an upper bound ten times smaller than the universal prediction. This can be explained by the low density of available decay states in systems of light atoms [G. Qu\'em\'ener, J.-M. Launay, and P. Honvault, Phys. Rev. A 75, 050701 (2007)], for which such collisions have not been studied before.
3. arXiv:1301.3553 [pdf, other]
Glassy dynamics and Landau-Zener phenomena in trapped quasi-one dimensional coupled Bose-Einstein condensates
Santiago F. Caballero-Benitez, Rosario Paredes
The purpose of this article is to address the dynamics of an interacting Bose-Einstein condensate confined in coupled one-dimensional Landau-Zener arrays under the influence of disorder and harmonic confinement. In particular, we concentrate in studying the interplay of disorder and interparticle interaction on the transfer of atoms depending on the speed of Landau-Zener sweeps. A dynamical phase diagram summarizing the final situation across ground state and inverse sweeps is given in terms of the effect of disorder, interaction and the speed of the sweeps.
4. arXiv:1301.3693 (cross-list from cond-mat.stat-mech) [pdf, ps, other]
Equilibrium and nonequilibrium entanglement properties of 2D and 3D Fermi gases
Jacopo Nespolo, Ettore Vicari
We investigate the entanglement properties of the equilibrium and nonequilibrium quantum dynamics of 2D and 3D Fermi gases, by computing entanglement entropies of extended space regions, which generally show multiplicative logarithmic corrections to the leading power-law behaviors, corresponding to the logarithmic corrections to the area law.
We consider 2D and 3D Fermi gases of N particles constrained within a limited space region, for example by a hard-wall trap, at equilibrium at T=0, i.e. in their ground state, and compute the first few terms of the asymptotic large-N behaviors of entanglement entropies and particle fluctuations of subsystems with some convenient geometries, which allow us to significantly extend their computation. Then, we consider their nonequilibrium dynamics after instantaneously dropping the hard-wall trap, which allows the gas to expand freely. We compute the time dependence of the von Neumann entanglement entropy of space regions around the original trap. We show that at small time it is characterized by the relation $S \approx \pi^2 V/3$ with the particle variance, and multiplicative logarithmic corrections to the leading power law, i.e. $S \sim t^{1-d}\ln(1/t)$.
Jan 16
1. arXiv:1301.3406 [pdf, ps, other]
Thermal spin fluctuations in spinor Bose-Einstein condensates
Marina Melé-Messeguer, Bruno Juliá-Díaz, Artur Polls, Luis Santos
We study the thermal activation of spin fluctuations in dynamically-stable spinor Bose-Einstein condensates. We analyze the specific cases of a non-dipolar spin-1 condensate in m = 0, where thermal activation results from spin-changing collisions, and of a Chromium condensate in the maximally stretched state m = -3, where thermal spin fluctuations are due to dipole-induced spin- relaxation. In both cases, we show that the low energy associated to the spinor physics may be employed for thermometry purposes down to extremely low temperatures, typically impossible to measure in BECs with usual thermometric techniques. Moreover, the peculiar dependence of the system's entropy with the applied Zeeman energy opens a possible route for adiabatic cooling.
2. arXiv:1301.3251 [pdf, ps, other]
Phase diagram of a rapidly-rotating two-component Bose gas
E. Ö. Karabulut, F. Malet, G. M. Kavoulakis, S. M. Reimann
We derive analytically the phase diagram of a two-component Bose gas confined in an anharmonic potential, which becomes exact and universal in the limit of weak interactions and small anharmonicity of the trapping potential. The transitions between the different phases, which consist of vortex states of single and multiple quantization, are all continuous because of the addition of the second component.
3. arXiv:1301.3470 (cross-list from cond-mat.str-el) [pdf, ps, other]
Mott insulators of ultracold fermionic alkaline earth atoms in three dimensions
Hao Song, Michael Hermele
We study a class of SU(N) Heisenberg models, describing Mott insulators of fermionic ultra-cold alkaline earth atoms on the three-dimensional simple cubic lattice. Based on an earlier semiclassical analysis, magnetic order is unlikely, and we focus instead on a solvable large-N limit designed to address the competition among non-magnetic ground states. We find a rich phase diagram as a function of the filling parameter k, composed of a variety of ground states spontaneously breaking lattice symmetries, and in some cases also time reversal symmetry. One particularly striking example is a state spontaneously breaking lattice rotation symmetry, where the cubic lattice breaks up into bilayers, each of which forms a two-dimensional chiral spin liquid state.
4. arXiv:1301.3139 (cross-list from cond-mat.stat-mech) [pdf, other]
Universal properties of the Higgs resonance in (2+1)-dimensional U(1) critical systems
Kun Chen, Longxiang Liu, Youjin Deng, Lode Pollet, Nikolay Prokof'ev
We present spectral functions for the magnitude squared of the order parameter in the scaling limit of the two-dimensional superfluid to Mott insulator quantum phase transition at constant density, which has emergent particle-hole symmetry and Lorentz invariance. The universal functions for the superfluid, Mott insulator, and normal liquid phases reveal a Higgs resonance which is relatively sharp and is followed by a damped oscillation (in the first two phases only) before saturating to the quantum critical plateau. In order to understand the counter-intuitive Higgs resonance in the insulating and normal phases, we invoke a picture of a scale-dependent Mexican hat. Our results are derived from analytically continued correlation functions obtained from path-integral Monte Carlo simulations of the Bose-Hubbard model.
Jan 15
1. arXiv:1301.2869 [pdf, ps, other]
Ultracold bosons with the synthetic three-dimensional spin-orbit coupling in an optical lattice
Dan-Wei Zhang, Ji-Pei Chen, Chuan-Jia Shan, Z. D. Wang, Shi-Liang Zhu
We study ultracold bosonic atoms with the synthetic three-dimensional spin-orbit (SO) coupling in a cubic optical lattice. In the superfluidity phase, the lowest energy band exhibits one, two or four pairs of degenerate single-particle ground states depending on the SO-coupling strengths, which can give rise to the condensate states with spin-stripes for the weak atomic interactions. In the deep Mott-insulator regime, the effective spin Hamiltonian of the system combines three-dimensional Heisenberg exchange interactions, anisotropy interactions and Dzyaloshinskii-Moriya interactions. Based on Monte Carlo simulations, we numerically demonstrate that the resulting Hamiltonian with an additional Zeeman field has a rich phase diagram with spiral, stripe, vortex crystal, and especially Skyrmion crystal spin-textures in each xy-plane layer. The obtained Skyrmion crystals can be tunable with square and hexagonal symmetries in a columnar manner along the z axis, and moreover are stable against the inter-layer spin-spin interactions in a large parameter region.
2. arXiv:1301.2803 [pdf, other]
Heating dynamics of bosonic atoms in a noisy optical lattice
Hannes Pichler, Johannes Schachenmayer, Andrew J. Daley, Peter Zoller
We analyze the heating of interacting bosonic atoms in an optical lattice due to intensity fluctuations of the lasers forming the lattice. We focus in particular on fluctuations at low frequencies below the band gap frequency, such that the dynamics is restricted to the lowest band. We derive stochastic equations of motion, and analyze the effects on different many-body states, characterizing heating processes in both strongly and weakly interacting regimes. In the limit where the noise spectrum is flat at low frequencies, we can derive an effective Master equation describing the dynamics. We compute heating rates and changes to characteristic correlation functions both in the perturbation theory limit, and using a full time-dependent calculation of the stochastic many-body dynamics in 1D based on time-dependent density-matrix-renormalization-group methods.
Jan 7 - Jan 11, Johannes Schachenmayer
Jan 11
1. arXiv:1301.2199 [pdf, ps, other]
Truncated many-body dynamics of interacting bosons: A variational principle with error monitoring
Kang-Soo Lee, Uwe R. FischerWe introduce a scheme to describe the evolution of an interacting system of bosons, for which the field operator expansion is truncated after a finite number of modes, in a rigourously controlled manner. Using McLachlan's principle of least error, we find a self-consistent set of equations for the many-body state. As a particular benefit, and in distinction to previously proposed approaches, our approach allows for the dynamical increase of the number of orbitals during the temporal evolution. The additional orbitals, determined by the condition of least error of the truncated evolution relative to the exact one, are obtained from an initial trial state by a method we call steepest constrained descent.
2. arXiv:1301.2159 [pdf, ps, other]
Bound states of Dipolar Bosons in One-dimensional Systems
A. G. Volosniev, J. R. Armstrong, D. V. Fedorov, A. S. Jensen, M. Valiente, N. T. ZinnerWe consider one-dimensional tubes containing bosonic polar molecules. The long-range dipole-dipole interactions act both within a single tube and between different tubes. We consider arbitrary values of the externally aligned dipole moments with respect to the symmetry axis of the tubes. The few-body structures in this geometry are determined as function of polarization angles and dipole strength by using both essentially exact stochastic methods and the harmonic approximation. The main focus is on the three, four, and five-body problems in two or more tubes. Our results indicate that in the weakly-coupled limit the inter-tube interaction is similar to a zero-range term with a suitable rescaled strength. This allows us to address the corresponding many-body physics of the system by constructing a model where bound chains with one molecule in each tube are the effective degrees of freedom. This model can be mapped onto one-dimensional Hamiltonians for which exact solutions are known.
Jan 10
1. arXiv:1301.1691 [pdf, ps, other]
Magnetic phases of mass- and population-imbalanced ultracold fermionic mixtures in optical lattices
Andrii Sotnikov, Michiel Snoek, Walter HofstetterWe study magnetic phases of two-component mixtures of ultracold fermions with repulsive interactions in optical lattices in the presence of both hopping and population imbalance by means of dynamical mean-field theory (DMFT). It is shown that these mixtures can have easy-axis antiferromagnetic, ferrimagnetic, charge-density wave and canted-antiferromagnetic order or be unordered depending on parameters of the system. We study the resulting phase diagram in detail and investigate the stability of the different phases with respect to thermal fluctuations. We also perform a quantitative analysis for a gas confined in a harmonic trap, both within the local density approximation and using a full real-space generalization of DMFT.
2. arXiv:1301.1963 [pdf, other]
Controlling spontaneous-emission noise in measurement-based feedback cooling of a Bose-Einstein Condensate
M. R. Hush, S. S. Szigeti, A. R. R. Carvalho, J. J. HopeA Bose-Einstein condensate (BEC) undergoing measurement-based feedback cooling is analysed with a full quantum-field simulation. Two experimental setups are considered: a BEC in a cavity and a trapped BEC undergoing phase-contrast imaging. An experimentally important parameter regime is found where the simulation results diverge from the predictions of both single-mode quantum models and multi-mode semiclassical models. In this regime, simple cooling schemes cannot control the quantum noise limited heating due to measurement backaction. We describe a feedback scheme that is specifically adapted to the measurement, which can still produce cooling.
3. arXiv:1301.1869 [pdf, ps, other]
First-order Superfluid-Mott-Insulator Transition for Quantum Optical Switching in Cavity QED Arrays with two cavity modes
Kenji Kamide, Makoto Yamaguchi, Takashi Kimura, Tetsuo OgawaWe theoretically investigated the ground states of coupled arrays of cavity quantum electrodynamical (CQED) systems in presence of two photon modes. Within the Gutzwiller-type variational approach, we found the first-order quantum phase transition between Mott insulating and superfluid phases as well as the conventional second-order one. The first-order phase transition was found only for a specific type of emitter models, and its physical origin is clarified based on the analytic arguments which are allowed in the perturbative and semiclassical limits. The first-order transition of the correlated photons is accompanied with discontinuous change in the emitter states, not only with the appearance of inter-cavity coherence of photons. Our result gives a clear insight on the condition for the firsy-order transition to occur, and can give a strategy for future design of quantum optical switching device with CQED arrays.
4. arXiv:1301.1766 [pdf, ps, other]
Nonlinear Spectroscopic Effects in Quantum Gases Induced by Atom-Atom Interactions
Alexander Safonov, Irina Safonova, Igor Yasnikov
We consider nonlinear spectroscopic effects - interaction-enhanced double resonance and spectrum instability - that appear in ultracold quantum gases owing to collisional frequency shift of atomic transitions and, consequently, due to the dependence of the frequencies on the population of various internal states of the particles. Special emphasis is put to two simplest cases, (a) the gas of two-level atoms and (b) double resonance in a gas of three-level bosons, in which the probe transition frequency remains constant.
Jan 9
1. arXiv:1301.1621 [pdf, other]
Non-universal bound states of two identical heavy fermions and one light particle
A. Safavi-Naini, Seth. T. Rittenhouse, D. Blume, H. R. SadeghpourWe study the behavior of the bound state energy of a system consisting of two identical heavy fermions of mass M and a light particle of mass m. The heavy fermions interact with the light particle through a short-range two-body potential with positive s-wave scattering length a_s. We impose a short-range boundary condition on the logarithmic derivative of the hyperradial wavefunction and show that, in the regime where Efimov states are absent, a non-universal three-body state "cuts through" the universal three-body states previously described by Kartavtsev and Malykh [O. I. Kartavtsev and A. V. Malykh, J. Phys. B 40, 1429 (2007)]. The presence of the non-universal state alters the behavior of the universal states in certain regions of the parameter space. We show that the existence of the non-universal state is predicted accurately by a simple quantum defect theory model that utilizes hyperspherical coordinates. An empirical two-state model is employed to quantify the coupling of the non-universal state to the universal states.
2. arXiv:1301.1353 [pdf, ps, other]
Parity violating superfluidity in ultra-cold fermions under the influence of artificial non-Abelian gauge fields
Kangjun Seo, Li Han, C. A. R. Sá de MeloWe discuss the creation of parity violating Fermi superfluids in the presence of non-Abelian gauge fields involving spin-orbit coupling and crossed Zeeman fields. We focus on spin-orbit coupling with equal Rashba and Dresselhaus (ERD) strengths which has been realized experimentally in ultra-cold atoms, but we also discuss the case of arbitrary mixing of Rashba and Dresselhaus (RD) and of Rashba-only (RO) spin-orbit coupling. To illustrate the emergence of parity violation in the superfluid, we analyze first the excitation spectrum in the normal state and show that the generalized helicity bands do not have inversion symmetry in momentum space when crossed Zeeman fields are present. This is also reflected in the superfluid phase, where the order parameter tensor in the generalized helicity basis violates parity. However, the pairing fields in singlet and triplet channels of the generalized helicity basis are still parity even and odd, respectively. Parity violation is further reflected on ground state properties such as the spin-resolved momentum distribution, and in excitation properties such as the spin-dependent spectral function and density of states.
3. arXiv:1301.1342 [pdf, ps, other]
Quantum simulation of many-body spin interactions with ultracold polar molecules
Hendrik WeimerWe present an architecture for the quantum simulation of many-body spin interactions based on ultracold polar molecules trapped in optical lattices. Our approach employs digital quantum simulation, i.e., the dynamics of the simulated system is reproduced by the quantum simulator in a stroboscopic pattern, and allows to simulate both coherent and dissipative dynamics. We discuss the realization of Kitaev's toric code Hamiltonian, a paradigmatic model involving four-body interactions, and we analyze the requirements for an experimental implementation.
4. arXiv:1301.1451 [pdf, other]
Cavity-Enhanced Long-Distance Coupling of an Atomic Ensemble to a Micromechanical Membrane
B. Vogell, K. Stannigel, P. Zoller, K. Hammerer, M. T. Rakher, M. Korppi, A. Jöckel, P. TreutleinWe discuss a hybrid quantum system where a dielectric membrane situated inside an optical cavity is coupled to a distant atomic ensemble trapped in an optical lattice. The coupling is mediated by the exchange of sideband photons of the lattice laser, and is enhanced by the cavity finesse as well as the square root of the number of atoms. In addition to observing coherent dynamics between the two systems, one can also switch on a tailored dissipation by laser cooling the atoms, thereby allowing for sympathetic cooling of the membrane. The resulting cooling scheme does not require resolved sideband conditions for the cavity, which relaxes a constraint present in standard optomechanical cavity cooling. We present a quantum mechanical treatment of this modular open system which takes into account the dominant imperfections, and identify optimal operation points for both coherent dynamics and sympathetic cooling. In particular, we find that ground state cooling of a cryogenically pre-cooled membrane is possible for realistic parameters.
5. arXiv:1301.1344 [pdf, ps, other]
Non-equilibrium Fractional Quantum Hall state of light
Mohammad Hafezi, Mikhail D. Lukin, Jacob M. Taylor
We investigate the quantum dynamics of systems involving small numbers of strongly interacting photons. Specifically, we develop an efficient method to investigate such systems when they are externally driven with a coherent field. Furthermore, we show how to quantify the many-body quantum state of light via correlation functions. Finally, we apply this method to two strongly interacting cases: the Bose-Hubbard and fractional quantum Hall models, and discuss an implementation of these ideas in atom-photon system.
Jan 8
1. arXiv:1301.1236 [pdf, other]
Resonant control of cold-atom transport through two optical lattices with a constant relative speed
M.T. Greenaway, A.G. Balanov, T.M. FromholdWe show theoretically that the dynamics of cold atoms in the lowest energy band of a stationary optical lattice can be transformed and controlled by a second, weaker, periodic potential moving at a constant speed along the axis of the stationary lattice. The atom trajectories exhibit complex behavior, which depends sensitively on the amplitude and speed of the propagating lattice. When the speed and amplitude of the moving potential are low, the atoms are dragged through the static lattice and perform drifting orbits with frequencies an order of magnitude higher than that corresponding to the moving potential. Increasing either the speed or amplitude of the moving lattice induces Bloch-like oscillations within the energy band of the static lattice, which exhibit complex resonances at critical values of the system parameters. In some cases, a very small change in these parameters can reverse the atom's direction of motion. In order to understand these dynamics we present an analytical model, which describes the key features of the atom transport and also accurately predicts the positions of the resonant features in the atom's phase space. The abrupt controllable transitions between dynamical regimes, and the associated set of resonances, provide a mechanism for transporting atoms between precise locations in a lattice: as required for using cold atoms to simulate condensed matter or as a stepping stone to quantum information processing. The system also provides a direct quantum simulator of acoustic waves propagating through semiconductor nanostructures in sound analogs of the optical laser (SASER).
2. arXiv:1301.1145 [pdf, ps, other]
Dynamical Properties of Quasi-One-Dimensional Boson-Fermion Mixtures of Atoms in a Toroidal Potential
Ryosuke Shibato, Takushi NishimuraWe theoretically investigate quantum-mechanical dynamics of quasi-one-dimensional boson-fermion mixtures of atomic gases trapped in a toroidal potential, where effective inter-atomic interactions are tunable and affect the dynamics. We especially focus on effects of quantum statistics and many-body correlations beyond the Hartree-Fock (HF) mean-field approximation on the dynamics. In order to predict the dynamics, we utilize the numerical exact diagonalization method and also reproduce the calculation in the HF approximation for comparison. The toroidal gases originally have a rotational symmetry in the toroidal direction. We firstly prepare a deformed ground state as an initial state by adding a weak potential deformed in the toroidal direction, and then remove the potential to start the dynamics. In the dynamics, number densities of the deformed gases exhibit oscillations as demonstrated in the present paper. As a result, we find out that the bosons and fermions show quite different behaviors owing to quantum statistics. In particular, the bosons exhibit a low-frequency oscillation in the strong boson-boson attraction regime owing to the many-body correlations, and it can not be reproduced in the HF approximation. The oscillation of the fermions is strongly influenced by that of the bosons through the boson-fermion interaction as a forced oscillator. In addition, we also discuss a relationship between the low-frequency oscillation and restoration of the broken symmetry.
3. arXiv:1301.1139 [pdf, ps, other]
Pauli paramagnetism of an ideal Fermi gas
Ye-Ryoung Lee, Tout T. Wang, Timur M. Rvachov, Jae-Hoon Choi, Wolfgang Ketterle, Myoung-Sun HeoWe show how to use trapped ultracold atoms to measure the magnetic susceptibility of a two-component Fermi gas. The method is illustrated for a non-interacting gas of $^6$Li, using the tunability of interactions around a wide Feshbach resonances. The susceptibility versus effective magnetic field is directly obtained from the inhomogeneous density profile of the trapped atomic cloud. The wings of the cloud realize the high field limit where the polarization approaches 100%, which is not accessible for an electron gas.
4. arXiv:1301.1267 [pdf, other]
Correlated Dirac Particles and Superconductivity on the Honeycomb Lattice
Wei Wu, Michael M. Scherer, Carsten Honerkamp, Karyn Le HurWe investigate the properties of the nearest-neighbor singlet pairing and the emergence of d-wave superconductivity in the doped honeycomb lattice considering the limit of large interactions and the $t-J_1-J_2$ model. First, by applying a renormalized mean-field procedure as well as slave-boson theories which account for the proximity to the Mott insulating state, we confirm the emergence of d-wave superconductivity in agreement with earlier works. We show that a small but finite $J_2$ spin coupling between next-nearest neighbors stabilizes d-wave symmetry compared to the extended s-wave scenario. At small hole doping, to minimize energy and to gap the whole Fermi surface or all the Dirac points, the superconducting ground state is characterized by a $d+id$ singlet pairing assigned to one valley and a $d-id$ singlet pairing to the other, which then preserves time-reversal symmetry. The slightly doped situation is distinct from the heavily doped case (around 3/8 and 5/8 filling) supporting a pure chiral $d+id$ symmetry and breaking time-reversal symmetry. Then, we apply the functional Renormalization Group and we study in more detail the competition between antiferromagnetism and superconductivity in the vicinity of half-filling. We discuss possible applications to strongly-correlated compounds with Copper hexagonal planes such as In$_3$Cu$_{2}$VO$_9$. Our findings are also relevant to the understanding of exotic superfluidity with cold atoms.
Jan 7
1. arXiv:1301.0718 [pdf, other]
Entangling two distinguishable matter-wave bright solitons via collisions
Bettina Gertjerenken, Thomas P. Billam, Caroline L. Blackley, C. Ruth Le Sueur, Lev Khaykovich, Simon L. Cornish, Christoph WeissWe investigate numerically the collisions of two distinguishable quantum matter-wave bright solitons in a one-dimensional harmonic trap. We show that such collisions can be used to generate mesoscopic Bell states which can reliably be distinguished from statistical mixtures. Calculation of the relevant s-wave scattering lengths reveals that such states could potentially be realized in quantum-degenerate mixtures of 85Rb and 133Cs. In addition to fully quantum simulations for two distinguishable two-particle solitons, we use a mean-field description supplemented by a stochastic treatment of quantum fluctuations in the soliton's center of mass: We demonstrate the validity of this approach by comparison to an effective potential treatment of the quantum many-particle problem.
2. arXiv:1301.0658 [pdf, ps, other]
Non-equilibrium spin dynamics and Zitterbewegung in quenched spin-orbit coupled Bose-Einstein condensates
Chunlei Qu, Chris Hamner, Ming Gong, Chuanwei Zhang, Peter EngelsSpin-orbit coupled ultra-cold atoms provide an intriguing new avenue for the study of rich spin dynamics in superfluids. In this Letter, we investigate non-equilibrium spin dynamics in a spin-orbit coupled Bose-Einstein condensate (BEC) that are induced by sudden quantum quenches of the Hamiltonian. We observe a broad range of spin dynamics on different time scales, ranging from short time Zitterbewegung oscillations between two spin-orbit coupled bands, to long time spin relaxation and heating of the BEC induced by the interactions between atoms. We also demonstrate how quantum quenches can be exploited to populate a higher spin-orbit band, and observe a subsequent dipole motion. Our experimental results are corroborated by a theoretical and numerical analysis and showcase the great flexibility that ultracold atoms provide for investigating non-equilibrium spin dynamics.
3. arXiv:1301.0800 [pdf, other]
Emergence of Topological and Strongly Correlated Ground States in trapped Rashba Spin-Orbit Coupled Bose Gases
B. Ramachandhran, Hui Hu, Han PuWe theoretically study an interacting few-body system of Rashba spin-orbit coupled two-component Bose gases confined in a harmonic trapping potential. We solve the interacting Hamiltonian at large Rashba coupling strengths using Exact Diagonalization scheme, and obtain the ground state phase diagram for a range of interatomic interactions and particle numbers. At small particle numbers, we observe that the bosons condense to an array of topological states with n+1/2 quantum angular momentum vortex configurations, where n = 0, 1, 2, 3... At large particle numbers, we observe two distinct regimes: at weaker interaction strengths, we obtain ground states with topological and symmetry properties that are consistent with mean-field theory computations; at stronger interaction strengths, we report the emergence of strongly correlated ground states.
Jan 1 - Jan 4, Stephan Langer
Tue Jan 1
1. arXiv:1212.6826 [pdf, ps, other]
Fulde-Ferrell-Larkin-Ovchinnikov Phases in Two-dimensional Spin-Orbit Coupled Degenerate Fermi Gases
Zhen Zheng, Ming Gong, Yichao Zhang, Xubo Zou, Chuanwei Zhang, Guangcan Guo
The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, a superconducting state with non-zero total momentum Cooper pairs in a strong magnetic field, was predicted more than 50 years ago and now becomes an important concept in many branches of physics. However, no unambiguous experimental evidences for the existence of FFLO phases have been observed yet. Recently, spin-imbalanced ultracold degenerate Fermi gases have emerged as a new powerful platform for the observation of FFLO phases due to their high experimental controllability and the lack of disorder. However, in three dimensional degenerate Fermi gases where the FFLO phases can be described within the simple mean field theory, the parameter region for the FFLO phases is too small to be observed in experiments. Recently, we showed that the Rashba type spin-orbit coupling and in-plane Zeeman field in three dimensional degenerate Fermi gases provide a more efficient way to create the FFLO phase because (1) the parameter region for the FFLO phase is greatly enlarged in the phase diagram and (2) the FFLO phase is stabilized due to the enhanced energy difference between FFLO phase and conventional Bardeen-Cooper-Schrieffer (BCS) phase. In this work we investigate the FFLO phase in two dimensional spin-orbit coupled degenerate Fermi gases using the mean field theory, with special concerns on the physical origin of the FFLO state. The basic properties of the FFLO phase are discussed and the phase diagram is obtained at zero temperature. The symmetry of different quantum phases is examined, which provide important basis for the experimental observation of FFLO phases using the time-of-flight imaging.
3. arXiv:1212.6884 [pdf, ps, other]
Quantum phase transition in an atom-molecule conversion system with atomic hopping
Ning-Ju Hui, Li-Hua Lu, Xiao-Qiang Xu, You-Quan Li
The quantum phase transition in an atom-molecule conversion system with atomic hopping between different hyperfine states is studied. In mean field approximation, we give the phase diagram whose phase boundary only depends on the atomic hopping strength and the atom-molecule energy detuning but not on the atomic interaction. Such a phase boundary is further confirmed by the fidelity of the ground state and the energy gap between the first-excited state and the ground one. In comparison to mean field approximation, we also study the quantum phase transition in full quantum method, where the phase boundary can be affected by the particle number of the system. Whereas, with the help of finite-size scaling behaviors of energy gap, fidelity susceptibility and the first-order derivative of entanglement entropy, we show that one can obtain the same phase boundary by the MFA and full quantum methods in the limit of $N\rightarrow \infty$. Additionally, our results show that the quantum phase transition can happens at the critical value of the atomic hopping strength even if the atom-molecule energy detuning is fixed on a certain value, which provides one a new way to control the quantum phase transition.
1. arXiv:1212.6934 [pdf, other]
Elementary excitations of ultracold soft-core bosons across the superfluid-supersolid phase transition
Tommaso Macri, Fabian Maucher, Fabio Cinti, Thomas Pohl
We investigate the zero-temperature excitation spectrum of two-dimensional soft-core bosons for a wide range parameters and across the phase transition from a superfluid to a supersolid state. Based on mean field calculations and recent Quantum Monte Carlo results, we demonstrate the applicability of the Bogoliubov-de Gennes equations, even at high interaction strengths where the system forms an insulating cluster crystal. Interestingly, our study reveals that the maximum energy of the longitudinal phonon band in the supersolid phase connects to the maxon energy of the superfluid at the phase transition.
Thu Jan 3
1. arXiv:1301.0308 [pdf, other]
Relaxation dynamics of conserved quantities in a weakly non-integrable one-dimensional Bose gas
Giuseppe Brandino, Jean-Sebastien Caux, Robert Konik
Comments: 11 pages, 6 figures
In this work we report preliminary results on the relaxational dynamics of one dimensional Bose gases, as described by the Lieb-Liniger model, upon release from a parabolic trap. We explore the effects of integrability and integrability breaking upon these dynamics by placing the gas post-release in an integrability breaking one-body cosine potential of variable amplitude. By studying the post-quench evolution of the conserved charges that would exist in the purely integrable limit, we begin to quantify the effects of the weak breaking of integrability on the long time thermalization of the gas.
2. arXiv:1301.0059 [pdf, ps, other]
All-time dynamics of continuous-time random walks on complex networks
Hamid Teimouri, Anatoly B. Kolomeisky
The concept of continuous-time random walks (CTRW) is a generalization of ordinary random walk models, and it is a powerful tool for investigating a broad spectrum of phenomena in natural, engineering, social and economic sciences. Recently, several theoretical approaches have been developed that allowed to analyze explicitly dynamics of CTRW at all times, which is critically important for understanding mechanisms of underlying phenomena. However, theoretical analysis has been done mostly for systems with a simple geometry. Here we extend the original method based on generalized master equations to analyze all-time dynamics of CTRW models on complex networks. Specific calculations are performed for models on lattices with branches and for models on coupled parallel-chain lattices. Exact expressions for velocities and dispersions are obtained. Generalized fluctuations theorems for CTRW models on complex networks are discussed.
3. arXiv:1301.0052 [pdf, ps, other]
Layer Antiferromagnetic State in Bilayer Graphene : A First-Principle Investigation
Yong Wang, Hao Wang, Jin-Hua Gao, Fu-Chun Zhang
The ground state of bilayer graphene is investigated by the density functional calculations with local spin density approximation. We find a ground state with layer antiferromagnetic ordering, which has been suggested by former studies based on simplified model. The calculations prove that the layer antiferromagnetic state (LAF) is stable even if the remote hopping and nonlocal Coulomb interaction are included. The gap of the LAF state is about 1.8 meV, comparable to the experimental value. The surface magnetism in BLG is of the order of $10^{-2} \mu_B /nm^2 $.
Fri Jan 4
1. arXiv:1301.0421 [pdf, ps, other]
Coherent spin dynamics of nanomolecules and magnetic nanoclusters
V. I. Yukalov, E. P. Yukalova
Spin dynamics of nanomolecules and nanoclusters are analyzed. The nanosizes of these objects make it possible to consider them as single-domain magnets with a large total spin, where the motion of the spins of all atoms, composing a nanocluster, occurs in a coherent way. Another meaning of coherence in spin dynamics is the coherent spin motion of several nanomolecules or nanoclusters. Different approaches for treating spin dynamics are compared and the main mechanisms influencing the spin motion are studied. Spin dynamics of separate magnetic nanomolecules and nanoclusters are investigated, as well as the spin dynamics of the ensembles of these nano-objects.
2. arXiv:1301.0440 [pdf, ps, other]
Low-loss tunable metamaterials using superconducting circuits with Josephson junctions
Philipp Jung, Susanne Butz, Sergey V. Shitov, Alexey V. Ustinov
We report on experiments with superconducting metamaterials containing Josephson junctions. In these structures, split-ring resonators used in conventional metamaterials are replaced by superconducting loops that are interrupted by Josephson junctions, so called rf-SQUIDs. Like the split-ring resonators, these elements can be seen as LC-resonators that couple to the magnetic field. The advantage of superconducting thin-film metamaterials is that, due to the tunable intrinsic inductance of the Josephson junction, the resonance frequency of the rf-SQUID can be changed by applying an external dc magnetic field. We present experimental results that demonstrate the tunability of the resonance frequency of these devices.
3. arXiv:1301.0389 [pdf, ps, other]
Nonequilibrium Rashba field driven domain wall motion in ferromagnetic nanowires
Martin Stier, Reinhold Egger, Michael Thorwart
We study the effects of spin-orbit interaction (SOI) on the current-induced motion of a magnetic (Bloch) domain wall in ultrathin ferromagnetic nanowires. The conspiracy of spin relaxation and SOI is shown to generate a novel strong nonequilibrium Rashba field, which is dominant even for moderate SOI. This field causes intricate spin precession and a transition from translatory to oscillatory wall dynamics with increasing SOI. We show that current pulses of different lengths can efficiently be used to control the domain wall motion.
