Sep 26 - Sep 30, Xiaopeng Li
Sep 30
1. arXiv:1109.6639 [pdf, other]
Title: Fidelity susceptibility in the two-dimensional spin-orbit models
Author: Wen-Long You, Yu-Li Dong
We study the quantum phase transitions in the two-dimensional spin-orbit models in terms of fidelity susceptibility and reduced fidelity susceptibility. An order-to-order phase transition is identified by fidelity susceptibility in the two-dimensional Heisenberg XXZ model with Dzyaloshinsky-Moriya interaction on a square lattice. The finite size scaling of fidelity susceptibility shows a power-law divergence at criticality, which indicates the quantum phase transition is of second order. Two distinct types of quantum phase transitions are witnessed by fidelity susceptibility in Kitaev-Heisenberg model on a hexagonal lattice. We exploit the symmetry of two-dimensional quantum compass model, and obtain a simple analytic expression of reduced fidelity susceptibility. Compared with the derivative of ground-state energy, the fidelity susceptibility is a bit more sensitive to phase transition. The violation of power-law behavior for the scaling of reduced fidelity susceptibility at criticality suggests that the quantum phase transition belongs to a first-order transition. We conclude that fidelity susceptibility and reduced fidelity susceptibility show great advantage to characterize diverse quantum phase transitions in spin-orbit models.
2. arXiv:1109.6635 [pdf, ps, other]
Title: All-Optical Production of a Lithium Quantum Gas Using Narrow-Line Laser Cooling
Author: P. M. Duarte, R. A. Hart, J. M. Hitchcock, T. A. Corcovilos, T.-L. Yang, A. Reed, R. G. Hulet
We have used the narrow $2S_{1/2} \rightarrow 3P_{3/2}$ transition in the ultraviolet (UV) to laser cool and magneto-optically trap (MOT) $^{6}$Li atoms. Laser cooling of lithium is usually performed on the $2S_{1/2} \rightarrow 2P_{3/2}$ (D2) transition, but unresolved hyperfine structure in the excited state hinders the attainment of sub-Doppler temperatures by polarization gradient cooling. Temperatures of roughly twice the Doppler limit, or ~300 {\mu}K for lithium, are typically achieved. The linewidth of the UV transition is seven times narrower than the D2 line, resulting in lower Doppler cooling temperatures. We show that a MOT operating on the UV transition reaches temperatures as low as 59 {\mu}K. Furthermore, we show that the light shift of the UV transition in an optical dipole trap at 1070 nm is small and blue-shifted, facilitating efficient loading from the UV MOT. Evaporative cooling of a two spin-state mixture of $^{6}$Li in the optical trap produces a quantum degenerate Fermi gas with $3 \times 10^{6}$ atoms in only 11 s.
3. arXiv:1109.6506 [pdf, ps, other]
Title: Collective quantum jumps of Rydberg atoms
Author: Tony E. Lee, H. Häffner, M. C. Cross
We study an open quantum system of atoms with long-range Rydberg interaction, laser driving, and spontaneous emission. Over time, the system occasionally jumps between a state of low Rydberg population and a state of high Rydberg population. The jumps are inherently collective and in fact exist only for a large number of atoms. We explain how entanglement and quantum measurement enable the jumps, which are otherwise classically forbidden.
Sep 29
1. arXiv:1109.6307 [pdf]
Title: To use or not to use cool superconductors?
Author: A. Gurevich
The high critical temperature and magnetic field in cuprates and Fe-based superconductors are not enough to assure applications at higher temperatures. Making these superconductors useful involves complex and expensive technologies to address many conflicting physics and materials requirements.
2. arXiv:1109.6219 [pdf, ps, other]
Title: A Quantized $ν=5/2$ State in a Two-Subband Quantum Hall System
Author: J. Nuebler, B. Friess, V. Umansky, B. Rosenow, M. Heiblum, K. v. Klitzing, J. Smet
The evolution of the fractional quantum Hall state at filling 5/2 is studied in density tunable two-dimensional electron systems formed in wide wells in which it is possible to induce a transition from single to two subband occupancy. In 80 and 60 nm wells, the quantum Hall state at 5/2 filling of the lowest subband is observed even when the second subband is occupied. In a 50 nm well the 5/2 state vanishes upon second subband population. We attribute this distinct behavior to the width dependence of the capacitive energy for intersubband charge transfer and of the overlap of the subband probability densities.
3. arXiv:1109.6149 [pdf, ps, other]
Title: Instability of the Mott or Lieb-Wu insulator caused by an infinitesimal perturbation
Author: Fusayoshi J. Ohkawa
The half-filled ground state of the Hubbard model in one dimension is studied by Kondo-lattice theory. Because of the Kondo effect, any insulating ground state with a complete gap open is unstable in the presence of an infinitesimal perturbation. This fact casts doubt on the claim by E. H. Lieb and F. Y. Wu, Phys. Rev. Lett. 20, 1445 (1968) that the half-filled ground state is the Mott insulator. Though the claim is based on a rigorous result given by the Bethe-ansatz solution, the rigorous result is simply a necessary condition for the ground state being an insulator.
Sep 28
1. arXiv:1109.5842 [pdf, ps, other]
Title: A proposal for the measurement of Rashba and Dresselhaus spin-orbit interaction strengths in a single sample
Author: Santanu K. Maiti, Shreekantha Sil, Arunava Chakrabarti
A tight-binding model of a mesoscopic ring is considered, in which both the Rashba and Dresselhaus spin-orbit interactions are present, but only in a subsection of the ring's perimeter. We exploit a hidden symmetry in the Hamiltonian to show that the spin current, as measured in the interaction-free zone of the ring's periphery, vanishes when the strength of the Dresselhaus interaction becomes equal to the strength of the Rashba term. As the latter is controllable via a gate voltage, the disappearance of the spin current gives a unique method to measure the strength of the Dresselhaus interaction. On the other hand, if the latter is already known, the former can be estimated using the same proposal. Our calculations are exact. We present numerical evaluation of the persistent spin current which support our analytical findings. The results are sustained even in the case of a disordered ring. An experiment in this regard will thus be challenging.
2. arXiv:1109.5811 [pdf, ps, other]
Title: Exotic Superfluidity in Spin-orbit Coupled Bose-Einstein Condensates
Author: Qizhong Zhu, Chuanwei Zhang, Biao Wu
With the recent experimental realization of artificial gauge field for ultracold bosonic atomic gases, there arises an issue of how the spin-orbit coupling affects superfluidity. We study the superfluidity of a spin-orbit coupled Bose-Einstein condensate (BEC) by computing its Bogoliubov excitations, which are found to consist of two branches: one is gapless and phonon-like at large wavelength; the other is typically gapped. These excitations imply a superfluidity that has two new features: {\it i}) it depends on not only whether the speed of the BEC exceeds a critical value $v_{\rm c}$, but also the direction of the cross product of the spin and the kinetic momentum of the BEC (i.e., {\it cross helicity}); {\it ii}) the critical speed $v_{\rm c}$ is in general not directly linked to the excitation properties of the system at the ground state. The latter is beyond Landau's argument of superfluidity based on the Galilean transformation.
3. arXiv:1109.5709 [pdf, other]
Title: Spin diffusion in trapped clouds of strongly interacting cold atoms
Author: G. M. Bruun, C. J. Pethick
We show that puzzling recent experimental results on spin diffusion in a strongly interacting atomic gas may be understood in terms of the predicted spin diffusion coefficient for a generic strongly interacting system. Three important features play a central role: a) Fick's law for diffusion must be modified to allow for the trapping potential, b) the diffusion coefficient is inhomogeneous, due to the density variations in the cloud and c) the diffusion approximation fails in the outer parts of the cloud, where the mean free path is long.
4. arXiv:1109.5703 [pdf, ps, other]
Title: Emergence of a Chiral Mott insulator in a fully frustrated Bose Hubbard model
Author: Arya Dhar, Maheswar Maji, Tapan Mishra, R. V. Pai, Subroto Mukerjee, Arun Paramekanti
Many striking new states of condensed matter arise from an interplay of strong correlations and frustration. A simple Hamiltonian which captures this interplay for bosons is the fully frustrated Bose-Hubbard (FFBH) model with half a flux quantum per lattice plaquette which leads to kinetic frustration via multiple minima in the band dispersion. We study the T=0 phase diagram of the FFBH model at integer filling on two-leg ladders, using a density matrix renormalization group approach as well as Monte Carlo calculations on an effective classical model. We discover, for intermediate correlation strengths, a remarkable Chiral Mott insulator (CMI) ground state. This state of quantum matter has a nonzero charge gap and supports staggered loop currents that spontaneously break time reversal symmetry. The CMI is sandwiched between a chiral superfluid with staggered currents at weak repulsion and an ordinary Mott insulator at strong coupling. We present a wavefunction for the CMI, discuss its physical picture as a vortex supersolid or an exciton condensate, and foresee its realization using Josephson junction arrays or cold atoms in an optical lattice.
Sep 27
1. arXiv:1109.5480 [pdf, other]
Title: Quantum crystals in a trapped Rydberg-dressed Bose-Einstein condensate
Author: C.-H. Hsueh, T.-C. Lin, T.-L. Horng, W. C. Wu
Comments: 4 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
Spontaneously crystalline ground states, called quantum crystals, of a trapped Rydberg-dressed Bose-Einstein condensate are numerically investigated. As a result described by a mean-field order parameter, such states simultaneously possess crystalline and superfluid properties. A hexagonal droplet lattice is observed in a quasi-two-dimensional system when dressing interaction is sufficiently strong. Onset of these states is characterized by a drastic drop of the non-classical rotational inertia proposed by Leggett [Phys. Rev. Lett. 25, 1543 (1970)]. In addition, an AB stacking bilayer lattice can also be attained. Due to an anisotropic interaction possibly induced by an external electric field, transition from a hexagonal to a nearly square droplet lattice is also observed.
Sep 26
1. arXiv:1109.5103 [pdf, ps, other]
Title: Effective action for strongly correlated electron systems
Author: A. Ferraz, E.A. Kochetov
The su(2|1) coherent-state path-integral representation of the partition function of the t - J model of strongly correlated electrons is derived at finite doping. The emergent effective action is compared to the one proposed earlier on phenomenological grounds by Shankar to describe holes in an antiferromagnet (Nucl.Phys. B330 (1990) 433). The t - J model effective action is found to have an important "extra" factor with no analogue in Shankar's action. It represents the local constraint of no double electron occupancy and reflects the rearrangement of the underlying phase-space manifold due to the presence of strong electron correlation. This important ingredient is shown to be essential to describe the physics of strongly correlated electron systems.
2. arXiv:1109.4954 [pdf, ps, other]
Title: Ground state phases of ultracold bosons with Rashba-Dresselhaus spin-orbit coupling
Author: Tomoki Ozawa, Gordon Baym
We study ultracold bosons in three dimensions with an anisotropic Rashba-Dresselhaus spin-orbit coupling. We first carry out the exact summation of ladder diagrams for the two boson t-matrix at zero energy. Then, with the t-matrix as the effective interaction, we find the ground state phase diagrams of bosons in mean field, as a function of the spin-orbit coupling, the anisotropy, and the scattering lengths between particles in the same and in different pseudospin states. The resulting phase diagrams have a much richer structures than those obtained using mean-field couplings, exhibiting three different phases: a plane wave condensate, a striped condensate, and an unstable phase. The differences between the present approach using the t-matrix compared using mean-field couplings is significant for large scattering lengths, large spin-orbit coupling strength, or small anisotropy.
Sep 19 - Sep 23, Zixu Zhang
Sep 23
1. arXiv:1109.4782 [pdf, ps, other]
Dipolar molecules in optical lattices revisited
Tomasz Sowiński, Omjyoti Dutta, Philipp Hauke, Luca Tagliacozzo, Maciej Lewenstein
We study the extended Bose--Hubbard model describing an ultra-cold gas of dipolar molecules in an optical lattice, taking into account all on-site and nearest-neighbor interactions, including occupation-dependent tunneling and pair tunneling terms. Using exact diagonalization and the multi-scale entanglement renormalization ansatz (MERA), we show that these terms can destroy insulating phases and lead to novel quantum phases. These considerable changes of the phase diagram have to be taken into account in upcoming experiments with dipolar molecules.
2. arXiv:1109.4902 [pdf, ps, other]
The last word in strong correlations
R. Shankar
Comments: Dedicated to Dieter Vollhardt on his 60th birthday
In the Fractional Quantum Hall Effect (FQHE), in the noninteracting limit, only a fraction $\nu $ of the Lowest Landau Level (LLL) is occupied, producing a huge degeneracy. Interactions lift this degeneracy and mix in higher LL's. In the limit in which we ignore all but the LLL (i.e., let the inverse electron mass ${1 \over m}\to \infty$), the kinetic energy is an irrelevant constant and the ratio of potential to kinetic energy is essentially infinite, making this the most strongly correlated problem imaginable. I give a telegraphic review of the Hamiltonian Theory of the FQHE developed with Ganpathy Murthy that deals with this problem with some success. A nodding acquaintance with FQHE physics is presumed.
Sep 22
1. arXiv:1109.4561 [pdf, ps, other]
Non-Abelian Braiding of Lattice Bosons
Eliot Kapit, Paul Ginsparg, Erich Mueller
We report on a numerical experiment in which we use time-dependent potentials to braid non-abelian quasiparticles. We consider lattice bosons in a uniform magnetic field within the fractional quantum Hall regime, where $\nu$, the ratio of particles to flux quanta, is near 1/2, 1 or 3/2. We introduce time-dependent potentials which move quasiparticle excitations around one another, explicitly simulating a braiding operation which could implement part of a gate in a quantum computation. We find that different braids do not commute for $\nu$ near $1$ and $3/2$, with Berry matrices respectively consistent with Ising and Fibonacci anyons. Near $\nu=1/2$, the braids commute.
2. arXiv:1109.4602 [pdf, other]
Few-body bound state stability of dipolar molecules in two dimensions
A. G. Volosniev, D. V. Fedorov, A. S. Jensen, N. T. Zinner
Bound structures among dipolar molecules in multilayers are a topic of great interest in the light of recent experiments that have demonstrated the feasibility of the setup. While it is known that two molecules in two adjacent layers will always bind, larger complexes have only been scarcely addressed thus far. Here we prove rigorously that three- and four-body states will never be bound when the dipoles are oriented perpendicular to the layers. The technique employed is general and can be used for more molecules/layers and other geometries. Our analytical findings are supported by numerical calculations for both fermionic and bosonic molecules. Furthermore, we calculate the reduction in intralayer repulsion necessary to bind large complexes and estimate the influence of bound complexes in systems with many layers.
Sep 21
None
Sep 20
1. arXiv:1109.3755 [pdf, other]
Density ordering instabilities of quasi-two-dimensional fermionic polar molecules in single-layer and multi-layer configurations: exact treatment of exchange interactions
Mehrtash Babadi, Eugene Demler
We study the in-plane and out-of-plane density ordering instabilities of quasi-two-dimensional fermionic polar molecules in single-layer and multi-layer configurations. We locate the soft modes by evaluating linear response functions within the conserving time-dependent Hartree-Fock (TDHF). The short-range exchange effects are taken into account by solving the Bethe-Salpeter integral equation numerically. An instability phase diagram is calculated for both single-layer and multi-layer systems and the unstable wave-vector is indicated. In all cases, the in-plane density wave instability is found to precede the out-of-plane instability. The unstable wave-vector is found to be approximately twice the Fermi wave-vector of one of the subbands at a time and can change discontinuously as a function of density and dipolar interaction strength. In multi-layer configurations, we find a large enhancement of density wave instability driven by dilute quasiparticles in the first excited subband. Finally, we provide a simple qualitative description of the phase diagrams using a RPA-like approach. Compared to previous works done within the RPA approximation, we find that inclusion of exchange interactions stabilize the normal liquid phase further and increase the critical dipolar interaction strength corresponding to the onset of density-wave instability by over a factor of two.
2. arXiv:1109.3970 [pdf, ps, other]
Condensate Fraction and Pair Coherence Lengths of Two-Dimension Fermi Gases with Spin-Orbit Coupling
Beibing Huang, Shaolong Wan
The effects of Rashba spin-orbit coupling on BCS-BEC crossover, the condensate fraction and pair coherence lengths for a two-component attractive Fermi gas in two dimension are studied. The results at $T=0K$ indicate that (1) when the strength of SOC is beyond a critical value, BCS-BEC crossover does not happen in a conventional sense; (2) SOC enhances the condensate fraction, but suppresses pair coherence lengths.
Sep 19
1. arXiv:1109.3534 [pdf, ps, other]
Coexistence of superconductivity and incommensurate magnetic order
Andrzej Ptok, Maciej M. Maska, Marcin Mierzejewski
The influence of incommensurate spin density waves (SDW) on superconductivity in unconventional superconductors is studied by means of the Bogolubov-de Gennes (BdG) equations. Exploiting translational symmetries of a magnetically ordered two-dimensional system we propose an approach that allows to solve the BdG equations on much larger clusters than it is usually possible for inhomogeneous systems. Applying this approach we demonstrate that the presence of incommensurate spin density waves induces real-space inhomogeneity of the superconducting order parameter even in the absence of external magnetic field. In this case a homogeneous order parameter of the Bardeen-Cooper-Schrieffer-type superconducting state is slightly modulated, or equivalently, a small fraction of the charge carriers form Cooper pairs with non-zero total momentum. However, when a sufficiently strong magnetic field is applied, the homogeneous component of the order parameter is suppressed and the system transits to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, where the order parameter oscillates changing sign. We show that for s-wave pairing the presence of external magnetic field diminishes the destructive influence of the SDW order on superconductivity. A simple explanation of this effect is also proposed.
2. arXiv:1109.3648 [pdf, ps, other]
Pair excitations and parameters of state of imbalanced Fermi gases at finite temperatures
S. N. Klimin (1), J. Tempere (1 and 2), Jeroen P. A. Devreese (1) ((1) Theorie van Kwantumsystemen en Complexe Systemen, Universiteit Antwerpen, Belgium, (2) Lyman Laboratory of Physics, Harvard University, USA)
The spectra of low-lying pair excitations for an imbalanced two-component superfluid Fermi gas are analytically derived within the path-integral formalism taking into account Gaussian fluctuations about the saddle point. The spectra are obtained for nonzero temperatures, both with and without imbalance, and for arbitrary interaction strength. On the basis of the pair excitation spectrum, we have calculated the thermodynamic parameters of state of cold fermions and the first and second sound velocities. The parameters of pair excitations show a remarkable agreement with the Monte Carlo data and with experiment.
Sep 12 - Sep 16, Bin Wang
Sep 16
1. arXiv:1109.3384 [pdf, other]
Title: Fractional domain walls from on-site softening in dipolar bosons
Author: Emma Wikberg, Jonas Larson, Emil J. Bergholtz, Anders Karlhede
We study dipolar bosons in a 1D optical lattice and identify a region in parameter space---strong coupling but relatively weak on-site repulsion---hosting a series of stable CDW states whose low-energy excitations, built from "fractional domain walls", are remarkably similar to those of non-abelian fractional quantum Hall states. Here, a conventional domain wall between translated CDW's may split by inserting strings of degenerate, but inequivalent, CDW states. Outside these insulating regions, we find numerous supersolids as well as a superfluid regime. The mentioned phases should be accessible experimentally, and in particular, the fractional domain walls can be created in the ground state using single-site addressing, i.e. by locally changing the chemical potential.
2. arXiv:1109.3252 [pdf, ps, other]
Title: Quantum phase-space analysis of population equilibration in multi-well ultracold atomic systems
Author: C.V. Chianca, M.K. Olsen
We examine the medium time quantum dynamics and population equilibration of two, three and four-well Bose-Hubbard models using stochastic integration in the truncated Wigner phase-space representation. We find that all three systems will enter at least a temporary state of equilibrium, with the details depending on both the classical initial conditions and the initial quantum statistics. We find that classical integrability is not necessarily a good guide as to whether equilibration will occur. We construct an effective single-particle reduced density matrix for each of the systems, using the expectation values of operator moments, and use this to calculate an effective entropy. Knowing the expected maximum values of this entropy for each system, we are able to quantify the different approaches to equilibrium.
3. arXiv:1109.3434 (cross-list from cond-mat.mes-hall) [pdf, other]
Title: Fractional Quantum Hall Effect of Lattice Bosons Near Commensurate Flux
Author: L. Hormozi, Gunnar Moller, Steven H. Simon
We study interacting bosons on a lattice in a magnetic field. When the number of flux quanta per plaquette is close to a rational fraction, the low energy physics is mapped to a multi-species continuum model: bosons in the lowest Landau level where each boson is given an internal degree of freedom, or pseudospin. We find that the interaction potential between the bosons involves terms that do not conserve pseudospin, corresponding to umklapp processes, which in some cases can also be seen as BCS-type pairing terms. We argue that in experimentally realistic regimes for bosonic atoms in optical lattices with synthetic magnetic fields, these terms are crucial for determining the nature of allowed ground states. In particular, we show numerically that certain paired wavefunctions related to the Moore-Read Pfaffian state are stabilized by these terms, whereas certain other wavefunctions can be destabilized when umklapp processes become strong.
Sep 15
1. arXiv:1109.3045 [pdf, ps, other]
Title: Finite temperature phase diagram of spin-1/2 bosons in two-dimensional optical lattice
Author: L. de Forges de Parny, F. Hébert, V.G. Rousseau, G.G. Batrouni
We study a two-species bosonic Hubbard model on a two-dimensional square lattice by means of quantum Monte Carlo simulations and focus on finite temperature effects. We show in two different cases, ferro- and antiferromagnetic spin-spin interactions, that the phase diagram is composed of solid Mott phases, liquid phases and superfluid phases. In the antiferromagnetic case, the superfluid (SF) is polarized while the Mott insulator (MI) and normal Bose liquid (NBL) phases are not. On the other hand, in the ferromagnetic case, none of the phases is polarized. The superfluid-liquid transition is of the Berezinsky-Kosterlitz-Thouless type whereas the solid-liquid passage is a crossover.
Sep 14
1. arXiv:1109.2820 [pdf, ps, other]
Title: BCS and BEC p-wave pairing in Bose-Fermi gases
Author: F. Matera, A. Dellafiore
The pairing of fermionic atoms in a mixture of atomic fermion and boson gases at zero temperature is investigated. The attractive interaction between fermions, that can be induced by density fluctuations of the bosonic background, can give rise to a superfluid phase in the Fermi component of the mixture. The atoms of both species are assumed to be in only one internal state, so that the pairing of fermions is effective only in odd-l channels. No assumption about the value of the ratio between the Fermi velocity and the sound velocity in the Bose gas is made in the derivation of the energy gap equation. The gap equation is solved without any particular "ansatz" for the pairing field or the effective interaction. The p-wave superfluidity is studied in detail. By increasing the strength and/or decreasing the range of the effective interaction a transition of the fermion pairing regime, from the Bardeen-Cooper-Schrieffer state to a system of tightly bound couples can be realized. These composite bosons behave as a weakly-interacting Bose-Einstein condensate.
Sep 13
1. arXiv:1109.2464 [pdf, ps, other]
Title: Density instabilities in a two-dimensional dipolar Fermi gas
Author: Meera M. Parish, Francesca M. Marchetti
We study the density instabilities of a two-dimensional gas of dipolar fermions with aligned dipole moments. We show that the Random Phase Approximation (RPA) for the density-density response function is never accurate for the dipolar gas. We incorporate correlations beyond RPA via an improved version of the Singwi-Tosi-Land-Sjolander scheme. In addition to density-wave instabilities, our formalism captures the collapse instability that is expected from Hartree-Fock calculations but is absent from RPA. Crucially, we find that when the dipoles are perpendicular to the layer, the system spontaneously breaks rotational symmetry and forms a stripe phase, in defiance of conventional wisdom.
Sep 12
1. arXiv:1109.2045 [pdf, ps, other]
Title: Trapped two-dimensional condensates with synthetic spin-orbit coupling
Author: Subhasis Sinha, Rejish Nath, Luis Santos
We study trapped 2D atomic Bose-Einstein condensates with spin-independent interactions in the presence of an isotropic spin-orbit coupling, showing that a rich physics results from the non-trivial interplay between spin-orbit coupling, confinement and inter-atomic interactions. For low interactions two types of half-vortex solutions with different winding occur, whereas strong-enough repulsive interactions result in a stripe-phase similar to that predicted for homogeneous condensates. Intermediate interaction regimes are characterized for large enough spin-orbit coupling by an hexagonally-symmetric phase with a triangular lattice of density minima similar to that observed in rapidly rotating condensates.
Sep 5 - Sep 9, Xiaopeng Li
Sep 9
1. arXiv:1109.1569 [pdf, other]
Title: Fractional Chern Insulators from the nth Root of Bandstructure
Author: John McGreevy, Brian Swingle, Ky-Anh Tran
We provide a parton construction of wavefunctions and effective field theories for fractional Chern insulators. We also analyze a strong coupling expansion in lattice gauge theory that enables us to reliably map the parton gauge theory onto the microsopic Hamiltonian. We show that this strong coupling expansion is useful because of a special hierarchy of energy scales in fractional quantum Hall physics. Our procedure is illustrated using the Hofstadter model and then applied to bosons at 1/2 filling and fermions at 1/3 filling in a checkerboard lattice model recently studied numerically. Because our construction provides a more or less unique mapping from microscopic model to effective parton description, we obtain wavefunctions in the same phase as the observed fractional Chern insulators without tuning any continuous parameters.
2. arXiv:1109.1589 [pdf, ps, other]
Title: Isolated quantum heat engine
Author: O. Fialko, D. Hallwood
A scheme to create a quantum heat engine is presented where the system is isolated from the environment. Bosonic atoms are confined to a double well potential created by splitting a harmonic trap with a focused laser that acts as a barrier. The system shows thermalization and is experimentally simple enough to model a heat engine cycle. One well acts as the system that does work and another well serves as the heat bath. Heat is transfered between the wells and the volume is changed by modifying the barrier position and confinement. The heat engine is shown to be reversible and this work represents the first demonstration of a heat engine described fully quantum mechanically.
3. arXiv:1109.1644 [pdf, ps, other]
Title: Beyond the Dirac phase factor: Dynamical Quantum Phase-Nonlocalities in the Schrödinger Picture
Author: Konstantinos Moulopoulos
Generalized solutions of the standard gauge transformation equations are presented and discussed in physical terms. They go beyond the usual Dirac phase factors and they exhibit nonlocal quantal behavior, with the well-known Relativistic Causality of classical fields affecting directly the phases of wavefunctions in the Schr\"odinger Picture. These nonlocal phase behaviors, apparently overlooked in path-integral approaches, give a natural account of the dynamical nonlocality character of the various (even static) Aharonov-Bohm phenomena, while at the same time they seem to respect Causality. For particles passing through nonvanishing magnetic or electric fields they lead to cancellations of Aharonov-Bohm phases at the observation point, generalizing earlier semiclassical experimental observations (of Werner & Brill) to delocalized (spread-out) quantum states. This leads to a correction of previously unnoticed sign-errors in the literature, and to a natural explanation of the deeper reason why certain time-dependent semiclassical arguments are consistent with static results in purely quantal Aharonov-Bohm configurations. These nonlocalities also provide a remedy for misleading results propagating in the literature (concerning an uncritical use of Dirac phase factors, that persists since the time of Feynman's work on path integrals). They are shown to conspire in such a way as to exactly cancel the instantaneous Aharonov-Bohm phase and recover Relativistic Causality in earlier " paradoxes" (such as the van Kampen thought-experiment), and to also complete Peshkin's discussion of the electric Aharonov-Bohm effect in a causal manner. The present formulation offers a direct way to address time-dependent single- vs double-slit experiments and the associated causal issues -- issues that have recently attracted attention, with respect to the inability of current theories to address them.
Sep 8
1. arXiv:1109.1523 [pdf, ps, other]
Title: Introduction to {\it Quantum Matter}
Author: Frank Wilczek
This paper records my opening remarks at Nobel Symposium 148, on Graphene and Quantum Matter, at Saltsj\"obaden, Sweden, in June 2010. After some broad comments on the quantum theory of matter as a frontier of physics, and some slightly more particular comments about re-quantization, I report on the universal geometry that arises in a refined discussion of quantum-mechanical level crossing.
2. arXiv:1109.1316 [pdf, ps, other]
Title: Field-Induced Freezing of a Quantum Spin-Liquid on the Kagome Lattice
Author:M. Jeong, F. Bert, P. Mendels, F. Duc, J. C. Trombe, M. A. de Vries, A. Harrison
We report 17O NMR measurements in the S=1/2 Cu2+ kagome antiferromagnet Herbertsmithite ZnCu3(OH)6Cl2 down to 45mK in magnetic fields ranging from 2T to 12T. While Herbertsmithite displays a gapless spin-liquid behavior in zero field, we uncover an instability toward a spin-solid phase at sub-kelvin temperature induced by an applied magnetic field. The latter phase shows largely suppressed moments $\lesssim 0.1\muB$ and gapped excitations. The H-T phase diagram suggests the existence of a quantum critical point at the small but finite magnetic field mu0 Hc=1.55(25)T. We discuss this finding in light of the perturbative Dzyaloshinskii-Moriya interaction which was theoretically proposed to sustain a quantum critical regime for the quantum kagome Heisenberg antiferromagnet model.
Sep 7
1. arXiv:1109.1012 [pdf, ps, other]
Title: Quantum Phase Transition in an Antiferromagnetic Spinor Bose-Einstein Condensate
Author: E. M. Bookjans, A. Vinit, C. Raman
We have experimentally observed the dynamics of an antiferromagnetic sodium Bose-Einstein condensate (BEC) quenched through a quantum phase transition. Using an off-resonant microwave field coupling the F = 1 and F = 2 atomic hyperfine levels, we rapidly switched the quadratic energy shift q from positive to negative values. At q = 0 the system undergoes a transition from a polar to antiferromagnetic phase. We measured the dynamical evolution of the population in the F = 1, m_F = 0 state in the vicinity of this transition point and observed a mixed state of all 3 hyperfine components for q < 0. We also observed the coarsening dynamics of the instability for q<0, as it nucleated small domains that grew to the axial size of the cloud.
2. arXiv:1109.0988 [pdf, ps, other]
Title: Disorder by order in graphene
Author: S. Das Sarma, E. H. Hwang, Qiuzi Li
We predict the existence of an intriguing "disorder by order" phenomenon in graphene transport where higher-quality (and thus more ordered) samples, while having higher mobility at high carrier density, will manifest more strongly insulating (and thus effectively more disordered) behavior as the carrier density is lowered compared with lower quality samples (with higher disorder) which exhibit an approximate resistivity saturation phenomenon at low carrier density near the Dirac point. This predicted behavior, which we believe to have recently been observed in an experiment at Manchester University, arises from the suppression of Coulomb disorder induced inhomogeneous puddles near the charge neutrality point in high quality graphene samples.
Sep 6
1. arXiv:1109.0632 [pdf, other]
Title: A Phenomenological Theory of the Anomalous Pseudogap Phase in Underdoped Cuprates
Author: T. M. Rice, Kai-Yu Yang, F. C. Zhang
The theoretical description of the anomalous properties of the pseudogap phase in the underdoped region of the cuprate phase diagram lags behind the progress in spectroscopic and other experiments. A phenomenological ansatz, based on analogies to the approach to Mott localization at weak coupling in lower dimensional systems, has been proposed by Yang, Rice and Zhang [Phys. Rev. B 73 (2006),174501]. This ansatz has had success in describing a range of experiments. The motivation underlying this ansatz is described and the comparisons to experiment are reviewed. Implications for a more microscopic theory are discussed together with the relation to theories that start directly from microscopic strongly coupled Hamiltonians.
Sep 5
1. arXiv:1109.0504 [pdf, ps, other]
Title: Kinetic energy driven superconductivity and superfluidity
Author: J. E. Hirsch
The theory of hole superconductivity proposes that superconductivity is driven by lowering of quantum kinetic energy and is associated with expansion of electronic orbits and expulsion of negative charge from the interior to the surface of superconductors and beyond. This physics provides a dynamical explanation of the Meissner effect. Here we propose that similar physics takes place in superfluid helium 4. Experimental manifestations of this physics in $^4He$ are the negative thermal expansion of $^4He$ below the $\lambda$ point and the "Onnes effect", the fact that superfluid helium will creep up the walls of the container and escape to the exterior. The Onnes effect and the Meissner effect are proposed to originate in macroscopic zero point rotational motion of the superfluids. It is proposed that this physics indicates a fundamental inadequacy of conventional quantum mechanics.
Conlusion: The considerations in this paper suggest that Schrodinger’s equation, as well as Dirac’s equation and Klein Gordon’s equation from which Schrodinger’s equation derives for fermions and bosons respectively, are inadequate to provide a general description of reality, because they do not predict rotational zero-point motion. If so, the correct equations remain to be discovered.
2. arXiv:1109.0406 [pdf, other]
Title: Luttinger-volume violating Fermi liquid in the cuprates
Author: Jia-Wei Mei, Shinji Kawasaki, Guo-Qing Zheng, Zheng-Yu Weng, Xiao-Gang Wen
Based on the detail analysis of the NMR experiments on Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+{\delta}}$ (La-Bi2201), we propose that the non-superconducting pseudogap phase in the cuprates is described as a Luttinger- volume violating Fermi liquid (LvvFL). The LvvFL is a zero temperature quantum liquid that does not break translational symmetry, and yet, the Fermi surface has an area smaller than the large one given by the Luttinger theorem. In the rigorous sign structure for the t-J model, only the doped holes carry the fermionic statistic signs. So the particle number enclosed by the small Fermi surface in the LvvFL equals the doping level p, not the total electron number ne = 1 - p. Both the phase string theory and the dopon theory can capture the sparse fermionic signs. For the dopon theory, we can obtain a semi-quantitative agreement with the NMR experiments.
