Dec 2010

Dec 27 - Dec 31, Xiaopeng Li

Dec 30
1. arXiv:1012.5543 [pdf, ps, other]
Title: Adaptive Lanczos-vector method for dynamic properties within the density-matrix renormalization group
Authors: P.E. Dargel, A. Honecker, R. Peters, R. M. Noack, T. Pruschke
Current widely-used approaches to calculate spectral functions using the density-matrix renormalization group in frequency space either necessarily include an artificial broadening (correction-vector method) or have limited resolution (time-domain density-matrix renormalization group with Fourier transform method). Here we propose an adaptive Lanczos-vector method to calculate the coefficients of a continued fraction expansion of the spectral function iteratively. We show that one can obtain a very accurate representation of the spectral function very efficiently, and that one can also directly extract the spectral weights and poles for the discrete system. As a test case, we study spinless fermions in one dimension and compare our approach to the correction vector method. 

2. arXiv:1012.5864 [pdf, other]
Title: Nearly-flat bands with nontrivial topology 
Authors: Kai Sun, Zhengcheng Gu, Hosho Katsura, S. Das Sarma

We report the theoretical discovery of a large class of 2D tight-binding models containing nearly-flat bands with nonzero Chern numbers. In contrast with previous studies, where nonlocal hoppings are usually required, the Hamiltonians of our models only require short-range hopping and have the potential to be realized in cold atomic gases. Due to the similarity with 2D continuum Landau levels, these topologically nontrivial nearly-flat bands may lead to the realization of fractional anomalous quantum Hall states and fractional topological insulators in real materials. Among the models we discover, the most interesting and practical one is a square-lattice three-band model which has only nearest-neighbor hopping. To understand better the physics underlying the topological flat band aspects, we also present the studies of a minimal two-band model on the checkerboard lattice. 

3. arXiv:1012.5589 [pdf, ps, other]
Title: Bilayer superfluidity of fermionic polar molecules: many body effects 
Authors: M.A. Baranov, A. Micheli, S. Ronen, P. Zoller
We study the BCS superfluid transition in a single-component fermionic gas of dipolar particles loaded in a tight bilayer trap, with the electric dipole moments polarized perpendicular to the layers. Based on the detailed analysis of the interlayer scattering, we calculate the critical temperature of the interlayer superfluid pairing transition when the layer separation is both smaller (dilute regime) and of the order or larger (dense regime) than the mean interparticle separation in each layer. Our calculations go beyond the standard BCS approach and include the many-body contributions resulting in the mass renormalization, as well as additional contributions to the pairing interaction. We find that the many-body effects have a pronounced effect on the critical temperature, and can either decrease (in the very dilute limit) or increase (in the dense and moderately dilute limits) the transition temperature as compared to the BCS approach.

4. arXiv:1012.5671 [pdf, ps, other]
Title: Tetrad magnetic orders and Topological Quantum Liquids with Non-Abelian Statistics 
Authors: Cenke Xu, Andreas W.W. Ludwig
"Tetrad order" refers to spin orders whose ground state manifolds are the configurations of three perpendicular vectors or nematic-directors. We study three types of tetrad orders, whose ground state manifolds are SO(3) = S^3/Z_2, S^3/Z_4, and S^3/Q_8, respectively. Here Q_8 stands for the finite, non-Abelian quaternion group of order eight. We demonstrate that after quantum disordering these three types of tetrad orders, the systems enter fully gapped liquid phases described by Z_2, Z_4 and non-Abelian quaternion gauge field theories, respectively. The latter case provides a realization of the non-Abelian Toric Code phase proposed by Kitaev based on a finite group G, where here G=Q_8. This topological phase possesses a 22-fold ground state degeneracy on the torus arising from the 22 excitations with non-Abelian Braiding which form the representations of the Drinfeld double of Q_8.

Dec 27
1. arXiv:1012.5432 [pdf, other]
Title: Towards Topological Quantum Computation? - Knotting and Fusing Flux Tubes 
Authors: Meagan B. Thompson
Models for topological quantum computation are based on braiding and fusing anyons (quasiparticles of fractional statistics) in (2+1)-D. The anyons that can exist in a physical theory are determined by the symmetry group of the Hamiltonian. In the case that the Hamiltonian undergoes spontaneous symmetry breaking of the full symmetry group G to a finite residual gauge group H, particles are given by representations of the quantum double $D(H)$ of the subgroup. The quasi-triangular Hopf Algebra $D(H)$ is obtained from Drinfeld's quantum double construction applied to the algebra $\textit{F}(H)$ of functions on the finite group H.
A major new contribution of this work is a program written in MAGMA to compute the particles (and their properties - including charge, flux, and spin) that can exist in a system with an arbitrary finite residual gauge group, in addition to the braiding and fusion rules for those particles. We compute explicitly the fusion rules for two non-abelian groups suggested for universal quantum computation: $S_3$ and $A_5$, and discover some interesting results and symmetries in the tables. The tables demonstrate that the anyons in physical theories based on $S_3$ and $A_5$ are all Majorana, but this is not the case for all finite groups. In addition, closed subsystems are analyzed with a view towards topological quantum computation. The probabilities of obtaining specific fusion products in quantum computation schemes are determined for theories based on finite groups. The MAGMA program includes a procedure to determine the probabilities for any finite group based on these results. In the appendices, a few other non-abelian groups that may be of interest - $S_4$, $A_4$, and $D_4$ - are included. Throughout, connections to possible experiments are mentioned.

Dec 20 - Dec 24, Zixu Zhang

Dec 24

1. arXiv:1012.5116 [pdf, ps, other]
Title: Measuring entanglement using quantum quenches
Authors: John Cardy
Comments: 5 pages, 2 figures 
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph) 

We show that block entanglement entropies in one-dimensional systems close to a quantum critical point can in principle be measured in terms of the population of low-lying energy levels following a certain type of local quantum quench. 

2. arXiv:1012.5163 [pdf, other]
Title: Charge-density-wave and topological transitions in interacting Haldane model
Authors: Lei Wang, Hao Shi, Shiwei Zhang, Xiaoqun Wang, Xi Dai, X. C. Xie
Comments: 4.5 pages, 6 figures 
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall) 

Haldane model is a noninteracting model for spinless fermions showing nontrivial topological properties. Effect of the electron-electron interaction on the topological phase poses an intriguing question. By means of the Hartree-Fock mean field, the exact diagonalization and the constrained-path Monte Carlo methods we mapped out the phase diagram of the interacting Haldane model. It is found that interaction breaks down the topological phase and drives the system into the charge-density-wave state. Sequence of the two transitions depends on the strength of next-nearest-neighbor hopping. Many-body Chern number and the charge excitation gap are used to characterize the topological transition. 

Dec 23

1. Bound Dimers in Bilayers of Cold Polar Molecules
Authors: A. G. Volosniev, N. T. Zinner, D. V. Fedorov, A. S. Jensen, B. Wunsch
Comments: 6 pages, 4 figures 
Subjects: Quantum Gases (cond-mat.quant-gas); Mathematical Physics (math-ph); Quantum Physics (quant-ph) 
The exploration of cold polar molecules in different geometries is a rapidly developing experimental and theoretical pursuit. Recently, the implementation of optical lattices has enabled confinement in stacks of planes, the number of which is also controllable. Here we consider the bound state structure of two polar molecules confined in two adjacent planes as function of the polarization angle of the dipole moment of the molecules. We present evidence for the existence of bound states for arbitrary dipole moments and polarization angles in this two-dimensional geometry. The spatial structure of the bound states is dominated by two-dimensional s- and p-waves, where the latter exceeds 40 percent over a large range of polarization angles for intermediate or strong dipole strength. Finally, we consider the influence of the dimer bound states on the potential many-body ground-state of the system. 

2. arXiv:1012.4618 (cross-list from quant-ph) [pdf, ps, other]
Title: Dissipation-induced correlations in 1D bosonic systems
Authors: M. Kiffner, M. J. Hartmann
Comments: 13 pages, 6 figures 
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas) 

The quantum dynamics of interacting bosons in a one-dimensional system is investigated numerically. We consider dissipative and conservative two-particle interactions, and integrate the master equation describing the system dynamics via a time-evolving block-decimation (TEBD) algorithm. Our numerical simulations directly apply to stationary-light polaritons in systems where atoms and photons are confined to the hollow core of a photonic crystal fibre. We show that a two-particle loss term can drive an initially uncorrelated state into a regime where correlations effectively inhibit the dissipation of particles. The correlations induced by two-particle losses are compared with those generated by an elastic repulsion. For the considered time range, we find a similar behaviour in local density-density correlations but differences in non-local correlations. 


Dec 22

1. arXiv:1012.4470 [pdf, ps, other]
Title: Symmetry fractional quantization in two dimensions
Authors: Hong Yao, Liang Fu, Xiao-Liang Qi
Comments: 4 pages, 1 figure 
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We introduce a solvable spin-rotational and time-reversal invariant spin-1 model in two dimensions. Depending on parameters, the ground state is an equal-weight superposition of all valence loops called "resonating valence loop" (RVL) or an equal-weight superposition of all valence bonds known as "resonating valence bond" (RVB). In RVL, ends of open loops are deconfined spinons of spin-1/2 that cannot be obtained by simple combinations of spin-1 -- a phenomenon of fractionalization; while in RVB, all quasiparticles carry an integer spin, hence no fractionalization. RVL and RVB are spin liquids with identical topological order but different spin-rotational and time-reversal symmetry quantum numbers of quasiparticles. We propose that quantized symmetry quantum number gives a systematic way to (partially) classify phases with identical topological order in dimensions greater than one. 

2. arXiv:1012.4569 [pdf, ps, other]
Title: Itinerant Ferromagnetism in ultracold Fermi gases
Authors: Henning Heiselberg
Comments: 8 pages, 6 figures 
Subjects: Quantum Gases (cond-mat.quant-gas)

Itinerant ferromagnetism in cold Fermi gases with repulsive interactions is studied applying the Jastrow-Slater approximation generalized to finite polarization and temperature. For two components at zero temperature a second order transition is found at $ak_F\simeq0.90$ compatible with QMC. Thermodynamic functions and observables such as the compressibility and spin susceptibility and the resulting fluctuations in number and spin are calculated. For trapped gases the resulting cloud radii and kinetic energies are calculated and compared to recent experiments. Spin polarized systems are recommended for effective separation of large ferromagnetic domains. Collective modes are predicted and tri-critical points are calculated for multi-component systems. 

3. arXiv:1012.4664 [pdf, ps, other]
Title: A new Fermi liquid: the normal phase of a strongly interacting gas of cold atoms
Authors: Sylvain Nascimbène (LKB - Lhomond), Nir Navon (LKB - Lhomond), Sebastiano Pilati, Frédéric Chevy (LKB - Lhomond), Stefano Giorgini, Antoine Georges (CPHT), Christophe Salomon (LKB - Lhomond) 
Subjects: Quantum Gases (cond-mat.quant-gas)

In 1956 Landau developed an elegant description of interacting Fermi systems at low temperature relying on the existence of long-lived quasiparticles. While this Fermi liquid theory (FLT) describes well Helium 3 and many solidstate materials above the superfluid temperature, there exist notable exceptions such as underdoped cuprates [1]. In dilute atomic fermionic gases, the Fermi liquid nature of the normal phase is currently under debate. On the one hand, recent photoemission spectroscopy experiments near the critical temperature were interpreted using a pseudogap model [2]. On the other hand, measurement of the temperature dependence of the specific heat displayed a linear behaviour compatible with Fermi liquid's prediction [3]. Here, we measure the magnetic susceptibility of a Fermi gas with tunable interaction in the low temperature limit and compare it to quantum Monte Carlo calculations. Experiment and theory are in excellent agreement and fully compatible with FLT. We also measure the condensation energy which is a key quantity measured in superconducting compounds. Temperature variation data and magnetic susceptibility are combined to deduce a full set of Fermi liquid parameters describing the microscopic quasiparticle properties of the normal phase. The photoemission spectrum calculated with these parameters is found in good agreement with the measurements reported in [2]. We conclude that all existing data to date on the normal phase of the unitary Fermi gas are well described by Landau's Fermi liquid theory. 

4. arXiv:1012.4723 [pdf, other]
Title: Fractional quantum Hall states at zero magnetic field
Authors: Titus Neupert, Luiz Santos, Claudio Chamon, Christopher Mudry
Comments: 4 pages, 1 figure 
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall) 

We present a simple prescription to flatten isolated Bloch bands with non-zero Chern number. We first show that approximate flattening of bands with non-zero Chern number is possible by tuning ratios of nearest-neighbor and next-nearest neighbor hoppings in the Haldane model and, similarly, in the chiral-pi-flux square lattice model. Then we show that perfect flattening can be attained with further range hoppings that decrease exponentially with distance. Finally, we compare and contrast the conditions to select the fractional quantum Hall effect in these lattice models with those for the conventional fractional quantum Hall effect (a two-dimensional continuum model with a uniform magnetic field). 

Dec 21
1. arXiv:1012.4028 [pdf, other]
Title: Ground-State Phase Diagram of the 1D t-J model
Authors:Alexander Moreno, Alejandro Muramatsu, Salvatore R. Manmana
Comments: 14 pages, submitted to Phys. Rev. B 
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We examine the ground-state phase diagram of the t-J model in one dimension by means of the Density Matrix Renormalization Group. The phase boundaries separating the repulsive from the attractive Luttinger-liquid (LL) phase, and also the boundaries of the spin-gap region and phase-separation, are determined on the basis of correlation functions and energy gaps. In particular, we shed light on a contradiction between variational and renormalization-group (RG) results about the extent of the spin-gap phase, that results larger than the variational but smaller than the RG one. Furthermore, we show that the spin-gap can reach a sizable value ($\sim 0.1 t$) at low enough filling, such that preformed pairs should be observable at temperatures below these energy scales. No evidence for a phase with clustering of more than two particles is found on approaching phase separation. 

2. arXiv:1012.4041 [pdf, other]
Title: A crossover from weak to strong pairing in unconventional superconductors
Authors: D. S. Inosov, J. T. Park, A. Charnukha, Yuan Li, A. V. Boris, B. Keimer, V. Hinkov
Comments: 13 pages, 3 figures. Tables in the Supplementary Information contain an exhaustive overview of the published gap measurements in Fe-based superconductors 
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el) 

Superconductors are classified by their pairing mechanism, i.e. the type of microscopic interactions that bind electrons into Cooper pairs, and by the pairing strength, measured as the ratio of the superconducting gap to the critical temperature, Tc. Conventional phonon-mediated superconductors are characterized by relatively weak pairing and low critical temperatures, whereas high-Tc layered copper-oxide-based ceramics typically exhibit stronger pairing. Therefore, we are used to viewing these two classes of materials as clearly distinct. Here we present an extensive comparison of the pairing strength among many single- and multiband superconductors, encompassing simple metals, high-Tc cuprates and the recently discovered Fe-based compounds. For Fe- and Cu-based materials, our analysis reveals a universal correlation between the gap ratio and Tc, which is not found in conventional superconductors. The superconducting pairing in ferropnictides ranges from weak, near the limit predicted by the Bardeen-Cooper-Schrieffer (BCS) theory, to strong, as in cuprates, bridging the gap between these two extremes. Our comparison emphasizes the internal differences within the Fe-based family and places it as a "missing link" between low- and high-Tc compounds. 

3. arXiv:1012.4417 [pdf, other]
Title: BEC-BCS crossover and universal relations in unitary Fermi gases
Authors: S. Gandolfi, K. E. Schmidt, J. Carlson
Comments: 5 pages, 4 figures 
Subjects: Quantum Gases (cond-mat.quant-gas)

The contact parameter in unitary Fermi Gases governs the short-distance, high-momentum, and high-energy properties of the system. We perform accurate quantum Monte Carlo calculations with highly optimized trial functions to precisely determine this parameter at T=0, demonstrate its universal application to a variety of observables, and determine the regions of momentum and energy over which the leading short-range behavior is dominant. We derive Tan's expressions for the contact parameter using just the short-range behavior of the ground-state many-body wave function, and use this behavior to calculate the two-body distribution function, one-body density matrix, and the momentum distribution of unitary Fermi gases; providing a precise value of the contact parameter that can be compared to experiments. 


Dec 20
1. arXiv:1012.3806 [pdf, other]
Title: Quantum dynamics at and near conformal quantum critical points
Authors: S.V. Isakov, P. Fendley, A.W.W. Ludwig, S. Trebst, M. Troyer
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Statistical Mechanics (cond-mat.stat-mech); High Energy Physics - Theory (hep-th) 

We explore the dynamical behavior at and near a special class of two-dimensional quantum critical points. Each is a conformal quantum critical point (CQCP), where in the scaling limit the equal-time correlators are those of a two-dimensional conformal field theory. The critical theories include the square-lattice quantum dimer model, the quantum Lifshitz theory, and a deformed toric code model. We show that under perturbation the latter flows toward the ordinary Lorentz-invariant (2+1) dimensional Ising critical point, illustrating that CQCPs are generically unstable. We exploit a correspondence between the classical and quantum dynamical behavior in such systems to perform an extensive numerical study of two lines of CQCPs in a quantum eight-vertex model, or equivalently, two coupled deformed toric codes. We find that the dynamical critical exponent z remains 2 along the U(1)-symmetric quantum Lifshitz line, while it continuously varies along the line with only Z_2 symmetry. This illustrates how two CQCPs can have very different dynamical properties, despite identical equal-time ground-state correlators. 
Dec 13 - Dec 17, Chungwei Lin

Dec 17
1. arXiv:1012.3690 (cross-list from quant-ph) [pdf, other]
Title: Stückelberg-Interferometry with ultra-cold atoms
Authors: Patrick Plötz, Sandro Wimberger
Comments: 6 pages, 4 figures 
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas) 
We show that and how ultra-cold atoms in an accelerated two-band lattice are a controlled realization of Landau--Zener--St\"uckelberg interferometry

2. arXiv:1012.3484 [pdf, ps, other]
Title: Electron-Electron Interactions in Graphene: Current Status and Perspectives
Authors: Valeri N. Kotov, Bruno Uchoa, Vitor M. Pereira, A. H. Castro Neto, F. Guinea
Comments: Submitted to Reviews of Modern Physics. 62 pages, 43 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We review the problem of electron-electron interactions in graphene. Starting from the screening of long range interactions in these systems, we discuss the existence of an emerging Dirac liquid of Lorentz invariant quasi-particles in the weak coupling regime, and strongly correlated electronic states in the strong coupling regime. We also analyze the analogy and connections between the many-body problem and the Coulomb impurity problem. The problem of the magnetic instability and Kondo effect of impurities and/or adatoms in graphene is also discussed in analogy with classical models of many-body effects in ordinary metals. We show that Lorentz invariance plays a fundamental role and leads to effects that span the whole spectrum, from the ultraviolet to the infrared. The effect of an emerging Lorentz invariance is also discussed in the context of finite size and edge effects as well as mesoscopic physics. We also briefly discuss the effects of strong magnetic fields in single layers and review some of the main aspects of the many-body problem in graphene bilayers. In addition to reviewing the fully understood aspects of the many-body problem in graphene, we show that a plethora of interesting issues remain open, both theoretically and experimentally, and that the field of graphene research is still exciting and vibrant. 



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Dec 16
1. arXiv:1012.3170 [pdf, ps, other]
Title: Chiral Rashba spin textures in ultra-cold Fermi gases
Authors: Jay D. Sau, Rajdeep Sensarma, Stephen Powell, I. B. Spielman, S. Das Sarma
Comments: 4.5 pages, 3 figures 
Subjects: Quantum Gases (cond-mat.quant-gas)
Spin-orbit coupling is an important ingredient in many recently discovered phenomena such as the spin-Hall effect and topological insulators. Of particular interest is topological superconductivity, with its non=Abelian quasiparticle statistics potential application in topological quantum computation. The absence of disorder in ultra-cold atomic systems make them ideal for quantum computation applications, however, the spin-orbit (SO) coupling schemes proposed so far for cold atomic systems are experimentally impractical owing to large spontaneous emission rates in the alkali fermions. In this paper, we propose a scheme to generate Rashba SO coupling with a straightforward extension of to an existing experiment. Our setup requires low laser powers and can be operated far from atomic resonance, reducing spontaneous emission. We show that this scheme can be used to generate a Fermi surface spin texture for $^{40}\rm{K}$ atoms, which is observable in time-of-flight measurements. The spin texture, together with conventional s-wave interactions (tunable by Feshbach resonance) leads to effective p-wave pairing and hence topological superconductivity and non-Abelian Majorana quasiparticles. 
2. arXiv:1012.3341 [pdf, ps, other]
Title: Supersolid phase in atomic gases with magnetic dipole interaction
Authors: Adam Bühler, Hans Peter Büchler
Comments: 4 pages, 3 figures 
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph) 
A major obstacle for the experimental realization of a supersolid phase with cold atomic gases in an optical lattice is the weakness of the nearest-neighbor interactions achievable via magnetic dipole-dipole interactions. In this letter, we show that using a large filling of atoms within each well the characteristic energy scales are strongly enhanced. Within this regime, the system is well described by the rotor model, and the qualitative behavior of the phase diagram derives from mean-field theory. We find a stable supersolid phase for realistic parameters with chromium atoms. 

3. arXiv:0911.1378 (cross-list from cond-mat.quant-gas) [pdf, other]
Title: Spontaneous interlayer superfluidity in bilayer systems of cold polar molecules
Authors: Roman M. Lutchyn, Enrico Rossi, S. Das Sarma
Comments: 7 pages, 5 figures, final version 
Journal-ref: Phys. Rev. A 82, 061604(R) (2010) 
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el) 
Quantum degenerate cold-atom gases provide a remarkable opportunity to study strongly interacting systems. Recent experimental progress in producing ultracold polar molecules with a net electric dipole moment opens up new possibilities to realize novel quantum phases governed by the long-range and anisotropic dipole-dipole interactions. In this work we predict the existence of experimentally observable novel broken-symmetry states with spontaneous interlayer coherence in cold polar molecules. These exotic states appear due to strong repulsive interlayer interactions and exhibit properties of superfluids, ferromagnets and excitonic condensates. 


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Dec 15
1. arXiv:1012.2885 [pdf, ps, other]
Title: Observation of shock waves in a strongly interacting Fermi gasAuthors: James Joseph, John E. Thomas, Manas Kulkarni, Alexander G. AbanovSubjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el) We study collisions between two strongly interacting atomic Fermi gas clouds. We observe exotic nonlinear hydrodynamic behavior, distinguished by the formation of a very sharp and stable density peak as the clouds collide and subsequent evolution into a box-like shape. We model the nonlinear dynamics of these collisions using quasi-1D hydrodynamic equations. Our simulations of the time-dependent density profiles agree very well with the data and provide clear evidence of shock wave formation in this universal quantum hydrodynamic system. 

2.arXiv:1012.3048 [pdf, ps, other]
Title: Weakly interacting Bose gas in the one-dimensional limit
Authors: P. Krüger, S. Hofferberth, I. E. Mazets, I. Lesanovsky, J. Schmiedmayer
Comments: to appear in Phys. Rev. Lett
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We prepare a chemically and thermally one-dimensional (1d) quantum degenerate Bose gas in a single microtrap. We introduce a new interferometric method to distinguish the quasicondensate fraction of the gas from the thermal cloud at finite temperature. We reach temperatures down to $kT\approx 0.5\hbar\omega_\perp$ (transverse oscillator eigenfrequency $\omega_\perp$) when collisional thermalization slows down as expected in 1d. At the lowest temperatures the transverse momentum distribution exhibits a residual dependence on the line density $n_{1d}$, characteristic for 1d systems. For very low densities the approach to the transverse single particle ground state is linear in $n_{1d}$. 

3. arXiv:1012.2930 [pdf, other]
Title: High temperature fractional quantum Hall states Authors: Evelyn Tang, Jia-Wei Mei, Xiao-Gang WenComments: 4 pages, 6 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el)We show that a suitable combination of geometric frustration, ferromagnetism and spin-orbit interactions can give rise to nearly flat bands with a large bandgap and non-zero Chern number. Partial filling of the flat band can give rise to FQH states at high temperatures (maybe even room temperature). While the identification of material candidates with suitable parameters remains open, our work indicates intriguing directions for exploration and synthesis. 

4. arXiv:1012.2960 [pdf, other]
Title: Collision-dominated spin transport in graphene and Fermi liquids Authors: Markus Mueller, Hai Chau NguyenSubjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall) In a clean Fermi liquid, due to spin up/spin down symmetry, the dc spin current driven by a magnetic field gradient is finite even in the absence of impurities. Hence, the spin conductivity sigma_s assumes a well-defined collision-dominated value in the disorder-free limit, providing a direct measure for the inverse strength of electron-electron interactions. In neutral graphene, with Fermi energy at the Dirac point, the Coulomb interactions remain unusually strong, such that the inelastic scattering rate comes close to a conjectured upper bound 1/\tau_{inel} <= k_B T/\hbar, similarly as in strongly coupled quantum critical systems. The strong scattering is reflected by a minimum of the spin conductivity at the Dirac point, where it reaches sigma_s = (0.121/alpha^2) * (mu_B^2/\hbar) at weak Coulomb coupling alpha. Up to the replacement of quantum units, e^2/\hbar -> mu_s^2/\hbar, this result equals the collision-dominated electrical conductivity obtained previously. This accidental symmetry is, however, broken to higher orders in the interaction strength. For gated graphene, and 2d metals in general, we show that the transport time is parametrically smaller than the collision time. We exploit this to compute the collision-limited sigma_s analytically as sigma_s= (1/C) * (mu/T)^2 * (mu_B^2/\hbar) with C=4 pi^2 alpha^2 [2/3 ln(1/(2\alpha))-1] for weak Coulomb coupling alpha. 

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Dec 14
1. arXiv:1012.2626 [pdf, other]
Title: Temperature dependence of the contact in a unitary Fermi gas Authors: E. D. Kuhnle, S. Hoinka, P. Dyke, H. Hu, P. Hannaford, C. J. ValeComments: 4 pages, 2 figuresSubjects: Quantum Gases (cond-mat.quant-gas) The contact ${\cal I}$, introduced by Tan, has emerged as a key parameter characterizing universal properties of strongly interacting Fermi gases. For ultracold Fermi gases near a Feshbach resonance, the contact depends upon two quantities: the interaction parameter $1/(k_F a)$, where $k_F$ is the Fermi wave-vector and $a$ is the $s$-wave scattering length, and the temperature $T/T_F$, where $T_F$ is the Fermi temperature. We present the first measurements of the temperature dependence of the contact in a unitary Fermi gas using Bragg spectroscopy. The contact is seen to follow the predicted decay with temperature and shows how pair-correlations at high momentum persist well above the superfluid transition temperature. 

2. arXiv:1012.2764 (cross-list from cond-mat.str-el) [pdf, ps, other]
Title: Self-consistent calculation of the single particle scattering rate in high $Tc$ cuprates
Authors: Abolhassan Vaezi
Comments: 6 pages, 2 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con)
The linear temperature dependence of the resistivity above the optimal doping is a longstanding problem in the field of high temperature superconductivity in cuprates. In this paper, we investigate the effect of gauge fluctuations on the momentum relaxation time and the transport scattering rate within the slave boson method. We use a more general slave treatment to resolve the ambiguity of decomposing the Heisenberg exchange term. We conclude that this term should be decomposed only in the Cooper channel. This results in the spinon mass inversely proportional to the doping. It is showed that solving the equation for the transport scattering rate self-consistently, we find a crossover temperature above which we obtain the linear temperature dependence of the electrical resistivity as well as the single particle scattering rate. It is also shown that this linear temperature dependence of the scattering rate in the pseudogap region explains the existence of the Fermi arcs with a length that linearly varies with temperature. 

3.arXiv:1012.2850 (cross-list from quant-ph) [pdf, ps, other]
Title: Generalized Bose-Einstein Condensation
Authors: William J. Mullin, Asaad R. Sakhel
Comments: 17 pages; 6 figures. Intended for American Journal of Physics
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech)
Generalized Bose-Einstein condensation (GBEC) involves condensates appearing simultaneously in multiple states. We analyze examples of the three types in an ideal Bose gas with different geometries. In Type I there is a discrete number of quantum states each having macroscopic occupation; Type II has condensation into a continuous band of states, with each state having macroscopic occupation; in Type III each state is microscopically occupied while the entire condensate band is macroscopically occupied. We begin by discussing Type I or "normal" BEC into a single state for an isotropic harmonic oscillator potential. Other geometries and external potentials are then considered: the "channel" potential (harmonic in one dimension and hard-wall in the other), which displays Type II, the "cigar trap" (anisotropic harmonic potential), and the "Casimir prism" (an elongated box), the latter two having Type III condensations. General box geometries are considered in an appendix. We particularly focus on the cigar trap, which Van Druten and Ketterle first showed had a two-step condensation: a GBEC into a band of states at a temperature $T_{c}$ and another "one-dimensional" transition at a lower temperature $T_{1}$ into the ground state. In a thermodynamic limit in which the ratio of the dimensions of the anisotropic harmonic trap is kept fixed, $T_{1}$ merges with the upper transition, which then becomes a normal BEC. However, in the thermodynamic limit of Beau and Zagrebnov, in which the ratio of the boundary lengths increases exponentially, $T_{1}$ becomes fixed at the temperature of a true Type I phase transition. The effects of interactions on GBEC are discussed briefly. 

4. arXiv:1012.2637 [pdf, ps, other]
Title: Mott Physics on Helical Edges of 2D Topological Insulators
Authors: Youhei Yamaji, Masatoshi Imada
Comments: 4 pages, 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Materials Science (cond-mat.mtrl-sci)
We study roles of electron correlations on topological insulators on the honeycomb lattice with the spin-orbit interaction. Accurate variational Monte Carlo calculations show that the increasing on-site Coulomb interactions cause a strong suppression of the charge Drude weight in the helical-edge metallic states leading to a presumable Mott transition from a conventional topological insulator to an edge Mott insulator before a transition to a bulk antiferromagnetic insulator. The intermediate bulk-topological and edge-Mott-insulator phase has a helical spin-liquid character with the protected time-reversal symmetry. 

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Dec 13
1. arXiv:1012.2293 [pdf, ps, other]
Title: The evolution from BCS to BEC superfluidity in the presence of disorder
Authors: Li Han, C. A. R. Sá de Melo
Subjects: Quantum Gases (cond-mat.quant-gas)
We describe the effects of disorder on the critical temperature of $s$-wave superfluids from the BCS to the BEC regime, with direct application to ultracold fermions. We use the functional integral method and the replica technique to study Gaussian correlated disorder due to impurities, and we discuss how this system can be generated experimentally. In the absence of disorder, the BCS regime is characterized by pair breaking and phase coherence temperature scales which are essentially the same allowing strong correlations between the amplitude and phase of the order parameter for superfluidity. As non-pair breaking disorder is introduced the largely overlapping Cooper pairs conspire to maintain phase coherence such that the critical temperature remains essentially unchanged, and Anderson's theorem is satisfied. However in the BEC regime the pair breaking and phase coherence temperature scales are very different such that non-pair breaking disorder can affect dramatically phase coherence, and thus the critical temperature, without the requirement of breaking tightly-bound fermion pairs simultaneously. In this case, Anderson's theorem does not apply, and the critical temperature can be more easily reduced in comparison to the BCS limit. Lastly, we find that the superfluid is more robust against disorder in the intermediate region near unitarity between the two regimes. 

2. arXiv:1012.2329 [pdf, ps, other]
Title: Tan's universal contact and collective oscillations of strongly interacting Fermi gases
Authors: Mark DelloStritto, Theja N. De Silva
Comments: 7 pages and 6 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We study strongly interacting two component Fermi gas near a Feshbach resonance. By using a ground state energy functional constructed based on asymptotic limits and Monte Carlo calculations, we calculate the contact, structure factor, and collective oscillation frequencies in the BCS-BEC crossover region. The calculated contact and structure factor show excellent agreements with recent experiments. We compare these results with a standard mean-filed theory and find that the contact is proportional to the square of superfluid order parameter. Further, we show that the upper bounds of the collective modes have universal form in the sense that they depend only on the contact and the homogenous energy. In other words, the collective modes of the Fermi atoms trapped near Feshbach resonance can be calculated without the explicit knowledge of trapping potential. 

3. arXiv:1007.4711 (replaced) [pdf, ps, other]
Title: Melting of stripe phases and its signature in the single-particle spectral function
Authors: Marcin Raczkowski, Fakher F. Assaad
Comments: 4 pages, 5 figures, minor changes, added Ref. 12
Journal-ref: Phys. Rev. B 82, 233101 (2010)
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
Motivated by the recent experimental data [Phys. Rev. B 79, 100502 (2009)] indicating the existence of a pure stripe charge order over unprecedently wide temperature range in La_{1.8-x}Eu_{0.2}Sr_xCuO_4, we investigate the temperature-induced melting of the metallic stripe phase. In spite of taking into account local dynamic correlations within a real-space dynamical mean-field theory of the Hubbard model, we observe a mean-field like melting of the stripe order irrespective of the choice of the next-nearest neighbor hopping. The temperature dependence of the single-particle spectral function shows the stripe induced formation of a flat band around the antinodal points accompanied by the opening a gap in the nodal direction.



Dec 6 - Dec 10, Xiaopeng Li

Dec 10
1. arXiv:1012.1861 [pdf, ps, other]
Title: Fractionalize This
Author: Philip Phillips
Precisely what are the electrons in a high-temperature superconductor doing before they superconduct? Strong electronic correlations may give rise to composite rather than fractionalized excitations, as is typical in other strongly coupled systems such as quark matter.

2. arXiv:1012.1984 [pdf, other]
Title: Nonequilibrium Transport of Quantum Molecular Chain in terms of the Complex Liouvillian Spectrum
Author: Satoshi Tanaka, Kazuki Kanki, Tomio Petrosky
Transport process in molecular chain in nonequilibrium stationary state is theoretically investigated. The molecule is interacting at its both ends with thermal baths which has different temperatures, while no dissipation mechanism is contained inside the molecular chain. We have first obtained the nonequilibrium stationary state outside the Hilbert space in terms of the complex spectral representation of Liouvillian. The nonequilibrium stationary state is obtained as an eigenstate of the Liouvillian which is constructed through the collision invariant of the kinetic equation. The eigenstate of the Liouvillian contains an information of spatial correlation between the molecular chain and the thermal baths. While energy flow in the nonequilibrium state which is due to the first order correlation can be described by Landauer formula, the particle current due to the second order correlation cannot be described by the Landauer formula. The present method provides a simple and perspective way to evaluate the energy transport of molecular chain under the nonequilibrium situation.


Dec 9
1. arXiv:1012.1846 [pdf, ps, other]
Title: Anomalies in the Fermi surface and band dispersion of quasi-one-dimensional CuO chains in the high-temperature superconductor YBa2Cu4O8
Author: Takeshi Kondo, R. Khasanov, J. Karpinski, S. M. Kazakov, N. D. Zhigadlo, Z. Bukowski, M. Shi, A. Bendounan, Y. Sassa, J. Chang, S. Pailhés, J. Mesot, J. Schmalian, H. Keller, A. Kaminski
We have investigated the electronic states in quasi one dimensional (1D) CuO chains by microprobe Angle Resolved Photoemission Spectroscopy. We find that the quasiparticle Fermi surface consists of six disconnected segments, consistent with recent theoretical calculations that predict the formation of narrow, elongated Fermi surface pockets for coupled CuO chains. In addition, we find a strong renormalization effect with a significant kink structure in the band dispersion. The properties of this latter effect [energy scale (~40 meV), temperature dependence and behavior with Zn-doping] are identical to those of the bosonic mode observed in CuO2 planes of high temperature superconductors, indicating they have a common origin.

2. arXiv:1012.1828 [pdf, ps, other]
Title: Compressibility and entropy of cold fermions in one dimensional optical lattices
Author: Andrew Snyder, Iori Tanabe, Theja De Silva 
We calculate several thermodynamic quantities of repulsively interacting one dimensional fermions. We solve the Hubbard model at both zero and finite temperature using the Bethe ansatz method. For arbitrary values of chemical potential, we calculate the particle number density, the double occupancy, various compressibilities, and the entropy as a function of temperature and interaction. We find that these thermodynamic quantities show characteristic behavior so that measurements of these quantities can be used as a detection of temperature, metal-insulator transition, and metalic and insulating phases in the trap environment.

Dec 8
1. arXiv:1012.1360 [pdf, ps, other]
Title: Disordered Bose-Hubbard model and its entanglement entropy: a variational approach
Author: Luca Dell'Anna, Michele Fabrizio
We study the superfluid-to-Bose glass transition in a disordered Bose-Hubbard model through a very simple variational wavefunction: a permanent of non-orthogonal single-particle wavefunctions that are variationally determined. The transition is identified by the behavior of the superfluid stiffness. We also introduce a less rigorous but very enlightening criterium for the transition, which is related to the overlap matrix among the single-particle wavefunctions that are used to built the permanent. We find that the two criteria agree quite well. We finally consider a further quantity, the bipartite entanglement entropy, which also provides a very good estimator for the superfluid-to-Bose glass transition. 

2. arXiv:1012.1485 [pdf, ps, other]
Title: Three Dimensional Ising Model, Percolation Theory and Conformal Invariance
Author: Abbas Ali Saberi, Horr Dashti-Naserabadi
The fractal structure and scaling properties of a 2d slice of the 3d Ising model is studied using Monte Carlo techniques. The percolation transition of geometric spin (GS) clusters is found to occur at the Curie point, reflecting the critical behavior of the 3d model. The fractal dimension and the winding angle statistics of the perimeter and external perimeter of the geometric spin clusters at the critical point suggest that, if conformally invariant in the scaling limit, they can be described by the theory of Schramm-Loewner evolution (SLE_\kappa) with diffusivity of \kappa=5 and 16/5, respectively, putting them in the same universality class as the interfaces in 2d tricritical Ising model. It is also found that the Fortuin-Kasteleyn (FK) clusters associated with the cross sections undergo a nontrivial percolation transition, in the same universality class as the ordinary 2d critical percolation. 

Dec 7

1. arXiv:1012.1299 [pdf, ps, other]
Title: Spin Susceptibility and Helical Magnetic Order at the Edge of Topological Insulators due to Fermi Surface Nesting
Author: J. H. Jiang, Si Wu
We study spin susceptibility and magnetic order at the edge of topological insulators when the Fermi surface is nested. We show that due to spin-momentum locking, the spin susceptibility function at nesting wavevector has a strong {\em helical} feature. It follows then, a {\em helical} spin density wave (SDW) state emerges at low temperature due to Fermi surface nesting. We present a mean field theory of the helical SDW state. From a mean field Zener theory, we predict a helical magnetic order on the surface state of the magnetically doped topological insulators when the Fermi surface is nested, where the Curie temperature is as high as $T_C\simeq 100$~K.
2. arXiv:1012.1294 [pdf, other]
Title: An analytical model of fractional overshooting
Author: A. Salman, A. I. Mese, M. B. Yucel, A. Siddiki
We predict resistance anomalies to be observed at high mobility two dimensional electron systems (2DESs) in the fractional quantized Hall regime, where the narrow (L <10 ?m) Hall bar is defined by top gates. An analytic calculation scheme is used to describe the formation of integral and fractional incompressible strips. We incorporate the screening properties of the 2DES, together with the effects of perpendicular magnetic field, to calculate the effective widths of the current carrying channels. The many-body effects are included to our calculation scheme through the energy gap obtained from the well accepted formulation of the composite fermions. We show that, the fractional incompressible strips at the edges, assuming different filling factors, become evanescent and co-exist at certain magnetic field intervals yielding an overshoot at the Hall resistance. Similar to that of the integral quantized Hall effect. We also provide a mechanism to explain the absence of 1/3 state at the Fabry-Perot interference experiments. Yet, an un-investigated sample design is proposed to observe and enhance the fragile effects like interference and overshooting based on our analytical model.

Dec 6

1. arXiv:1012.0823 [pdf, other]
Title: Off-equilibrium relaxational dynamics with improved Ising Hamiltonian
Author: Mario Collura
We study the off-equilibrium relaxational dynamics at criticality in the three-dimensional Blume-Capel model whose static critical behaviour belongs to the 3d-Ising universality class. Using "improved" Hamiltonian (the leading corrections to scaling have vanishing amplitude) we perform Monte Carlo simulations of the relaxational dynamics after a quench from $T=\infty$ to $T_c$. Analysing the off-equilibrium dynamics at $T_c$ we obtain an estimate of the dynamical critical exponent $z=2.020(8)$ that is perfectly consistent with the Field Theory predictions.