October2010

flat
=Oct 25 - Oct 29, Xiaopeng Li=

Title: Universal parity effects in the entanglement entropy of XX chains with open boundary conditions
Authors: Maurizio Fagotti, Pasquale Calabrese

We consider the Renyi entanglement entropies in the one-dimensional XX spin-chains with open boundary conditions in the presence of a magnetic field. In the case of a semi-infinite system and a block starting from the boundary, we derive rigorously the asymptotic behavior for large block sizes on the basis of a recent mathematical theorem for the determinant of Toeplitz plus Hankel matrices. We conjecture a generalized Fisher-Hartwig form for the corrections to the asymptotic behavior of this determinant that allows the exact characterization of the corrections to the scaling at order o(1/l) for any n. By combining these results with conformal field theory arguments, we derive exact expressions also in finite chains with open boundary conditions and in the case when the block is detached from the boundary.

Oct 28
===**1.__[|arXiv:1010.5519]__ [ [|pdf], [|ps] , [|other] ]** === **Title:** Integrability, Non-integrability and confinement Authors: Giuseppe Mussardo

Comments: Invited talk at StatPhys24, Cairns (Australia) 2010. 27 pages, 16 figures Subjects: **Statistical Mechanics (cond-mat.stat-mech)** ; High Energy Physics - Theory (hep-th); Mathematical Physics (math-ph) We discuss the main features of quantum integrable models taking the classes of universality of the Ising model and the repulsive Lieb-Liniger model as paradigmatic examples. We address the breaking of integrability by means of two approaches, the Form Factor Perturbation Theory and semiclassical methods. Each of them has its own advantage. Using the first approach, one can relate the confinement phenomena of topological excitations to the semi-locality of the operator which breaks integrability. Using the second approach, one can control the bound states which arise in each phase of the theory and predict that their number cannot be more than two.

**Oct 27**
**Title: Monte Carlo simulation with Tensor Network states** Authors: Ling Wang, Iztok Pizorn, Frank Verstraete
 * 1.** ** [|arXiv:1010.5450] [ [|pdf], [|other] ]**

It is demonstrated that Monte Carlo sampling can be used to efficiently extract the expectation value of projected entangled pair states with large virtual bond dimension. We use the simple update rule introduced by Xiang et al. to obtain the tensors describing the ground state wavefunction of the antiferromagnetic Heisenberg model and evaluate the finite size energy and staggered magnetization for square lattices with periodic boundary conditions of sizes up to L=16 and virtual bond dimensions up to D=16. The finite size magnetization errors are 0.003(2) and 0.013(2) at D=16 for a system of size L=8,16 respectively. Finite D extrapolation provides exact finite size magnetization for L=8, and reduces the magnetization error to 0.005(3) for L=16, significantly improving the previous state of the art results.

Oct 26
1. ** [|arXiv:1010.5169] [ [|pdf], [|other] ]** **Title: Plasmon mass and Drude weight in strongly spin-orbit-coupled 2D electron gases** Authors: Amit Agarwal, Stefano Chesi, T. Jungwirth, Jairo Sinova, G. Vignale, Marco Polini ** Spin-orbit-coupled two-dimensional electron gases (2DEGs) are a textbook example of helical Fermi liquids, i.e. quantum liquids in which spin (or pseudospin) and momentum degrees-of-freedom at the Fermi surface have a well-defined correlation. Here we study the long-wavelength plasmon dispersion and the Drude weight of archetypical spin-orbit-coupled 2DEGs. We first show that these measurable quantities are sensitive to electron-electron interactions due to broken Galileian invariance and then discuss in detail why the popular random phase approximation is not capable of describing the collective dynamics of these systems even at very long wavelengths. This work is focussed on presenting approximate microscopic calculations of these quantities based on the minimal theoretical scheme that captures the basic physics correctly, i.e. the time-dependent Hartree-Fock approximation. We find that interactions enhance the "plasmon mass" and suppress the Drude weight. Our findings can be tested by inelastic light scattering, electron energy loss, and far-infrared optical-absorption measurements. **

2.** [|arXiv:1010.5147] [ [|pdf], [|other] ]** **Title: Signatures of Wigner Crystallization in Epitaxially Grown Nanowires** Authors: L. H. Kristinsdóttir, J. C. Cremon, H. A. Nilsson, H. Q. Xu, L. Samuelson, H. Linke, A. Wacker, S. M. Reimann It was predicted by Wigner in 1934 that the electron gas will undergo a transition to a crystallized state when its density is very low. Whereas significant progress has been made towards the detection of electronic Wigner states, their clear and direct experimental verification still remains a challenge. Here we address signatures of Wigner crystal formation in the transport properties of InSb nanowire quantum dot systems, where a few electrons may form localized states depending on the size of the dot (i.e. the electron density). By a configuration interaction approach combined with an appropriate transport formalism, we are able to predict the transport properties of these systems, in excellent agreement with experimental data. We identify specific signatures of Wigner state formation, such as the strong suppression of the antiferromagnetic coupling, and are able to detect the onset of Wigner crystallization, both experimentally and theoretically, by studying different dot sizes.

<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Oct 25
<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif;"> 1. ** [|arXiv:1010.4712] ** **[** ** [|pdf] ** **,** ** [|other] ** **]** <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;">**Title: Dynamical approach to the Casimir effect** <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;">Authors: Pablo Rodriguez-Lopez, Ricardo Brito, Rodrigo Soto <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Casimir forces can appear between intrusions placed in different media driven by several fluctuation mechanisms, either in equilibrium or out of it. Herein, we develop a general formalism to obtain such forces from the dynamical equations of the fluctuating medium, the statistical properties of the driving noise, and the boundary conditions of the intrusions (which simulate the interaction between the intrusions and the medium). As a result, an explicit formula for the Casimir force over the intrusions is derived. This formalism contains the thermal Casimir effect as a particular limit and generalizes the study of the Casimir effect to such systems through their dynamical equations, with no appeal to their Hamiltonian, if any exists. In particular, we study the Casimir force between two infinite parallel plates with Dirichlet or Neumann boundary conditions, immersed in several media with finite correlation lengths (reaction--diffusion system, liquid crystals, and two coupled fields with non-Hermitian evolution equations). The driving Gaussian noises have vanishing or finite spatial or temporal correlation lengths; in the first case, equilibrium is reobtained and finite correlations produce nonequilibrium dynamics. The results obtained show that, generally, nonequilibrium dynamics leads to Casimir forces, whereas Casimir forces are obtained in equilibrium dynamics if the stress tensor is anisotropic.

=Oct 18 - Oct 22, Zixu Zhang=

Title: Fermionic trimers in spin-dependent optical lattices
Authors: Giuliano Orso, Evgeni Burovski, Thierry Jolicoeur Comments: 8 pages, 5 figures, submitted to a special few-body issue of the Journal of the French Academy of Sciences (CRAS) Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el) We investigate the formation of three-body bound states (trimers) in two-component Fermi gases confined in one dimensional optical lattice with spin-dependent tunneling rates. The binding energy and the effective mass of the trimer are obtained from the solution of the Mattis integral equation generalized to the case of unequal Bloch masses. We show that this equation admits multiple solutions corresponding to excited bound states, which are only stable for large mass asymmetry.

Title: Fragmentation and the Bose-glass phase transition of the disordered 1D Bose gas
Authors: Luca Fontanesi, Michiel Wouters, Vincenzo Savona Comments: 5 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas); Disordered Systems and Neural Networks (cond-mat.dis-nn) We investigate the superfluid-insulator quantum phase transition in a disordered 1D Bose gas in the mean field limit, by studying the probability distribution of the density. The superfluid phase is characterized by a vanishing probability to have zero density, whereas a nonzero probability marks the insulator phase. This relation is derived analytically, and confirmed by a numerical study. This fragmentation criterion is particularly suited for detecting the phase transition in experiments. When a harmonic trap is included, the transition to the insulating phase can be extracted from the statistics of the local density distribution.

Title: Single Impurity In Ultracold Fermi Superfluids
Authors: Lei Jiang, Leslie O. Baksmaty, Hui Hu, Yan Chen, Han Pu Comments: 4 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas) Impurities can be used as probes to detect material properties and to understand quantum phenomena. Here we study the effect of a single classical impurity in ultracold $s$-wave Fermi superfluids. The impurity can be either magnetic or non-magnetic. Although a non-magnetic impurity does not change macroscopic properties of the system, a magnetic impurity can induce a mid-gap bound state located inside the pairing gap. In addition, magnetic impurity can locally induce population imbalance in the system, potentially providing a method to realize FFLO-like state in a controlled way. We also propose a modified RF spectroscopy to measure the local density of states, as a cold-atom analog of STM.

Title: Spin relaxation and band excitation of a dipolar BEC in 2D optical lattices
Authors: B. Pasquiou, G. Bismut, E. Maréchal, P. Pedri, L. Vernac, O. Gorceix, B. Laburthe-Tolra Comments: 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) We observe interband transitions mediated by the dipole-dipole interaction for an array of 1D quantum gases of chromium atoms, trapped in a 2D optical lattice. Interband transitions occur when dipolar relaxation releases an energy which matches or overcomes the lattice band gap. We analyze the role of tunneling in higher lattice bands on this process. We compare the experimental dipolar relaxation rate with a calculation based on a multiple Fermi Golden Rule approach, when the lattice sites are symmetric, and the magnetic field is parallel to the lattice axis. We also show that an almost complete suppression of dipolar relaxation is obtained below a magnetic field threshold set by the depth of the lattice: 1D quantum gases in an excited Zeeman state then become metastable.

Title: Feshbach resonances in the 6Li-40K Fermi-Fermi mixture: Elastic versus inelastic interactions
Authors: D. Naik, A. Trenkwalder, C. Kohstall, F. M. Spiegelhalder, M. Zaccanti, G. Hendl, F. Schreck, R. Grimm, T. M. Hanna, P. S. Julienne Comments: Submitted to EPJD, EuroQUAM special issues "Cold Quantum Matter - Achievements and Prospects" Subjects: Quantum Gases (cond-mat.quant-gas) We present a detailed theoretical and experimental study of Feshbach resonances in the 6Li-40K mixture. Particular attention is given to the inelastic scattering properties, which have not been considered before. As an important example, we thoroughly investigate both elastic and inelastic scattering properties of a resonance that occurs near 155 G. Our theoretical predictions based on a coupled channels calculation are found in excellent agreement with the experimental results. We also present theoretical results on the molecular state that underlies the 155G resonance, in particular concerning its lifetime against spontaneous dissociation. We then present a survey of resonances in the system, fully characterizing the corresponding elastic and inelastic scattering properties. This provides the essential information to identify optimum resonances for applications relying on interaction control in this Fermi-Fermi mixture.

Title: Geometric phases and competing orders in two dimensions
Authors: Liang Fu, Subir Sachdev, Cenke Xu Comments: 36 pages, 5 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) We discuss the problem of characterizing "quantum disordered" ground states, obtained upon loss of antiferromagnetic order on general lattices in two spatial dimensions, with arbitrary electronic band structure. A key result is the response in electron bilinears to the skyrmion density in the local antiferromagnetic order, induced by geometric phases. We also discuss the connection to topological terms obtained under situations where the electronic spectrum has a Dirac form.

Title: Andreev reflection between a normal metal and the FFLO superconductor II: a self-consistent approach
Authors: J. Kaczmarczyk, M. Sadzikowski, J. Spałek Comments: 15 pages, 6 figures, accepted in Physica C Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el) We consider Andreev reflection in a two dimensional junction between a normal metal and a heavy fermion superconductor in the Fulde-Ferrell (FF) type of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. We assume s-wave symmetry of the superconducting gap. The parameters of the superconductor: the gap magnitude, the chemical potential, and the Cooper pair center-of-mass momentum Q, are all determined self-consistently within a mean-field (BCS) scheme. The Cooper pair momentum Q is chosen as perpendicular to the junction interface. We calculate the junction conductance for a series of barrier strengths. In the case of incoming electron with spin \sigma = 1 only for magnetic fields close to the upper critical field H_{c2}, we obtain the so-called Andreev window i.e. the energy interval in which the reflection probability is maximal, which in turn is indicated by a peak in the conductance. The last result differs with other non-self-consistent calculations existing in the literature.

Title: Phase Separation in Bose-Bose Mixtures in an Optical Lattice
Authors: Takeshi Ozaki, Tetsuro Nikuni Comments: 12 pages, 8 figures Subjects: Other Condensed Matter (cond-mat.other) We study the ground-state properties of mixtures of strongly interacting bosonic atoms in an optical lattice. Applying a mean-field approximation to the Hubbard model for Bose-Bose mixtures, we calculate the densities and superfluid order parameters for both species. Due to the repulsive interaction between the two species, the system exhibits phase separation. First, in the extreme limit of the zero-hopping case, we derive analytical expressions for the phase boundaries. In particular, we derive the conditions for phase separation in the Mott insulator phase. We find that the conditions for the phase separation depend on the on-site interactions as well as the occupation numbers. In particular, we show that the coexisting state appears by varying the on-site inter-species interaction. We also show the phase diagram of the finite hopping case. Second, we calculate the spatial density profile of $^{87}$Rb-$^{41}$K mixtures in the combined potential of a parabolic trap and an optical lattice using the local density approximation. We fixed the number of $^{87}$Rb and varied the number of $^{41}$K, and used the parameters estimated by experiments. We show that the phase separated $^{87}$Rb-$^{41}$K mixtures distribute like in a parabolic trap case. Furthermore, we find that phase separated mixtures distribute a nesting structure.

Title: Controlling spin motion and interactions in a one-dimensional Bose gas
Authors: P. Wicke, S. Whitlock, N.J. van Druten Comments: 15 pages, 5 figures Subjects: Quantum Gases (cond-mat.quant-gas) Experiments on ultracold gases offer unparalleled opportunities to explore quantum many-body physics, with excellent control over key parameters including temperature, density, interactions and even dimensionality. In some systems, atomic interactions can be adjusted by means of magnetic Feshbach resonances, which have played a crucial role in realizing new many-body phenomena. However, suitable Feshbach resonances are not always available, and they offer limited freedom since the magnetic field strength is the only control parameter. Here we show a new way to tune interactions in one-dimensional quantum gases using state-dependent dressed potentials, enabling control over non-equilibrium spin motion in a two-component gas of 87Rb. The accessible range includes the point of spin-independent interactions where exact quantum many-body solutions are available and the point where spin motion is frozen. This versatility opens a new route to experiments on spin waves, spin-"charge" separation and the relation between superfluidity and magnetism in low-dimensional quantum gases.

=Oct 11 - Oct 15, Chungwei Lin=

Title: Quantum Criticality of 1D Attractive Fermi Gas
Authors: Xiwen Guan, Tin-Lun Ho Subjects: Quantum Gases (cond-mat.quant-gas) We obtain analytical equation of state for one-dimensional strongly attractive Fermi gas for all parameter regime in current experiments. The critical properties of the system are described by that of free fermions and that mixtures of fermions with mass $m$ and $2m$. We also show how these critical properties of bulk systems can be revealed from the density profile of trapped Fermi gas and can be used to determine the T=0 phase boundary.

Title: Inter-band coupling induced novel condensates in a double-well lattice
Authors: Qi Zhou, J. V. Porto, S. Das Sarma Comments: 4 pages, 2 figures Subjects: Quantum Gases (cond-mat.quant-gas) We predict novel inter-band physics for bosons in a double-well lattice. An intrinsic coupling between the s and px band due to interaction gives rise to larger Mott regions on the phase diagram at even fillings than the ones at odd fillings. On the other hand, the ground state can form various types of condensates, including a mixture of single-particle condensates of both bands, a mixture of a single-particle condensate of one band and a pair-condensate of the other band, and a pair-condensate composed of one particle from one band and one hole from the other band. The predicted phenomena should be observable in current experiments on double-well optical lattices.

Title: Collapse and revival oscillations as a probe for the tunneling amplitude in an ultra-cold Bose gas
Authors: F. Alexander Wolf, Itay Hen, Marcos Rigol Comments: 10 pages, 12 figures Journal-ref: Phys. Rev. A 82, 043601 (2010) Subjects: Quantum Gases (cond-mat.quant-gas) We present a theoretical study of the quantum corrections to the revival time due to finite tunneling in the collapse and revival of matter wave interference after a quantum quench. We study hard-core bosons in a superlattice potential and the Bose-Hubbard model by means of exact numerical approaches and mean-field theory. We consider systems without and with a trapping potential present. We show that the quantum corrections to the revival time can be used to accurately determine the value of the hopping parameter in experiments with ultracold bosons in optical lattices. Comment: Non-equilibrium. Rabi Oscillation.

Title: Measurement of an Efimov trimer binding energy in a three-component mixture of 6Li
Authors: Shuta Nakajima, Munekazu Horikoshi, Takashi Mukaiyama, Pascal Naidon, Masahito Ueda Comments: 4 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) The binding energy of an Efimov trimer state was precisely determined via radio-frequency association. It is found that the measurement results significantly shift with temperature, but that the shift can be made negligible at the lowest temperature in our experiment. The obtained trimer binding energy reveals a significant deviation from the nonuniversal theory prediction based on a three-body parameter with a monotonic energy dependence. Comment: 3-body bound state. 3-pair of two-body interactions are same.

Title: Detecting the Amplitude Mode of Strongly Interacting Lattice Bosons by Bragg Scattering
Authors: U. Bissbort, Y. Li, S. Götze, J. Heinze, J. S. Krauser, M. Weinberg, C. Becker, K. Sengstock, W. Hofstetter Comments: 4 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas) We report the first detection of the Higgs-type amplitude mode using Bragg spectroscopy in a strongly interacting condensate of ultracold atoms in an optical lattice. By the comparison of our experimental data with a spatially esolved, time-dependent dynamic Gutzwiller calculation, we obtain good quantitative agreement. This allows for a clear identification of the amplitude mode, showing that it can be detected with full momentum resolution by going beyond the linear response regime. A systematic shift of the sound and amplitude modes' resonance frequencies due to the finite Bragg beam intensity is observed.

Title: Mean-field theory for the Mott insulator-paired superfluid transition in the two-species Bose-Hubbard model
Authors: M. Iskin Comments: 3 pages with 2 figures Subjects: Quantum Gases (cond-mat.quant-gas) The standard mean-field theory for the Mott insulator-superfluid phase transition is not sufficient to describe the Mott insulator-paired superfluid phase transition. Therefore, by restricting the two-species Bose-Hubbard Hamiltonian to the subspace of paired particles, and using perturbation theory, here we derive an analytic mean-field expression for the Mott insulator-paired superfluid transition boundary.

Title: Functional renormalization for the BCS-BEC crossover
Authors: Michael M. Scherer, Stefan Floerchinger, Holger Gies Comments: 18 pages, 6 figures, 2 tables, invited review for a Theme Issue of the Phil. Trans. A on new developments of the functional RG Subjects: Quantum Gases (cond-mat.quant-gas) We review the functional renormalization group (RG) approach to the BCS-BEC crossover for an ultracold gas of fermionic atoms. Formulated in terms of a scale-dependent effective action, the functional RG interpolates continuously between the atomic or molecular microphysics and the macroscopic physics on large length scales. We concentrate on the discussion of the phase diagram as a function of the scattering length and the temperature which is a paradigm example for the non-perturbative power of the functional RG. A systematic derivative expansion provides for both a description of the many-body physics and its expected universal features as well as an accurate account of the few-body physics and the associated BEC and BCS limits.

Title: Quasi-Nambu-Goldstone Modes in Bose-Einstein Condensates
Authors: Shun Uchino, Michikazu Kobayashi, Muneto Nitta, Masahito Ueda Comments: 4 pages, 2 figures Subjects: Quantum Gases (cond-mat.quant-gas); High Energy Physics - Theory (hep-th); Nuclear Theory (nucl-th) We show that quasi-Nambu-Goldstone (QNG) modes, which play prominent roles in high energy physics but have been elusive experimentally, can be realized with atomic Bose-Einstein condensates. The QNG modes emerge when the symmetry of a ground state is larger than that of the Hamiltonian. When they appear, the conventional order parameter manifold (OPM) should be enlarged. Consequently topological defects that are stable within the conventional OPM become unstable and decay by emitting the QNG modes. Contrary to conventional wisdom, however, we show that the topological defects are stabilized by quantum fluctuations that make the QNG modes massive, thereby suppressing their emission.

Title: Nature of the spin liquid state of the Hubbard model on honeycomb lattice
Authors: B. K. Clark, D. A. Abanin, S. L. Sondhi Comments: 5 pages, 6 figures Recent numerical work (Nature 464, 847 (2010)) indicates the existence of a spin liquid phase (SL) that intervenes between the antiferromagnetic and semimetallic phases of the half filled Hubbard model on a honeycomb lattice. To better understand the nature of this exotic phase, we study the quantum $J_1-J_2$ spin model on the honeycomb lattice,which provides an effective description of the Mott insulating region of the Hubbard model. Employing the variational Monte Carlo approach, we analyze the phase diagram of the model, finding a phase transition between antiferromagnet and an unusual $Z_2$ SL state at $J_2/J_1\approx 0.08$, which we identify as the SL phase of the Hubbard model. At higher $J_2/J_1\gtrsim 0.3$ we find a transition to a dimerized state with spontaneously broken rotational symmetry.