arXiv subjects

Fei Zhou

Publications and source records attributed to Fei Zhou.

At least 163 records · Page 9Linked to original sources

arXiv2008

Tunable Quantum Fluctuation-Controlled Coherent Spin Dynamics

Temporal evolution of a macroscopic condensate of ultra cold atoms is usually driven by mean field potentials, either due to scattering between atoms or due to coupling to external fields; and coherent quantum dynamics have been observed in various cold-atom experiments. In this article, we report results of studies of a class of quantum spin dynamics which are purely driven by zero point quantum fluctuations of spin collective coordinates. Unlike the usual mean-field coherent dynamics, quantum fluctuation-controlled spin dynamics or QFCSD studied here are very sensitive to variation of quantum fluctuations and can be tuned by four to five order of magnitude using optical lattices. They have unique dependence on optical lattice potential depths and quadratic Zeeman fields. QFCSD can be potentially used to calibrate quantum fluctuations and investigate correlated fluctuations and various universal scaling properties near quantum critical points.

cond-mat.other↗

arXiv2007

Quantum fluctuation-induced uniaxial and biaxial spin nematics

It is shown that zero point quantum fluctuations (ZPQFs) completely lift the accidental continuous degeneracy that is found in mean field analysis of quantum spin nematic phases of hyperfine spin 2 cold atoms. The result is two distinct ground states which have higher symmetries: a uniaxial spin nematic and a biaxial spin nematic with dihedral symmetry ${Dih}_4$. There is a novel first order quantum phase transition between the two phases as atomic scattering lengths are varied. We find that the ground state of $^{87}Rb$ atoms should be a uniaxial spin nematic. We note that the energy barrier between the phases could be observable in dynamical experiments.

cond-mat.other↗

arXiv2007

Discrete symmetries and 1/3-quantum vortices in condensates of F=2 cold atoms

In this Letter we study discrete symmetries of mean field manifolds of condensates of F=2 cold atoms, and various unconventional quantum vortices. Discrete quaternion symmetries result in two species of spin defects that can only appear in integer vortices while {\em cyclic} symmetries are found to result in a phase shift of $2π/3$ (or $4π/3$) and therefore 1/3- (or 2/3-) quantum vortices in condensates. We also briefly discuss 1/3-quantum vortices in condensates of trimers.

cond-mat.other↗

arXiv2006

Configurational electronic entropy and the phase diagram of mixed-valence oxides: the case of Li$_x$FePO$_4$

We demonstrate that configurational electronic entropy, previously neglected, in {\it ab initio} thermodynamics of materials can qualitatively modify the finite-temperature phase stability of mixed-valence oxides. While transformations from low-T ordered or immiscible states are almost always driven by configurational disorder (i.e. random occupation of lattice sites by multiple species), in FePO$_4$--LiFePO$_4$ the formation of a solid solution is almost entirely driven by electronic, rather than ionic configurational entropy. We argue that such an electronic entropic mechanism may be relevant to most other mixed-valence systems.

cond-mat.mtrl-sci↗

arXiv2006

Quantum insulating states of F=2 cold atoms in optical lattices

In this Letter we study various spin correlated insulating states of F=2 cold atoms in optical lattices. We find that the effective spin exchange interaction due to virtual hopping contains an {\em octopole} coupling between two neighboring lattice sites. Depending on scattering lengths and numbers of particles per site the ground states are either rotationally invariant dimer or trimer Mott insulators or insulating states with various spin orders. Three spin ordered insulating phases are ferromagnetic, cyclic and nematic Mott insulators. We estimate the phase boundaries for states with different numbers of atoms per lattice site.

cond-mat.other↗

arXiv2006

Cyclic Lattice Feshbach Resonances

In this Letter we illustrate the possible cyclic fermion pairing states across Feshbach resonances in optical lattices. In cyclic fermion pairing, the pairing amplitude exhibits an oscillatory behavior as the detuning varies. We estimate the quasi-particle gaps in different regimes of the resonances.

cond-mat.other↗

arXiv2006

Fermion pairing with population imbalance: energy landscape and phase separation in a constrained Hilbert subspace

In this Letter we map out the mean field energy potential landscape of fermion pairing states with population imbalance near broad Feshbach Resonances. We apply the landscape to investigate the nature of phase separation, when the Hilbert space is subject to the constraint of constant population imbalance. We calculate the scattering length dependence of the critical population imbalance for various phase separated states across Feshbach resonances.

cond-mat.other↗

arXiv2005

Quantum dynamics, particle delocalization and instability of Mott states: the effect of fermion-boson conversion on Mott states

We study the quantum dynamics of superfluids of bosons hybridized with Cooper pairs near Feshbach resonances and the influence of fermion-boson conversion on Mott states. We derive a set of equations of motion which describe novel low energy dynamics in superfluids and obtain a new distinct branch of {\em gapped} collective modes in superfluids which involve anti-symmetric phase oscillations in fermionic and bosonic channels. We also find that Mott states in general are unstable with respect to fermion-boson conversion; particles become delocalized and the off-diagonal long-range order of superfluids can be developed when a finite conversion is present. We further point out a possible hidden order in Mott states. It is shown that the quantum dynamics of Fermi-Bose states can be characterized by either an effective coupled $U(1)\otimes U(1)$ quantum rotor Hamiltonian in a large-N limit or a coupled XXZ $\otimes$ XXZ spin Hamiltonian in a single-orbit limit.

cond-mat.str-el↗

arXiv2005

Coarse-graining protein energetics in sequence variables

We show that cluster expansions (CE), previously used to model solid-state materials with binary or ternary configurational disorder, can be extended to the protein design problem. We present a generalized CE framework, in which properties such as energy can be unambiguously expanded in the amino-acid sequence space. The CE coarse grains over nonsequence degrees of freedom (e.g., side-chain conformations) and thereby simplifies the problem of designing proteins, or predicting the compatibility of a sequence with a given structure, by many orders of magnitude. The CE is physically transparent, and can be evaluated through linear regression on the energies of training sequences. We show, as example, that good prediction accuracy is obtained with up to pairwise interactions for a coiled-coil backbone, and that triplet interactions are important in the energetics of a more globular zinc-finger backbone.

physics.bio-ph↗

arXiv2005

Mott states under the influence of fermion-boson conversion: invasion of superfluidity

I study the influence of fermion-boson conversion near Feshbach resonances on Mott states of Cooper pairs and demonstrate possible invasion of superfluidity. The quantum dynamics of Fermi-Bose gases is studied using both an effective coupled $U(1)\otimes U(1)$ quantum rotor Hamiltonian and a coupled XXZ $\otimes$ XXZ spin Hamiltonian. I also point out two distinct branches of collective modes in superfluid states, one of which involves anti-symmetric phase oscillations in fermionic and bosonic channels and is {\em always} gapped because of fermion-boson conversion.

cond-mat.other↗

arXiv2005

Magnetically Stabilized Order. II: Critical states and algebraically ordered nematic spin liquids in one-dimensional optical lattices

We investigate the Zeeman-field-driven quantum phase transitions between singlet spin liquids and algebraically ordered O(2) nematic spin liquids of spin-one bosons in one-dimensional optical lattices. We find that the critical behavior is characterized by condensation of hardcore bosons instead of ideal magnons in high dimensional lattices. Critical exponents are strongly renormalized by hardcore interactions and critical states are equivalent to the free Fermion model up to the Friedal oscillations. We also find that the algebraically ordered nematic spin liquids close to critical points are fully characterized by the Luttinger liquid dynamics with Luttinger liquid parameters magnetically tunable. The Bethe Ansatz solution has been applied to determine the critical magnetization and nematic correlations.

cond-mat.stat-mech↗

arXiv2005

The Li intercalation potential of LiMPO4 and LiMSiO4 olivines with M = Fe, Mn, Co, Ni

The Li intercalation potential of LiMPO4 and LiMSiO4 compounds with M = Fe, Mn, Co, and Ni is computed with the GGA+U method. It is found that this approach is considerably more accurate than standard LDA or GGA methods. The calculated potentials for LiFePO4, LiMnPO4 and LiCoOPO4 agree to within 0.1 V with experimental results. The LiNiPO4 potential is predicted to be above 5 V. The potentials of the silicate materials are all found to be rather high, but LiFeSiO4 and LiCoSiO4 have negligible volume change upon Li extraction.

cond-mat.mtrl-sci↗

arXiv2005

The electronic structure and band gap of LiFePO4 and LiMnPO4

Materials with the olivine LixMPO4 structure form an important new class of materials for rechargeable Li batteries. There is significant interest in their electronic properties because of the importance of electronic conductivity in batteries for high rate applications. The density of states of LixMPO4 (x = 0, 1 and M = Fe, Mn) has been determined with the ab initio GGA+U method, appropriate for these correlated electron systems. Computed results are compared with the optical gap of LiFePO4, as measured using UV-Vis-NIR diffuse reflectance spectroscopy. The results obtained from experiment (3.8-4.0 eV) and GGA+U computations (3.7 eV) are in very good agreement. However, standard GGA, without the same level of treatment of electron correlation, is shown to make large errors in predicting the electronic structure. It is argued that olivines are likely to be polaronic conductors with extrinsically determined carrier levels and that their electronic conductivity is therefore not simply related to the band gap.

cond-mat.mtrl-sci↗

arXiv2005

First-principles prediction of redox potentials in transition-metal compounds with LDA+U

First-principles calculations within the Local Density Approximation (LDA) or Generalized Gradient Approximation (GGA), though very successful, are known to underestimate redox potentials, such as those at which lithium intercalates in transition metal compounds. We argue that this inaccuracy is related to the lack of cancellation of electron self-interaction errors in LDA/GGA and can be improved by using the DFT+$U$ method with a self-consistent evaluation of the $U$ parameter. We show that, using this approach, the experimental lithium intercalation voltages of a number of transition metal compounds, including the olivine Li$_{x}$MPO$_{4}$ (M=Mn, Fe Co, Ni), layered Li$_{x}$MO$_{2}$ ($x=$Co, Ni) and spinel-like Li$_{x}$M$_{2}$O$_{4}$ (M=Mn, Co), can be reproduced accurately.

cond-mat.mtrl-sci↗

arXiv2005

Emergent Geometric Hamiltonian and Insulator-Superfluid Phase Transitions

I argue that certain bosonic insulator-superfluid phase transitions as an interaction constant varies are driven by emergent geometric properties of insulating states. The {\em renormalized} chemical potential and distribution of disordered bosons define the geometric aspect of an effective low energy Hamiltonian which I employ to study various resonating states and quantum phase transitions. In a mean field approximation, I also demonstrate that the quantum phase transitions are in the universality class of a percolation problem.

cond-mat.dis-nn↗

arXiv2004

Topological spin pumps I

We have established a semiclassical kinetic approach for various spin correlated pumping phenomena incorporating spin rotation in wave functions into transport equations. We employ this technique to study topological pumps and illustrate spin pumping in a few models where various spin configurations or topological motors drive adiabatic pumps. In the Rashba model we find that a topological spin pump is driven by a meron with positive one half skyrmion charge, the size of which can be controlled by external gates or Zeeman fields. In the Dresselhaus model on the other hand, electron spins are pumped out by a negative meron. We examine effects of Zeeman fields on topological spin pumping and responses of fermi seas in various pumps. The phenomena of topological pumping are attributed to topological Stern-Gerlach splitting and occur in a transverse direction along which charge pumping currents either vanish or are negligible. The transport equations established here might also be applied for the studies of anomalous Hall effect and spin Hall effect as demonstrated in one of appendices.

cond-mat.mes-hall↗

arXiv2004

Microscopic Wave Functions of Spin Singlet and Nematic Mott States of Spin-One Bosons in High Dimensional Bipartite Lattices

We present microscopic wave functions of spin singlet Mott insulating states and nematic Mott insulating states. We also investigate quantum phase transitions between the spin singlet Mott phase and the nematic Mott phase in both large-N limit and small-N limit (N being the number of particles per site) in high dimensional bipartite lattices. In the mean field approximation employed in this article we find that phase transitions are generally weakly first order.

cond-mat↗

arXiv2003

Berry's Phases of Ground States of Interacting Spin-One Bosons: Chains of Monopoles and Monosegments

We study Berry's connection potentials of many-body ground states of spin-one bosons with antiferromagnetic interactions in adiabatically varying magnetic fields. We find that Berry's connection potentials are generally determined by, instead of usual singular monopoles, linearly positioned monosegments each of which carries one unit of topological charge; in the absence of a magnetic field gradient this distribution of monosegments becomes a linear chain of monopoles. Consequently, Berry's phases consist of a series of step functions of magnetic fields; a magnetic field gradient causes rounding of these step-functions. We also calculate Berry's connection fields, profiles of monosegments and show that the total topological charge is conserved in a parameter space.

cond-mat↗