arXiv subjects

Fei Zhou

Publications and source records attributed to Fei Zhou.

At least 109 records · Page 6Linked to original sources

arXiv2020

Tricritical physics in two-dimensional $p$-wave superfluids

We study effects of quantum fluctuations on two-dimensional $p+ip$ superfluids near resonance. In the standard paradigm, phase transitions between superfluids and zero density vacuum are continuous. When strong quantum fluctuations near resonance are taken into account, the line of continuous phase transitions terminates at two multicritical points near resonance, between which the transitions are expected to be first-order ones. The size of the window where first-order phase transitions occur is shown to be substantial when the coupling is strong. Near first-order transitions, superfluids self-contract due to phase separations between superfluids and vacuum.

cond-mat.quant-gas↗

arXiv2019

Compressive sensing lattice dynamics. II. Efficient phonon calculations and long-range interactions

We apply the compressive sensing lattice dynamics (CSLD) method to calculate phonon dispersion for crystalline solids. While existing methods such as frozen phonon, small displacement, and linear response are routinely applied for phonon calculations, they are considerable more expensive or cumbersome to apply to certain solids, including structures with large unit cells or low symmetry, systems that require more expensive electronic structure treatment, and polar semiconductors/insulators. In the latter case, we propose an approach based on a corrected long-range force constant model with proper treatment of the acoustic sum rule and the symmetric on-site force constant matrix. Our approach is demonstrated to be accurate and efficient for these systems through case studies of NaCl, CeO$_2$, Y$_3$Al$_5$O$_{12}$ and La$_2$Fe$_{14}$B.

cond-mat.mtrl-sci↗

arXiv2019

Compressive sensing lattice dynamics. I. General formalism

{\it Ab initio\} calculations have been successfully used for evaluating lattice dynamical properties of solids within the (quasi-)harmonic approximation (i.e., assuming non-interacting phonons with infinite lifetimes), but it remains difficult to treat anharmonicity in all but the simplest compounds. We detail a systematic information theory based approach to deriving {\it ab initio\} anharmonic force constants: compressive sensing lattice dynamics (CSLD). The non-negligible terms that are necessary to reproduce the first-principles calculated interatomic forces are automatically selected by minimizing the $\ell_1$ norm (sum of absolute values) of the scaled force constants. By using efficient sampling of the configuration space using a modest number of atomic configurations with quasi-random displacements, CSLD is well suited for deriving accurate anharmonic potentials for complex multicomponent compounds with large unit cells. We demonstrate the power and generality of CSLD by calculating the phonon lifetimes and thermal transport properties of Type-I Si clathrates.

physics.comp-ph↗

arXiv2019

Spectral Regularization for Combating Mode Collapse in GANs

Despite excellent progress in recent years, mode collapse remains a major unsolved problem in generative adversarial networks (GANs).In this paper, we present spectral regularization for GANs (SR-GANs), a new and robust method for combating the mode collapse problem in GANs. Theoretical analysis shows that the optimal solution to the discriminator has a strong relationship to the spectral distributions of the weight matrix.Therefore, we monitor the spectral distribution in the discriminator of spectral normalized GANs (SN-GANs), and discover a phenomenon which we refer to as spectral collapse, where a large number of singular values of the weight matrices drop dramatically when mode collapse occurs. We show that there are strong evidence linking mode collapse to spectral collapse; and based on this link, we set out to tackle spectral collapse as a surrogate of mode collapse. We have developed a spectral regularization method where we compensate the spectral distributions of the weight matrices to prevent them from collapsing, which in turn successfully prevents mode collapse in GANs. We provide theoretical explanations for why SR-GANs are more stable and can provide better performances than SN-GANs. We also present extensive experimental results and analysis to show that SR-GANs not only always outperform SN-GANs but also always succeed in combating mode collapse where SN-GANs fail. The code is available at https://github.com/max-liu-112/SRGANs-Spectral-Regularization-GANs-.

cs.LG↗

arXiv2019

Semi-parametric Bayesian variable selection for gene-environment interactions

Many complex diseases are known to be affected by the interactions between genetic variants and environmental exposures beyond the main genetic and environmental effects. Study of gene-environment (G$\times$E) interactions is important for elucidating the disease etiology. Existing Bayesian methods for G$\times$E interaction studies are challenged by the high-dimensional nature of the study and the complexity of environmental influences. Many studies have shown the advantages of penalization methods in detecting G$\times$E interactions in "large p, small n" settings. However, Bayesian variable selection, which can provide fresh insight into G$\times$E study, has not been widely examined. We propose a novel and powerful semi-parametric Bayesian variable selection model that can investigate linear and nonlinear G$\times$E interactions simultaneously. Furthermore, the proposed method can conduct structural identification by distinguishing nonlinear interactions from main-effects-only case within the Bayesian framework. Spike and slab priors are incorporated on both individual and group levels to identify the sparse main and interaction effects. The proposed method conducts Bayesian variable selection more efficiently than existing methods. Simulation shows that the proposed model outperforms competing alternatives in terms of both identification and prediction. The proposed Bayesian method leads to the identification of main and interaction effects with important implications in a high-throughput profiling study with high-dimensional SNP data.

stat.ME↗

arXiv2019

Cusps near Topological Phase Transitions: Signatures of Majorana fermions and interactions with fluctuations

In this article, we study non-analyticities or cusps near a topological phase transition driven by changes of global topologies rather than by the formation of conventional (local) order. The particular phase transitions between topologically ordered superfluids and non-topological superfluids studied in this article are always accompanied by the emergence or disappearance of Majorana boundary modes, while at the critical point these boundary modes are liberated and evolve into the bulk. Proliferation of massless Majorana fermions at the transition leaves unique thermodynamic signatures. For a very broad class of isotropic superfluids or superconductors (with or without time reversal symmetry), the resultant transitions in $d$ spatial dimensions will be of $(d+1)$th order. The $(d+1)$th order derivatives of the grand potential are either discontinuous in even dimensions or logarithmically divergent in odd dimensions. Bulk quantities such as compressibility can develop a cusp along with the change of topology of the superfluids. We compute quantitatively the cusp structures and further investigate the effect of interactions induced by massive fluctuation fields. These cusps are direct manifestations of Majorana fermions and their interactions, and can be potentially considered as an alternative approach for detecting these puzzling particles.

cond-mat.str-el↗

arXiv2019

Quantum Many-Body Conformal Dynamics: Symmetries, Geometry, Conformal Tower States, and Entropy Production

In this article we study the quench dynamics of Galilean and scale invariant many-body systems which can be prepared using interacting atomic gases. The far-away from equilibrium dynamics are investigated by employing $m$-body density matrices, which are most conveniently defined in terms of a special basis - the conformal tower states. We explicitly illustrate that, although during the initial stage of the dynamics all symmetries can be broken and absent in the unitary evolution because of the initialization of the state, there is always an emergent conformal symmetry in the long time limit. The emergence of this dynamic conformal symmetry is robust, and always occurs; it uniquely defines the characteristics of the asymptotic dynamics at a scale invariant fixed point. As an immediate application of the asymptotic dynamics of the microscopic density matrices, we have focused on the effects of this emergent conformal symmetry on two observables: the moment of inertia tensor, $I_{ij}(t)$, $i,j=x,y,z$, and the entropy density field, $S({\bf r}, t)$, in the hydrodynamic flow of strongly interacting particles. We show that the long time behaviour of these observables is completely set by conformal symmetry, while the leading long time corrections depend on interference effects between different conformal tower states. The emergent conformal symmetry naturally leads to entropy conservation, and {\em conformal cooling}, an energy conserving cooling of a strongly interacting gas during free expansion. When the interaction Hamiltonian breaks the scale symmetry, we further demonstrate that there is a direct cause-effect relation between conformal symmetry breaking in the long time limit, and a non-vanishing entropy production. This suggests that the entropy production rate is a natural {\em parameter} for categorizing the breaking of conformal symmetry.

cond-mat.quant-gas↗

arXiv2019

Achieving continuously tunable critical exponents for long-range spin systems simulated with trapped ions

Quantum phase transitions are usually classified into discrete universality classes that typically only depend on symmetries and spatial dimensionalities. In this Letter, we demonstrate an opportunity to continuously vary the critical exponents or universalities by tuning experimental parameters in a given physical system. Particularly, we show that critical exponents in long-range spin systems simulated in ion traps can be easily tuned with laser detuning. We suggest that such experiments also effectively simulate some aspects of critical phenomena in conventional spin systems but in artificial non-integer spatial dimensions.

quant-ph↗

arXiv2018

Non-Perturbative Dynamical Effects in Nearly Scale Invariant Systems: The Action of Breaking Scale Invariance

In this work we develop a general formalism that categorizes the action of broken scale invariance on the non-equilibrium dynamics of non-relativistic quantum systems. This approach is equally applicable to both strongly and weakly interacting systems. We show that any small deviation from the strongly interacting fixed point, in three spatial dimensions, leads to non-pertubative effects in the long time dynamics, dramatically altering the dynamics observed at the scale invariant fixed point. As a concrete example, we apply this approach to the non-equilibrium dynamics for the interacting two-body problem, and for a non-interacting quantum gas in the presence of an impurity, both in three spatial dimensions. Slightly away from the resonantly-interacting scale invariant fixed point, we show that the dynamics are altered by a non-perturbative log-periodic beat. The presence of the beat depends only on deviating from the resonant fixed point, while the frequency depends on the microscopic parameters of the system.

cond-mat.quant-gas↗

arXiv2018

Dynamical violation of scale invariance and the dilaton in a cold Fermi gas

We use the large N approximation to find an exotic phase of a cold, two-dimensional, N-component Fermi gas which exhibits dynamically broken approximate scale symmetry. We identify a particular weakly damped collective excitation as the dilaton, the pseudo-Goldstone boson associated with the broken approximate scale symmetry. We argue that the symmetry breaking phase is stable for a range of parameters of the theory and there is a fluctuation induced first order quantum phase transition between the normal and the scale symmetry breaking phases which can be driven by tuning the chemical potential. We find that the compressibility of the gas at its lower critical density is anomalously large, 2N times that of a perfect gas at the same density.

cond-mat.quant-gas↗

arXiv2018

Two-dimensional Fermi gases near a p-wave resonance: effect of quantum fluctuations

We study the stability of p-wave superfluidity against quantum fluctuations in two-dimensional Fermi gases near a p-wave Feshbach resonance . An analysis is carried out in the limit when the interchannel coupling is strong. By investigating the effective potential for the pairing field via the standard loop expansion, we show that a homogeneous p-wave pairing state becomes unstable when two-loop quantum fluctuations are taken into account. This is in contrast to the previously predicted $p + ip$ supefluid in the weak-coupling limit [V. Gurarie et al., Phys. Rev. Lett. 94, 230403 (2005)]. It implies a possible onset of instability at certain intermediate interchannel coupling strength. Alternatively, the instability can also be driven by lowering the particle density. We also discuss the validity of our analysis.

cond-mat.quant-gas↗

arXiv2018

A unified treatment of derivative discontinuity, delocalization and static correlation effects in density functional calculations

We propose a method that incorporates explicit derivative discontinuity of the total energy with respect to the number of electrons and treats both delocalization and static correlation effects in density functional calculations. Our approach is motivated by the exact behavior of the ground state total energy of electrons and involves minimization of the exchange-correlation energy with respect to the Fock space density matrix. The resulting density matrix minimization (DMM) model is simple to implement and can be solved uniquely and efficiently. In a case study of KCuF$_3$, a prototypical Mott-insulator with strong correlation, LDA+DMM correctly reproduced the Mott-Hubbard gap, magnetic ordering and Jahn-Teller distortion.

cond-mat.str-el↗

arXiv2018

Integrating quantum key distribution with classical communications in backbone fiber network

Quantum key distribution (QKD) provides information-theoretic security based on the laws of quantum mechanics. The desire to reduce costs and increase robustness in real-world applications has motivated the study of coexistence between QKD and intense classical data traffic in a single fiber. Previous works on coexistence in metropolitan areas have used wavelength-division multiplexing, however, coexistence in backbone fiber networks remains a great experimental challenge, as Tbps data of up to 20 dBm optical power is transferred, and much more noise is generated for QKD. Here we present for the first time, to the best of our knowledge, the integration of QKD with a commercial backbone network of 3.6 Tbps classical data at 21 dBm launch power over 66 km fiber. With 20 GHz pass-band filtering and large effective core area fibers, real-time secure key rates can reach 4.5 kbps and 5.1 kbps for co-propagation and counter-propagation at the maximum launch power, respectively. This demonstrates feasibility and represents an important step towards building a quantum network that coexists with the current backbone fiber infrastructure of classical communications.

quant-ph↗

arXiv2017

Quantum criticality of bosonic systems with the Lifshitz dispersion

We study the quantum criticality of the Lifshitz $φ^4$-theory below the upper critical dimension. Two fixed points, one Gaussian and the other non-Gaussian, are identified with zero and finite interaction strengths, respectively. At zero temperature the particle density exhibits different power-law dependences on the chemical potential in the weak and strong interaction regions. At finite temperatures, critical behaviors in the quantum disordered region are mainly controlled by the chemical potential. In contrast, in the quantum critical region critical scalings are determined by temperature. The scaling ansatz remains valid in the strong interaction limit for the chemical potential, correlation length, and particle density, while it breaks down in the weak interaction one. As approaching the upper critical dimension, physical quantities develop logarithmic dependence on dimensionality in the strong interaction region. These results are applied to spin-orbit coupled bosonic systems, leading to predictions testable by future experiments.

cond-mat.str-el↗

arXiv2016

Universal scaling of the electronic and the elastic energies of small polarons revealed by high-throughput first-principles calculations

Formation of self-trapped holes (STH) in a comprehensive list of scintillator materials, including halides and chalcogenides, are studied using an accurate and computationally efficient first-principles method, the polaron self-interaction correction (pSIC). The key characteristics of small hole polarons, including their geometries, energies and degree of localization, are found vastly different from halides to oxides to systems with open-shell cations. Nevertheless, we find a universal linear relation between the energy gap separating the bound hole level from the valence band maximum and the elastic energy associated with the lattice displacement field that accompanies the polaron.

cond-mat.mtrl-sci↗

arXiv2016

Integrated four-channel all-fiber up-conversion single-photon-detector with adjustable efficiency and dark count

Up-conversion single photon detector (UCSPD) has been widely used in many research fields including quantum key distribution (QKD), lidar, optical time domain reflectrometry (OTDR) and deep space communication. For the first time in laboratory, we have developed an integrated four-channel all-fiber UCSPD which can work in both free-running and gate modes. This compact module can satisfy different experimental demands with adjustable detection efficiency and dark count. We have characterized the key parameters of the UCSPD system.

physics.ins-det↗

arXiv2016

Scale Invariant Dynamics and Universal Quantum Beats in Bose Gases

We study the signature of scale invariance in the far-from-equilibrium quantum dynamics of two dimensional Bose gases. We show that the density profile displays a scale invariant logarithmic singularity near the center. In addition, the density oscillates due to quantum beats with universal structures. Namely, the frequencies of the beats can be connected with one another by a universal discrete scale transformation induced by the scale invariance. The experimental applicability of these results is then discussed.

cond-mat.quant-gas↗

arXiv2016

Ab-initio prediction of fast non-equilibrium transport of nascent polarons in SrI$_2$: A key to high-performance scintillation

The excellent light yield proportionality of europium-doped strontium iodide (SrI$_2$:Eu) has resulted in state-of-the-art $γ$-ray detectors with remarkably high energy resolution, far exceeding that of most halide compounds. In this class of materials the formation of self-trapped hole polarons is very common. However, polaron formation is usually expected to limit carrier mobilities and has been associated with poor scintillator light-yield proportionality and resolution. Here, using a recently developed first-principles method, we perform an unprecedented study of polaron transport in SrI$_2$, both for equilibrium polarons, as well as nascent polarons immediately following a self-trapping event. As a result, we put forward a rationale for the unexpected high energy resolution of SrI$_2$. We identify nine stable hole polaron configurations, which consist of dimerized iodine pairs with polaron binding energies of up to 0.5\,eV. They are connected by a complex potential energy landscape that comprises 66 unique nearest-neighbor migration paths. \textit{Ab initio} molecular dynamics simulations reveal that a large fraction of polarons is born into configurations that migrate practically barrier free at room temperature. As a result, carriers created during $γ$-irradiation can quickly diffuse away reducing the chance for non-linear recombination reactions that are the primary culprits for non-proportionality and resolution reduction. This leads to the conclusion that the flat, albeit complex, landscape for polaron migration in SrI$_2$ is key for understanding its outstanding performance. This insight provides important guidance not only for the future development of high-performance scintillators but also of other materials, for which large polaron mobilities are crucial such as batteries and solid state ionic conductors.

cond-mat.mtrl-sci↗