arXiv subjects

Chen-How Huang

Publications and source records attributed to Chen-How Huang.

15 recordsLinked to original sources

arXiv2026

Symmetry Classification of Non-Reciprocal Responses in Multiterminal Ring Devices

We present a symmetry-based framework to classify the non-reciprocal responses of multiterminal ring quantum devices. The device is modeled as a ring of $n$ vertices, where a binary variable $e_k\in\{+1,-1\}$ on each bond encodes the preferred direction of signal flow between terminals. Non-reciprocity corresponds to a preferred current configuration on the ring, and the symmetry group of the device partitions all $2^n$ configurations into equivalence classes(orbits) characterized by a topological winding number $W$. Using the minimal non-trivial case $n=3$, we establish two results independent of microscopic details. First, lifting the degeneracy within an orbit generates non-reciprocal responses. For $n=3$ this requires simultaneous breaking of both time-reversal $T$ and spatial inversion $I$. Breaking either alone is insufficient. Second, the residual geometry symmetry after $T$ and $I$ are broken determines which responses are observable. For an isosceles triangular geometry, only two types of response are allowed: uniform circulation (all bonds carrying current in the same direction) and semi-circulation with the reversed bond on the geometrically distinct base. Semi-circulation with the reversed bond on either equal leg is symmetry-forbidden. Both predictions are validated using a minimal toy model of three quantum dots coupled to superconducting baths, which demonstrates a reactive quantum circulator response.

cond-mat.mes-hall↗

arXiv2026

Mapping the Stability of Spin Qubits in Superconducting Pseudogap Systems

Superconducting spin qubits, realized as Yu-Shiba-Rusinov spin-doublet states in quantum-dot-superconductor systems, represent a cornerstone of current research in quantum technologies. We analyze these ground states of quantum impurities in superconducting pseudogap systems, namely systems with a pseudogap tunneling density of states $\rho(\epsilon) \sim |\epsilon|^r$ for energies $|\epsilon|\gg \Delta$ ($\Delta$ being a $s$-wave pairing potential). For $r=1$, these hosts are realized as Dirac materials (graphene or 3D topological insulator surfaces) in proximity to conventional superconductors, or as $d+i s$ superconductors. Using effective field theory and numerical renormalization group, we map the phase diagram against the pseudogap exponent $r > 0$ and particle-hole symmetry-breaking perturbations. At particle-hole symmetry, increasing $r$ also increases the critical value, $J_c$, of the Kondo coupling that triggers the transition from spin doublet to singlet. Unlike the gapless pseudogap Kondo systems, numerical and analytical evidence suggest that Andreev reflection stabilizes a singlet ground state at $J$ for all $r > 0$. Breaking particle-hole symmetry -- by potential scattering or chemical potential -- eventually restores the transition at lower $J_c$. Our results indicate that coupling to superconducting hosts with large pseudogap exponents enhances the stability of spin qubits at large Kondo coupling.

cond-mat.mes-hall↗

arXiv2026

Ground States and Excitations of Magnetic Impurities in Pseudogap Superconducting Systems

Combining effective field theory and numerical renormalization-group (NRG), we study the ground-state phase diagram and single-particle excitations of a spin-$\tfrac{1}{2}$ impurity in a superconducting system with a tunneling density of states behaving as $\rho(\epsilon) \sim |\epsilon|^{r}$, for $|\epsilon|\gg \Delta$ ($\Delta$ being the $s$-wave pairing potential). We focus on the properties of the doublet-singlet transition at large Kondo coupling. The effective field theory for the singlet phase is inferred from a strong coupling expansion in the Kondo coupling. For $\Delta \neq 0$, it contains a local pairing term which drives the system into a spin-singlet phase with enhanced paring correlations. We study how the singet-doublet phase boundary is affected by particle-hole symmetry breaking perturbations such as a scattering potential and/or the chemical potential. Results for the $T$-matrix spectral function are also reported near the transition both at particle-hole symmetry and away from it. It is shown that the singlet-doublet transition can be induced by the chemical potential rather than the Kondo coupling strength. At particle-hole symmetry, a resonance-like feature is observed for $r= 1$ and related to a two-quasiparticle excitation using a single-site model which is derived from effective field theory.

cond-mat.mes-hall↗

arXiv2026

Who can compete with quantum computers? Lecture notes on quantum inspired tensor networks computational techniques

This is a set of lectures on tensor networks with a strong emphasis on the core algorithms involving Matrix Product States (MPS) and Matrix Product Operators (MPO). Compared to other presentations, particular care has been given to disentangle aspects of tensor networks from the quantum many-body problem: MPO/MPS algorithms are presented as a way to deal with linear algebra on extremely (exponentially) large matrices and vectors, regardless of any particular application. The lectures include well-known algorithms to find eigenvectors of MPOs (the celebrated DMRG), solve linear problems, and recent learning algorithms that allow one to map a known function into an MPS (the Tensor Cross Interpolation, or TCI, algorithm). The lectures end with a discussion of how to represent functions and perform calculus with tensor networks using the "quantics" representation. They include the detailed analytical construction of important MPOs such as those for differentiation, indefinite integration, convolution, and the quantum Fourier transform. Three concrete applications are discussed in detail: the simulation of a quantum computer (either exactly or with compression), the simulation of a quantum annealer, and techniques to solve partial differential equations (e.g. Poisson, diffusion, or Gross-Pitaevskii) within the "quantics" representation. The lectures have been designed to be accessible to a first-year PhD student and include detailed proofs of all statements.

quant-ph↗

arXiv2025

Superconducting Spin-Singlet QuBit in a Triangulene Spin Chain

Chains of triangular nanographene (triangulene), recently identified as realizing the valence-bond solid phase of a spin-1 chain, offer a promising platform for quantum information processing. We propose a spin-singlet qubit based on these chains grown on a superconducting substrate. Using the numerical renormalization group (NRG), we identify a manifold consisting of the two lowest-lying, spin-singlet states isolated from doublet states of opposite fermion parity, which undergo an avoided crossing. A qubit utilizing these states is thus protected from random Zeeman and/or spin-orbit coupling. Despite the unavoidable effect of quasiparticle poisoning on qubit performance, the isolation of the singlet states offers additional protection. In addition, we introduce a mesoscopic device architecture, based on a triple quantum dot coupled to a superconducting junction, that quantum simulates the spin chain and enables control and readout of the qubit. An effective two-level description of the device is validated using time-dependent NRG.

cond-mat.mes-hall↗

arXiv2024

A Large-$N$ Approach to Magnetic Impurities in Superconductors

Quantum spin impurities coupled to superconductors are under intense investigation for their relevance to fundamental research as well as the prospects to engineer novel quantum phases of matter. Here we develop a large-$N$ mean-field theory of a strongly coupled spin-$\tfrac{1}{2}$ quantum impurity in a conventional $s$-wave superconductor. The approach is benchmarked against Wilson's numerical renormalization group (NRG). While the large-$N$ method is not applicable in the weak-coupling regime where the Kondo temperature $T_K$ is smaller than the superconducting gap $\Delta$, it performs very well in the strong coupling regime where $T_K \gtrsim \Delta$, thus allowing to obtain a reasonably accurate description of experimentally relevant quantities. The latter includes the energy of the Yu-Shiba-Rusinov subgap states, their spectral weight, as well as the local density of continuum states. The method provides a reliable analytical tool that complements other perturbative and non-perturbative methods, and can be extended to more complex impurity models for which NRG may be not easily applicable.

cond-mat.mes-hall↗

arXiv2024

Competition of Exchange and Correlation Energies in Two-Dimensional $N$-component Electron Gas Ferromagnetism

Motivated by recent observations of symmtry broken phases in lightly-doped multilayer graphene, we investigate magnetic phase transitions in a generalized electron gas model with four-component electron spin. This model simplifies the problem with a parabolic dispersion band, abstracting away the details of the graphene band structure to focus solely on the effects of the Coulomb interaction. We report four findings: 1) In the Hartree-Fock approximation, we observe that the paramagnetic state undergoes a sequence of density-driven, first-order phase transitions, progressively depopulating electrons from each spin component until achieving complete polarization within a very narrow density window where $1.2<r_s<2$ ($r_s$ being the electron gas parameter). 2) Further incorporating the correlation energy via the Bohm-Pines random-phase approximation shows that the cascade of transitions obtained within Hartree-Fock approximation is replaced by a single ferromagnetic phase transition at $r_s = 6.12$. 3) The disappearance of cascade is due to the correlation energy difference between the four-component paramagnetic state and symmetry-broken phases, which is nearly an order of magnitude more negative than the corresponding Hartree-Fock energy difference for $1.2 < r_s < 2$. 4) The transition from the paramagnetic state to the fully polarized state at $r_s=6.12$ is governed by the balance between exchange and correlation energies, a competition that cannot be captured by mean-field approximations to models featuring effective (density-dependent) delta-function interactions, such as the Stoner model. We use the insights from our model to comment on the phase diagram of multilayer graphene electron gas.

cond-mat.str-el↗

arXiv2024

A Microscopic Description for Two Body Loss in Cold Atoms Near Feshbach Resonances with Strong Spontaneous Emission

We study the two body loss dynamics of fermionic cold atoms near $s$- and $p$-wave Feshbach resonances with a microscopic Keldysh path integral formalism and compare the result to the macroscopic phenomenological loss rate equation. The microscopic loss rate equation is an integral-differential equation of the momentum distribution that depends on the functional form of the loss rate coefficient. For $s$-wave resonance, the microscopic theory yields the same result as the phenomenological equation. However, the calculation of $p$-wave resonance shows a discrepancy between the two descriptions for an quantum-degenerate gas. This discrepancy originates from the functional form of the loss coefficient which is associated with the microscopic loss mechanism of the two body loss and is neglected in the phenomenological equations where the coefficient is typically a constant. We find the discrepancy between the microscopic theory and the phenomenological description is smeared by thermal average at high temperature, $T\gtrsim T_F$ where $T_F$ is the Fermi temperature.

cond-mat.quant-gas↗

arXiv2024

Probing Magnetic and Triplet Correlations in Spin-Split Superconductors with Magnetic Impurities

A superconductor (SC) in proximity to a ferromagnetic insulator (FMI) is predicted to exhibit mixed singlet and triplet pair correlations. The magnetic proximity effect of FMI spin-splits the energy of Bogoliubov excitations and leads to a spin polarization at the surface for superconducting films thinner than the superconducting coherence length. In this work, we study manifestations of these phenomena in the properties of a magnetic impurity coupled via Kondo coupling to this FMI/SC system. Using the numerical renormalization group (NRG) method, we compute the properties of the ground state and low-lying excited states of a model that incorporates the Kondo interaction and a Ruderman-Kittel-Kasuya-Yosida (RKKY)-like interaction with the surface spin polarization. Our main finding is an energy splitting of the lowest even fermion-parity states caused by the proximity to the FMI. As the Kondo coupling increases, the splitting grows and saturates to a universal value equal to twice the exchange field of the FMI. We introduce a two-site model that can be solved analytically and provides a qualitative understanding of this and other NRG results. In addition, using perturbation theory we demonstrate that the mechanism behind the splitting involves the RKKY field and the triplet correlations of the spin-split superconductor. A scaling analysis combined with NRG shows that the splitting can be written as a single-parameter scaling function of the ratio of the Kondo temperature and the superconducting gap, which is also numerically obtained.

cond-mat.mes-hall↗

arXiv2023

Modeling Particle Loss in Open Systems using Keldysh Path Integral and Second Order Cumulant Expansion

For open quantum systems, integration of the bath degrees of freedom using the second order cumulant expansion in the Keldysh path integral provides an alternative derivation of the effective action for systems coupled to general baths. The baths can be interacting and not necessarily Markovian. Using this method in the Markovian limit, we compute the particle loss dynamics in various models of ultra-cold atomic gases including a one-dimensional Bose-Hubbard model with two-particle losses and a multi-component Fermi gas with interactions tuned by an optical Feshbach resonance. We explicitly demonstrate that the limit of strong two-body losses can be treated by formulating an indirect loss scheme to describe the bath-system coupling. The particle-loss dynamics thus obtained is valid at all temperatures. For the one-dimensional Bose-Hubbard model, we compare it to solutions of the phenomenological rate equations. The latter are shown to be accurate at high temperatures.

cond-mat.quant-gas↗

arXiv2023

Itinerant Ferromagnetism in SU(N)-Symmetric Fermi Gases at Finite Temperature: First Order Phase Transitions and Time-Reversal Symmetry

At temperatures well below the Fermi temperature $T_F$, the coupling of magnetic fluctuations to particle-hole excitations in a two-component Fermi gas makes the transition to itinerant ferromagnetism a first order phase transition. This effect is not described by the paradigm of Landau's theory of phase transitions, which assumes the free energy is an analytic function of the order parameter and predicts a second order phase transition. On the other hand, despite that larger symmetry often introduces larger degeneracies in the low-lying states, here we show that for a Fermi gas with SU($N > 2$)-symmetry in three space dimensions the ferromangetic phase transition is first order in agreement with the predictions of Landau's theory [M. A. Cazalilla \emph{et al}. New J. of Phys. {\bf 11} 103033 (2009)]. By performing unrestricted Hartree-Fock calculations for an SU($N > 2$)-symmetric Fermi gas with short range interactions, we find the order parameter undergoes a finite jump across the transition. In addition, we do not observe any tri-critical point up to temperatures $T \simeq 0.5\: T_F$, for which the thermal smearing of the Fermi surface is subtantial. Going beyond mean-field, we find that the coupling of magnetic fluctuations to particle-hole excitations makes the transition more abrupt and further enhances the tendency of the gas to become fully polarized for smaller values of $N$ and the gas parameter $k_F a_s$. In our study, we also clarify the role of time reversal symmetry in the microscopic Hamiltonian and obtain the temperature dependence of Tan's contact. For the latter, the presence of the tri-critical point for $N = 2$ leads to a more pronounced temperature dependence around the transition than for SU($N > 2$)-symmetric gases.

cond-mat.quant-gas↗

arXiv2021

Topological Lifshitz Transitions, Orbital Currents, and Interactions in Low-dimensional Fermi Gases in Synthetic Gauge Fields

Low-dimensional systems of interacting fermions in a synthetic gauge field have been experimentally realized using two-component ultra-cold Fermi gases in optical lattices. Using a two-leg ladder model that is relevant to these experiments, we have studied the signatures of topological Lifshitz transitions and the effects of the inter-species interaction $U$ on the gauge-invariant orbital current in the regime of large intra-leg hopping $Ω$. Focusing on non-insulating regimes, we have carried out numerically exact density-matrix renormalization-group (DMRG) calculations to compute the orbital current at fixed particle number as a function of the interaction strength and the synthetic gauge flux per plaquette. Signatures of topological Lifshitz transitions where the number Fermi points changes are found to persist even in the presence of very strong repulsive interactions. This numerical observation suggests that the orbital current can be captured by an appropriately renormalized mean-field band structure, which is also described here. Quantitative agreement between the mean-field and the DMRG results in the intermediate interaction regime where $U \lesssim Ω$ is demonstrated. We also have observed that interactions can change the sign of the current susceptibility at zero field and induce Lifshitz transitions between two metallic phases, which is also captured by the mean-field theory. Correlation effects beyond mean-field theory in the oscillations of the local inter-leg current are also reported. We argue that the observed robustness against interactions makes the orbital current a good indicator of the topological Lifshitz transitions.

cond-mat.quant-gas↗

arXiv2019

Suppression and Control of Pre-thermalization in Multi-component Fermi Gases Following a Quantum Quench

We investigate the mechanisms of control and suppression of pre-thermalization in $N$-component alkaline earth gases. To this end, we compute the short-time dynamics of the instantaneous momentum distribution and the relative population for different initial conditions after an interaction quench, accounting for the 11 peffect of initial interactions. We find that switching on an interaction that breaks the SU$(N)$ symmetry of the initial Hamiltonian, thus allowing for the occurrence of spin-changing collisions, does not necessarily lead to a suppression of pre-thermalization. However, the suppression will be most effective in the presence of SU$(N)$-breaking interactions provided the number of components $N \ge 4$ and the initial state contains a population imbalance that breaks the SU$(N)$ symmetry. We also find the conditions on the imbalance initial state that allow for a pre-thermal state to be stabilized for a certain time. Our study highlights the important role played by the initial state in the pre-thermalization dynamics of multicomponent Fermi gases. It also demonstrates that alkaline-earth Fermi gases provide an interesting playground for the study and control of pre-thermalization.

cond-mat.quant-gas↗

arXiv2019

Total Energy Dynamics and Asymptotics of the Momentum Distribution Following an Interaction Quench in a Two-component Fermi Gas

The absence of a characteristic momentum scale in the pseudo-potential description of atomic interaction in ultracold (two-component Fermi) gases is known to lead to divergence in perturbation theory. Here we show that they also plague the calculation of the dynamics of the total energy following a quantum quench. A procedure to remove the divergence is devised, which provides finite answers for the time-evolution of the total energy after a quench in which the interaction strength is ramped up according to an arbitrary protocol. An important result of this analysis is the time evolution of the asymptotic tail of the momentum distribution (related to Tan's contact) and the contact for a linear interaction ramp are obtained, as a function of the interaction ramp time in the crossover from the sudden quench to the adiabatic limit are reported. In sudden quench limit, the contact, following a rapid oscillation, reaches a stationary value which is different from the equilibrium one. In the adiabatic limit, the contact grows quadratically in time and later saturates to its equilibrium value for the final value of the scattering length.

cond-mat.quant-gas↗

arXiv2015

Measuring the second order correlation function and the coherence time using random phase modulation

A new approach to measure the second order correlation function $g^{(2)}$ and the coherence time was investigated. The $g^{(2)}$ was calculated from the photon pair time interval distribution by direct numerical self-convolution with the high order correction. The accuracy of this method was examined using an optical fiber based Hanbury-Brown-Twiss interferometer with a pseudo-thermal light source. We found that the significance of the high order correction is related to the factor $\bar{I}τ_{c}$, which is the overlapping of the photon wave packets. A novel technique was also demonstrated to measure the coherence time $τ_c$ of a light source using the random phase modulation. In comparison with the conventional self-heterodyne detection, this method is more suitable for a weak light source with a long coherence time.

physics.optics↗