arXiv subjects

Miguel A. Cazalilla

Publications and source records attributed to Miguel A. Cazalilla.

At least 19 recordsLinked to original sources

arXiv2026

Superconductor-"Metal" Transition of One-dimensional Interacting Bosons with Ohmic Quantum Dissipation

The phase diagram of a system of interacting bosons (Cooper pairs) hoping on a one-dimensional (1D) lattice with onsite phase dissipation describing the Josephson tunneling to a nearby diffusive normal-metal electrode is studied. Starting from the system at commensurate lattice filling, it is shown by a combination of analytical techniques that the phase diagram contains two quantum phases: A dissipative Bose-Einstein condensate (D-BEC) or superconductor with long-range phase coherence, and a dissipative Mott insulator (D-Mott) or "metal" with exponentially decaying phase correlations in space and local imaginary-time correlations decaying as the local pairing correlations of the electrode. The D-Mott/metal phase can be described as a 1D array of dissipative boson puddles, weakly coupled by Josephson tunneling. The puddle size roughly corresponds to the length scale beyond which phase slips suppress phase coherence. The dissipative time-dependent Ginsburg-Landau theory phenomenologically used by Sachdev, Werner, and Troyer [Phys. Rev. Lett. {\bf 92} 237003 (2004)] for the superconductor-metal transition in quasi-1D wires is derived from this microscopic puddle picture. Thus, the criticality of the D-Mott/D-BEC transition is shown to belong to the Wilson-Fisher universality class with dynamical exponent $z\approx 2$. At small doping, the D-Mott/metal phase remains stable due to its finite compressibility, which is computed to leading order in a perturbation expansion of the dissipation strength and the inter-puddle Josephson coupling. At larger doping, using a mapping to a pseudospin chain combined with bosonization, the D-BEC/superconductor phase is the ground state for non-vanishing but arbitrarily small dissipation. Similarities and differences with deconfinement transition of an array 1D bosonic Mott insulators in anisotropic optical lattices are also discussed.

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 $ρ(ε) \sim |ε|^r$ for energies $|ε|\gg Δ$ ($Δ$ 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 $ρ(ε) \sim |ε|^{r}$, for $|ε|\gg Δ$ ($Δ$ 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 $Δ\neq 0$, 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.

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arXiv2025

Phase-induced switching of ferromagnetic insulators in Josephson spin valves

We study the Josephson effect in junctions composed of two ferromagnetic insulator/diffusive superconductor bilayers separated by an insulating barrier. By computing the free energy of the system, we identify two distinct contributions: (i) The work performed by a current source to create a supercurrent through the junction, and (ii) an antiferromagnetic coupling between ferromagnetic insulators, mediated by the superconducting condensate across the insulating barrier. The competition between these contributions allows for switching between parallel and antiparallel configurations of the magnetizations of the ferromagnetic insulators. We explicitly show that the switching occurs at finite temperatures and for superconducting phase differences satisfying $π/2 < ϕ< 3π/2$. Importantly, this effect can be realized in ferromagnetic insulators with sufficiently large easy-plane anisotropy energy. Using realistic junction parameters, we demonstrate that the switching can be controlled by phase bias and triggered by half-flux-quantum voltage pulses or external magnetic field pulses on the microsecond timescale. These results provide a route towards controllable Josephson-based superconducting memory devices based on EuS/Al heterostructures.

cond-mat.supr-con↗

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 $Δ$, it performs very well in the strong coupling regime where $T_K \gtrsim Δ$, 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.

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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.

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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

Theory of a Single Magnetic Impurity on a Thin Metal Film in Proximity to a Superconductor

We argue that the formation of Yu-Shiba-Rusinov excitations in proximitized thin films is largely mediated by a type of Andreev-bound state named after de Gennes and Saint James. This is shown by studying an experimentally motivated model and computing the overlap of the wave functions of these two subgap states. We find the overlap stays close to unity even as the system moves away from weak coupling across the parity-changing quantum phase transition. Based on this observation, we introduce a single-site model of the bound state coupled to a quantum spin. The adequacy of this description is assessed by reintroducing the coupling to the continuum as a weak perturbation and studying its scaling flow using Anderson's poor man's scaling.

cond-mat.supr-con↗

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↗

arXiv2022

Pair excitations of a quantum spin on a proximitized superconductor

A magnetic impurity interacting with a superconductor develops a rich excitation spectrum formed by superposition of quasiparticles and spin states, which appear as Yu-Shiba-Rusinov and spin-flip excitations in tunneling spectra. Here, we show that tunneling electrons can also excite a superconducting pair-breaking transition in the presence of magnetic impurities, which is hidden for electrons on bare superconductors. Combining scanning tunneling spectroscopy with theoretical modeling, we map the excitation spectrum of a Fe-porphyrin molecule on the Au/V(100) proximitized surface into a manifold of many-body excitations and follow their behavior across a parity-changing transition. Pair excitations emerge in the tunneling spectra as peaks outside the gap in the strong interaction regime, scaling with the pair correlation. Our results unravel the quantum nature of magnetic impurities on superconductors and prove that pair excitations are parity detectors for magnetic impurities.

cond-mat.mes-hall↗

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↗

arXiv2020

Unambiguous Electrical Detection of Spin-Charge Conversion in Lateral Spin-Valves

Efficient detection of spin-charge conversion is crucial for advancing our understanding of emergent phenomena in spin-orbit-coupled nanostructures. Here, we provide proof of principle of an electrical detection scheme of spin-charge conversion that enables full disentanglement of competing spin-orbit coupling transport phenomena in diffusive lateral channels i.e. the inverse spin Hall effect (ISHE) and the spin galvanic effect (SGE). A suitable detection geometry in an applied oblique magnetic field is shown to provide direct access to spin-charge transport coefficients through a simple symmetry analysis of the output non-local resistance. The scheme is robust against tilting of the spin-injector magnetization, disorder and spurious non-spin related contributions to the non-local signal, and can be used to probe spin-charge conversion effects in both spin-valve and hybrid optospintronic devices.

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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

Theory of Spin Injection in Two-dimensional Metals with Proximity-Induced Spin-Orbit Coupling

Spin injection is a powerful experimental probe into a wealth of nonequilibrium spin-dependent phenomena displayed by materials with spin-orbit coupling (SOC). Here, we develop a theory of coupled spin-charge diffusive transport in two-dimensional spin-valve devices. The theory describes a realistic proximity-induced SOC with both spatially uniform and random components of the SOC due to adatoms and imperfections, and applies to the two dimensional electron gases found in two-dimensional materials and van der Walls heterostructures. The various charge-to-spin conversion mechanisms known to be present in diffusive metals, including the spin Hall effect and several mechanisms contributing current-induced spin polarization are accounted for. Our analysis shows that the dominant conversion mechanisms can be discerned by analyzing the nonlocal resistance of the spin-valve for different polarizations of the injected spins and as a function of the applied in-plane magnetic field.

cond-mat.mes-hall↗

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↗

arXiv2019

Control of Spin Diffusion and Suppression of the Hanle Effect by the Coexistence of Spin and Valley Hall Effects

In addition to spin, electrons in many materials possess an additional pseudo-spin degree of freedom known as 'valley'. In materials where the spin and valley degrees of freedom are weakly coupled, they can be both excited and controlled independently. In this work, we study a model describing the interplay of the spin and valley Hall effects in such two-dimensional materials. We demonstrate the emergence of an additional longitudinal neutral current that is both spin and valley polarized. The additional neutral current allows to control the spin density by tuning the magnitude of the valley Hall effect. In addition, the interplay of the two effects can suppress the Hanle effect, that is, the oscillation of the nonlocal resistance of a Hall bar device with in-plane magnetic field. The latter observation provides a possible explanation for the absence of the Hanle effect in a number of recent experiments. Our work opens also the possibility to engineer the conversion between the valley and spin degrees of freedom in two-dimensional materials.

cond-mat.mes-hall↗

arXiv2018

Electronic Structure Theory of Strained Two-Dimensional Materials with Hexagonal Symmetry

We derive electronic tight-binding Hamiltonians for strained graphene, hexagonal boron nitride and transition metal dichalcogenides based on Wannier transformation of {\it ab initio} density functional theory calculations. Our microscopic models include strain effects to leading order that respect the hexagonal crystal symmetry and local crystal configuration, and are beyond the central force approximation which assumes only pair-wise distance dependence. Based on these models, we also derive and analyze the effective low-energy Hamiltonians. Our {\it ab initio} approaches complement the symmetry group representation construction for such effective low-energy Hamiltonians and provide the values of the coefficients for each symmetry-allowed term. These models are relevant for the design of electronic device applications, since they provide the framework for describing the coupling of electrons to other degrees of freedom including phonons, spin and the electromagnetic field. The models can also serve as the basis for exploring the physics of many-body systems of interesting quantum phases.

cond-mat.mes-hall↗

arXiv2018

Nontrivial interplay of strong disorder and interactions in quantum spin-Hall insulators doped with dilute magnetic impurities

We investigate nonperturbatively the effect of a magnetic dopant impurity on the edge transport of a quantum spin Hall (QSH) insulator. We show that for a strongly coupled magnetic dopant located near the edge of a system, a pair of transmission anti-resonances appear. When the chemical potential is on resonance, interaction effects broaden the anti-resonance width with decreasing temperature, thus suppressing transport for both repulsive and moderately attractive interactions. Consequences for the recently observed QSH insulating phase of the $1$-T$^{\prime}$ of WTe$_2$ are briefly discussed.

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