arXiv subjects

Jahanfar Abouie

Publications and source records attributed to Jahanfar Abouie.

At least 19 recordsLinked to original sources

arXiv2026

Scarred discrete time crystal in a periodically driven dimerized spin chain

We investigate the emergence of a scarred discrete time crystal (SDTC) phase in a periodically driven dimerized spin chain. While generic interacting Floquet systems are expected to thermalize according to the eigenstate thermalization hypothesis (ETH), we demonstrate that this system hosts quantum many-body scars (QMBS) that induce a regime of weak ergodicity breaking. Through an analysis of Floquet level statistics, entanglement entropy, and eigenstate fidelity, we identify a manifold of low-entanglement states characterized by semi-Poisson statistics embedded within an otherwise thermal spectrum. These scarred states support robust subharmonic oscillations with period doubling, signaling the spontaneous breaking of discrete time-translation symmetry. We show that the SDTC response is robust against a variety of initial state configurations, demonstrating its stability beyond fine-tuned conditions. A finite-size scaling analysis reveals that the time-crystalline lifetime grows with system size within the range accessible to our exact-diagonalization calculations. However, drawing on the general phenomenology of approximate many-body scars, we expect that hybridization between Floquet scars and the thermal continuum will eventually curtail this growth, causing the lifetime to saturate at system sizes beyond our current numerical reach. This characterizes the SDTC as a long-lived metastable dynamical regime rather than a strictly stable thermodynamic phase, providing a comprehensive framework for understanding the interplay between periodic driving and constrained many-body dynamics in disorder-free systems.

cond-mat.str-el↗

arXiv2026

Dynamical susceptibility and quantum Fisher information in the Su-Schrieffer-Heeger model with Hatsugai-Kohmoto interactions

We investigate the dynamical spin and charge susceptibilities and the associated quantum Fisher information in a class of interacting lattice models, with a primary focus on the Su-Schrieffer-Heeger model in the presence of Hatsugai-Kohmoto interactions. To provide a rigorous analytical benchmark, we contrast the response properties of the SSH-HK system with those of the single-band Hubbard and SSH-Hubbard models, treated within the random-phase approximation. While standard Hubbard-type interactions typically suppress excitation strength, we demonstrate that the SSH-HK model displays qualitatively distinct physical behavior arising from the interplay between SSH dimerization and the momentum-diagonal nature of the HK interaction. Leveraging the exact solvability of the HK term, we derive closed-form expressions for the dynamical susceptibility, revealing unique filling-controlled characteristics such as a finite response at zero wave vector and a pronounced restructuring of spectral weight across integer and fractional filling sectors. We show that the quantum Fisher information, defined as the frequency integral of the imaginary part of the susceptibility, serves as an efficient probe of these filling sectors, exhibiting distinct piecewise behavior that distinguishes integer from fractional fillings. Notably, our results indicate that the quantum Fisher information remains insensitive to topological transitions within uniform-density regimes, highlighting the limitations of standard dynamical response functions in characterizing band topology. These findings establish the SSH-HK model as a powerful analytical platform for exploring the competition between topology and strong correlations, demonstrating how dynamical susceptibilities and the quantum Fisher information provide complementary, experimentally accessible probes of many-body physics.

cond-mat.str-el↗

arXiv2026

Emergence of prethermal time quasicrystalline order in a quasiperiodically driven non-interacting spin chain

We study prethermal time quasicrystalline (TQC) order in a quasiperiodically driven chain of non-interacting spin-1/2 particles. The drive consists of two parts, switched on and off periodically with frequency $ω_d$: (i) disordered Ising interactions, with exchange couplings chosen from a symmetric interval $[-J/2, J/2]$, allowing random antiferromagnetic or ferromagnetic nearest-neighbor couplings, together with a random transverse field; and (ii) a rotating transverse magnetic field with frequency $Ω$. The ratio $ω_d/Ω$ is chosen to be irrational, producing multiple incommensurate frequencies and yielding quasiperiodic dynamics beyond Floquet theory. Using exact diagonalization, we analyze the time autocorrelation function, dynamical structure factor, and entanglement entropy (EE). In the high-frequency regime, robust spectral peaks at incommensurate frequencies (not integer multiples of the fundamental drives) signal quasiperiodic time-translation symmetry breaking (QTTSB). The EE exhibits sublinear power-law growth followed by a prethermal plateau, indicating suppressed resonant heating due to an energy scale mismatch. The nonequilibrium lifetime increases rapidly with driving frequency. Unlike symmetric disorder sampling, an asymmetric distribution of the Ising exchange couplings induces collective spin rigidity, enhancing the system's resistance to heating. The TQC phase remains stable against next-nearest-neighbor (NNN) exchange perturbations and rotational imperfections, with robustness comparable to discrete time crystals (TCs) under periodic driving. Our results establish this quasiperiodically driven system as a platform for long-lived nonequilibrium temporal order, revealing the interplay of disorder, collective rigidity, and quasiperiodic driving.

cond-mat.dis-nn↗

arXiv2026

Analytic Theory and cQED Implementation of a Two-Qubit Refrigerator: Sub-100 mK Cavity Cooling from a 4 K Bath

We develop a theoretical framework for cooling a microwave cavity mode using a Poisson stream of internally correlated pairs of two-level systems and analyze its performance under realistic dissipation. Starting from a Lindblad model of a phonon-tethered cavity interacting with sequentially injected atom pairs, we derive closed-form expressions for the steady-state cavity occupation and effective temperature. Two coupling geometries are examined: a one-atom configuration, where only one member of each pair interacts with the cavity, and a two-atom configuration, where both atoms couple collectively. The single-atom model enables cooling below the phonon bath but not below the reservoir temperature, whereas the two-atom scheme exhibits enhanced refrigeration - pair correlations modify the cavity's upward and downward transition rates so that the steady-state temperature can fall well below that of the reservoir for weak phonon damping. We map the parameter space including detuning, coupling strength, damping, and intra-pair exchange, identifying cooling valleys near resonance and the crossover between reservoir- and phonon-dominated regimes. The two-atom configuration thus realizes a genuine quantum-enhanced cooling mechanism absent in the single-atom case. We further outline an experimental implementation using two superconducting qubits repeatedly prepared, coupled, and reset inside a 3D cavity. Realistic reset and flux-tuning protocols support MHz-rate interaction cycles, enabling engineered reservoirs to impose cavity temperatures of 50-120 mK even when the cryostat is at ~1 K, offering a pathway to autonomous, on-chip refrigeration of microwave modes in scalable quantum hardware.

quant-ph↗

arXiv2026

Reservoir-Engineered Refrigeration of a Superconducting Cavity with Double-Quantum-Dot Spin Qubits

We present an analytically tractable theory of reservoir-engineered refrigeration of a superconducting microwave cavity and map it onto a realistic solid-state implementation based on gate-defined double-quantum-dot (DQD) spin qubits. Treating the DQD not as a spectroscopic element but as a tunable engineered reservoir, we show how gate control of populations, coherences, linewidths, and detuning defines an effective photon birth-death process with predictable detailed balance. This framework yields closed-form expressions for the cavity steady state, identifies cooling bounds and detuning-dependent refrigeration valleys, and clarifies when refrigeration can drive the cavity below both the bath temperature and the DQD setpoint. By distinguishing refreshed (collision-like) and persistent reservoir regimes, we show how memory effects, saturation, and dark-state formation constrain cooling in realistic devices, while collective bright-mode coupling in a two-dot configuration can enhance refrigeration subject to mismatch and dephasing, as confirmed by numerical Lindblad simulations demonstrating targeted millikelvin cavity cooling relevant for cryogenic circuit-QED architectures.

quant-ph↗

arXiv2025

Spin Glasses: Disorder, Frustration, and Nonequilibrium Complexity

Spin glasses occupy a unique place in condensed matter: they freeze collectively while remaining struc-turally disordered, and they exhibit slow, history-dependent dynamics that reflect an exceptionally rug-ged free-energy landscape. This review provides an integrated account of spin-glass physics, emphasiz-ing how microscopic ingredients (quenched randomness, frustration, competing exchange interactions, and random fields) conspire to produce macroscopic glassiness. We begin with the canonical Edwards-Anderson and Sherrington-Kirkpatrick formulations to introduce the central theoretical ideas that recur across the literature: extensive degeneracy, metastability, and the emergence of long relaxation times that manifest as aging, memory, and rejuvenation under standard experimental protocols. We then summarize the principal routes used to characterize spin-glass freezing, combining thermodynamic signatures with dynamical probes that reveal the separation of timescales and the sensitivity to thermal and magnetic histories. Building on these foundations, we draw connections across experimental material classes (me-tallic alloys, insulating oxides, and geometrically frustrated systems) by emphasizing how intrinsic ver-sus induced disorder and competing interaction networks shape the observed phenomenology. Recent advances in reentrant and room-temperature spin-glass materials are highlighted as a rapidly developing direction that tests the limits of established paradigms and motivates new materials-driven questions. The review concludes by connecting modern computational developments, including machine-learning phase identification and neural-network analogies, to longstanding challenges in classification, univer-sality, and out-of-equilibrium behavior, and by outlining emerging opportunities at the boundary between classical and quantum spin glasses.

cond-mat.dis-nn↗

arXiv2025

From time crystals to time quasicrystals: Exploring quasiperiodic phases in transverse field Ising chains

Time quasicrystals (TQCs) represent a compelling extension of the concept of time crystals (TCs). While TCs break discrete time-translation symmetry by exhibiting a periodic response at a subharmonic of the driving frequency, TQCs display a more complex temporal order. They respond at multiple incommensurate frequencies, values that are not integer multiples of the fundamental driving frequency, resulting in quasiperiodic dynamics. In this work, we investigate the emergence of a TQC in a disordered quantum Ising chain subjected to a quasiperiodic transverse field. Using exact diagonalization, we find that the transverse magnetization exhibits quasiperiodic oscillations which persist over extended prethermal timescales before eventual decay. This indicates that the TQC exists as a long-lived, prethermal dynamical phase rather than a true equilibrium state. We further assess the robustness of this prethermal TQC against interactions and driving imperfections, confirming its stability under realistic experimental conditions. Finite-size analysis reveals that the prethermal TQC lifetime exhibits minimal dependence on system size. Additionally, we explore the emergence of TQCs in the same chain under symmetric sampling of exchange couplings. Our results demonstrate that the TQC phase is highly sensitive to both the choice of coupling distribution and the values of the driving frequencies. These findings highlight promising experimental prospects for realizing TQCs in cold atomic systems and quantum simulators, providing valuable insights into their stability, dynamical properties, and potential for exploring novel non-equilibrium quantum phases.

cond-mat.str-el↗

arXiv2025

What Really Drives Thermopower: Specific Heat or Entropy as the Unifying Principle Across Magnetic, Superconducting, and Nanoscale Systems

Thermopower, a key parameter in thermoelectric performance, is often linked to either specific heat or entropy, yet the fundamental quantity that governs it has remained elusive. In this work, we present a unified theoretical framework that identifies entropy per carrier, not specific heat, as the universal driver of thermopower across both closed and open systems. Using thermodynamic identities and the Onsager-Kelvin relation, we show that thermopower is universally proportional to entropy per carrier, while its apparent proportionality to specific heat arises only in systems where the specific heat follows a continuous power-law temperature dependence. To extend this framework to magnetic systems, we derive a general expression for magnon-drag thermopower that holds in both Newtonian (massive, parabolic) and relativistic (massless, linear) magnon regimes. In particular, we reformulate the momentum balance using a relativistic energy-momentum tensor, resolving conceptual inconsistencies in prior models that relied on ill-defined magnon masses in antiferromagnets. Our framework is further illustrated through three representative systems: (i) magnetic materials, where magnon and paramagnon entropy sustain thermopower across TC and TN; (ii) superconducting Nb, where anomalous thermopower emerges from entropy carried by Bogoliubov quasiparticles near TC; and (iii) a single-molecule junction, where entropy from occupation-number fluctuations governs thermopower in an open quantum system. We validate our unifying principle by comparing it with experimental data: thermopower measurements of superconducting niobium reveal the role of quasiparticle entropy near the critical temperature, and literature-reported specific heat data from a wide range of ferromagnetic and antiferromagnetic materials demonstrate consistent entropy-based scaling across magnetic transitions.

cond-mat.mtrl-sci↗

arXiv2025

Emergence of Topological Non-Fermi Liquid Phases in a Modified Su-Schrieffer-Heeger Chain with Long-Range Interactions

In this study, we investigate the emergence of a topological non-Fermi liquid (NFL) phase in a modified Su-Schrieffer-Heeger (SSH) chain model subjected to long-range interactions characterized by the Hatsugai-Kohmoto (HK) model. While Fermi liquid theory has been instrumental in understanding low temperature properties of metals, it fails to account for the complex behaviors exhibited by strongly correlated systems, where interactions lead to emergent phenomena such as non-Fermi liquid behavior. Our analysis reveals that the SSH-HK model supports a rich ground state phase diagram, exhibiting distinct NFL phases marked by many body Zak phases of $2π$ and $0$, corresponding to topological and trivial NFL states, respectively. We demonstrate that the topological NFL state manifests unique electronic polarization characteristics akin to those in the non-interacting SSH model. Through exact diagonalization of the interacting SSH-HK Hamiltonian, we explore the spectral functions and density of states, revealing significant departures from traditional quasiparticle behavior in various particle number sectors. Our findings extend the understanding of topological non-Fermi liquids and their potential implications for high-temperature superconductivity and other correlated electron systems, highlighting the intricate interplay between topology and strong electron correlations.

cond-mat.str-el↗

arXiv2025

Enhancing Magnetic Coupling in MN4-Graphene via Strain Engineering

MN4-embedded graphene (MN4-G) layers, incorporating transition metal elements (M), represent a class of experimentally accessible two-dimensional materials with significant potential for stable nanoscale magnetization. In these systems, magnetic exchange interactions are primarily governed by Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, exhibiting an anomalously prolonged decay of r to the power of (-n), where r is the M-M separation distance and n is between 0.5 and 2. This study investigates the impact of strain on the electronic and magnetic properties of MN4-G layers using ab-initio density functional theory (DFT). A novel strain-engineering approach is developed by applying controlled tension or compression to the layers. Our findings reveal that strain significantly modulates the strength, amplitude, and decay rate of the RKKY coupling. Notably, the CoN4-G layer demonstrates a pronounced enhancement in RKKY coupling strength, oscillation amplitude, and reduced decay rate under strain. Conversely, the CuN4-G layer exhibits distinct behavior, maintaining decoupled spin chains and invariant electronic and magnetic properties despite applied strain. This work underscores the tunability of magnetic interactions in MN4-G layers via strain engineering, providing insights into the design of strain-controlled magnetic materials for next-generation spintronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Valence-Bond Solid phases in the spin-$1/2$ Kekule-Heisenberg model

We map out the ground state phase diagram of the isotropic Kekule'-Kitaev model on the honeycomb lattice in the presence of the Heisenberg exchange couplings. Our study relies on large-scale tensor network simulations based on graph-based projected entangled pair state (gPEPS) approach in the thermodynamic limit. We find that on top of the quantum spin liquid (QSL) and conventional magnetically ordered phases which are typical of the Kitaev-Heisenberg model, the Kekule'-Heisenberg phase diagram, hosts two plaquette valance bond solid (VBS) phases with vanishing magnetic order. While the VBS phases preserve the symmetries of the original Hamiltonian, they differ markedly from the Kitaev spin liquid by having decorated plaquette ordering which is distinguished by a plaquette order parameter.

cond-mat.str-el↗

arXiv2024

Uncovering Electronic Exchange Behavior: Exploring Insights from Simple Models

Exchange couplings are fundamental to our understanding of many physical phenomena in condensed matter physics and material science. Model systems provide a controlled environment to investigate such phenomena, effectively. In this study, we employ first-principle calculations based on density functional theory and Green's function (GF) method to explore the impact of chemical structure on the sign and magnitude of exchange coupling, systematically. By designing model systems with bcc-Fe bulk doped with nonmagnetic X= (H, B, C, N, O, and F) atoms, we examine the effects of different ligands on the behavior of Fe-Fe exchange coupling, and demonstrate that the chemical environment surrounding the metal atom significantly influences the Fe-Fe exchange coupling. Our results highlight the tunability of exchange coupling based on Fe-dopant bond length(s), where the nature of ligand atoms and their electron correlation play a crucial role. This work illuminates the complex relationship between structure, and magnetism in magnetic materials, providing insights into the development of high-performance magnetic materials.

cond-mat.mtrl-sci↗

arXiv2024

Microwave-Induced Cooling in Double Quantum Dots: Achieving Millikelvin Temperatures to Reduce Thermal Noise around Spin Qubits

Spin qubits in gate-defined quantum dots (QDs) are emerging as a leading technology due to their scalability and long coherence times. However, maintaining these qubits at ultra-low temperatures typically requires complex cryogenic systems. This paper proposes a novel gate-defined double quantum dot (DQD) cooling system, where the DQDs act as refrigerants to reduce the local phonon environment around computational qubits. The cooling process occurs in two distinct stages: the first step involves microwave-induced state depopulation combined with fast cyclic detuning to transfer the DQD's population to the ground state, effectively lowering the DQD's temperature. In the second step, the cooled DQD interacts with and absorbs phonons resonant with the DQD spin energy, thereby filtering out these phonons that contribute to spin-lattice relaxation in the surrounding environment. This study focuses on the first step, presenting detailed calculations and numerical results that demonstrate the feasibility of achieving local DQD temperatures below 10 mK at a bath temperature of 1 K. The sensitivity of the cooling performance to detuning energy, magnetic field strength, and diabatic return time is analyzed, while the phonon filtering in the second step will require further investigation.

cond-mat.mes-hall↗

arXiv2024

Entanglement in Quantum Dots: Insights from Dynamic Susceptibility and Quantum Fisher Information

This study investigates the entanglement properties of quantum dots (QDs) under a universal Hamiltonian where the Coulomb interaction between particles (electrons or holes) decouples into a charging energy and an exchange coupling term. While this formalism typically decouples the charge and spin components, the confinement-induced energy splitting can induce unexpected entanglement in the system. By analyzing the dynamic susceptibility and quantum Fisher information (QFI), we uncover intriguing behaviors influenced by exchange constants, temperature variations, and confinement effects. In Ising QDs, far below the Stoner instability point where the QD is in a disordered paramagnetic phase, temperature reductions unexpectedly lead to decreased entanglement, challenging conventional expectations. Conversely, anisotropic Heisenberg models exhibit enhanced entanglement near isotropic points. Our findings highlight the intricate interplay between exchange interactions and entanglement in QDs, laying the groundwork for future studies on topological entanglement and the influence of entanglement on material properties. Overall, this work contributes to advancing our understanding of entanglement in QDs and its potential applications in quantum technologies.

quant-ph↗

arXiv2023

Paramagnon Heat Capacity and Anomalous Thermopower in Anisotropic Magnetic Systems: Understanding Inter-Layer Spin Correlations in a Magnetically Disordered Phase

The interplay between entropy transport and charge carriers-paramagnon interaction in the Onsager linear system has been a subject of debate due to the limited theoretical and experimental understanding of paramagnon heat capacity. In this study, we investigate this interplay in an anisotropic layered magnetic system using cluster mean-field theory with spin quantum correlations. By examining spin correlation functions between different spins with various types of clustering, we derive the spin correlation function as a function of distance and temperature for the inter-layer clusters both below and above the magnetic order phase transition. Our analysis reveals that paramagnons characterized by pronounced spin correlations among inter-layer nearest-neighbor spins exhibit a non-zero heat capacity, providing valuable insights into the dynamics of entropy transport. The findings align with experimental observations, lending strong support to the validity of the paramagnon drag thermopower concept. This study sheds light on the intricate dynamics and thermodynamic properties of paramagnons, advancing our understanding of entropy transport in complex systems.

cond-mat.str-el↗

arXiv2022

MN4 Embedded Graphene Layers: Tunable Decay Rate of RKKY Interaction

One of the most important tasks in the development of high-performance spintronic devices is the preparation of two dimensional (2D) magnetic layers with long-range exchange interactions. MN4 embedded graphene (MN4-G) layers, with M being transition metal elements, are experimentally accessible 2D layers, which exhibit interesting magnetic properties. In this paper, by employing the spin-polarized density functional theory (SP-DFT), we study MN4-G layers with a special focus on the behavior of the indirect M-M exchange interactions, and demonstrate that the MN4-Gs with M = Fe, Mn and Co, are 2D anisotropic magnetic layers with Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. By examining the electronic configurations of the M atoms for various M-M spacers, we demonstrate that the RKKY interaction in such layers are tunable and exhibiting an unusual prolonged decay (r to the power of minus n, n between 0.5 and 2). In addition, we investigate the influence of the CuN4 moiety in the graphene host, and show that, in contrary to the other MN4-Gs, the 2D CuN4-G layer behaves as decoupled one-dimensional spin chains, regardless of the spacer lengths.

cond-mat.mtrl-sci↗

arXiv2019

Entanglement conductance as a characterization of delocalized-localized phase transition in free fermion models

We study entanglement Hamiltonian (EH) associated with the reduced density matrix of free fermion models in delocalized-localized Anderson phase transition. We show numerically that the structure of the EH matrix differentiates the delocalized from the localizedphase. In the delocalized phase, EH becomes a long-range Hamiltonian but is short-range in the localized phase, no matter what the configuration of the system's Hamiltonian is (whether it is long or short range). With this view, we introduce the entanglement conductance (EC), which quantifies how much EH is long-range and propose it as an alternative quantity to measure entanglement in the Anderson phase transition, by which we locate the phase transition point of some one-dimensional free fermion models; and also by applying the finite size method to the EC, we find three-dimensional Anderson phase transition critical disorder strength.

cond-mat.str-el↗

arXiv2017

Inhomogeneous hard-core bosonic mixture with checkerboard supersolid phase: Quantum and thermal phase diagram

We introduce an inhomogeneous bosonic mixture composed of two kinds of hard-core and semi-hard-core bosons with different nilpotency conditions and demonstrate that in contrast with the standard hard-core Bose-Hubbard model, our bosonic mixture with nearest- and next-nearest-neighbor interactions on a square lattice develops the checkerboard supersolid phase characterized by the simultaneous superfluid and checkerboard solid orders. Our bosonic mixture is created from a two-orbital Bose-Hubbard model including two kinds of bosons: a single-orbital boson and a two-orbital boson. By mapping the bosonic mixture to an anisotropic inhomogeneous spin model in the presence of a magnetic field, we study the ground-state phase diagram of the model by means of cluster mean field theory and linear spin-wave theory and show that various phases such as solid, superfluid, supersolid, and Mott insulator appear in the phase diagram of the mixture. Competition between the interactions and magnetic field causes the mixture to undergo different kinds of first- and second-order phase transitions. By studying the behavior of the spin-wave excitations, we find the reasons of all first- and second-order phase transitions. We also obtain the temperature phase diagram of the system using cluster mean field theory. We show that the checkerboard supersolid phase persists at finite temperature comparable with the interaction energies of bosons.

cond-mat.other↗