arXiv subjects

Cătălin Paşcu Moca

Publications and source records attributed to Cătălin Paşcu Moca.

At least 19 recordsLinked to original sources

arXiv2026

Robust Spin Universality Across a Ballistic-to-Diffusive Transport Crossover

Whether a full counting statistical distribution preserves its universality class across a coherent-to-incoherent crossover is a fundamental question in open quantum systems. We resolve this for the infinite-$U$ Hubbard chain, showing that local density dephasing changes transport exponents without destroying the underlying non-Gaussian M-Wright spin fluctuations. Utilizing the exact spin-charge composition law inherent to impenetrable dynamics, we track the charge sector from ballistic flow to diffusive hydrodynamics. This dissipative crossover shifts the characteristic spin-transfer width from $t^{1/4}$ to $t^{1/8}$. Despite this altered scaling time, the central spin characteristic functions uniformly collapse onto a singular M-Wright scaling function, what we confirm using analytics as well as numerics.

cond-mat.quant-gas↗

arXiv2026

Spin-Charge Subordination in the Infinite-$U$ $SU(N)$ Hubbard Chain

We obtain the exact full counting statistics of charge and flavor transport in the one-dimensional infinite-$U$ $SU(N)$ Hubbard model. The no-passing constraint freezes the ordered flavor sequence while the particle coordinates evolve as free spinless fermions. Consequently, charge transfer is governed by a free-fermion determinant, whereas flavor transfer is exactly subordinated to the number of particles crossing the observation cut. For an arbitrary traceless Cartan generator this yields an exact relation between the charge and flavor cumulants. The complete asymptotic flavor distribution is a universal non-Gaussian M-Wright law. Together, these results establish spin--charge subordination as a kinematic mechanism for anomalously slow internal-state transport in one-dimensional impenetrable quantum gases.

cond-mat.quant-gas↗

arXiv2026

Interaction-induced asymmetry in infinite-temperature dynamical correlations of hard-core anyons

We study dynamical correlations of interacting hard-core anyons on a one-dimensional lattice at infinite temperature. This is a setting in which the many-body spectrum is independent of the statistical phase $θ$, while dynamical correlators remain sensitive to $θ$ through nonlocal Jordan-Wigner strings. We compute single-particle Green's functions, spectral functions, and density-density correlators, thereby separating the effects of fractional statistics on one-body coherence from those on density transport in a maximally mixed ensemble. In the noninteracting case $V=0$, high-temperature averaging leads to inversion-symmetric Green's functions for all $θ$ despite the presence of anyonic strings. Finite nearest-neighbor interactions $V$ generate, however, a pronounced left-right asymmetry in the Green's functions for $0<θ<π$, with the strongest chirality appearing at intermediate couplings $V\sim J$ where interactions and hopping compete most effectively. In this regime, the Green's function decays exponentially in time with a statistical-angle-dependent decay rate. At strong coupling, the dynamics crosses over to an atomic-limit regime in which the dependence on $θ$ is reduced. Here, the Green's function decays universally as $t^{-1}$ and the corresponding spectral function displays a three-band structure. In contrast, density-density correlations are insensitive to statistics and recover the known infinite-temperature transport regimes of the XXZ chain, including ballistic, superdiffusive, and diffusive behaviors. These results identify dynamical correlation functions as direct probes of fractional statistics in high-entropy quantum systems.

cond-mat.str-el↗

arXiv2026

Dynamic scaling and Family-Vicsek universality in the Hubbard model at infinite temperature

We study Family-Vicsek scaling of charge, spin, and energy fluctuations in the one-dimensional Hubbard model at infinite temperature. Using a quantum generating function approach, we compute time-dependent cumulants of transferred conserved quantities and analyze how the corresponding roughness depends on subsystem size and time. We start by focusing on a single interface at half the chain and determine the transport exponents. Then we turn to fluctuations of a small finite interval and study the Family-Vicsek universality of fluctuations over an extended timescale. We find that the long-time scaling behavior is controlled by integrability. In the free limit, charge, spin, and energy all display ballistic transport. In the interacting integrable Hubbard chain, charge and spin cross over to a KPZ scaling regime, while the energy sector remains ballistic. Once integrability is broken by a next-nearest-neighbor density interaction, the long-time dynamics becomes diffusive in all sectors. In every case we also observe a short-time microscopic regime with apparently universal ballistic growth before the hydrodynamic scaling window sets in. The Family-Vicsek setup allows us to determine the growth, the saturation as well as the dynamical exponents.

cond-mat.str-el↗

arXiv2026

Dynamical correlations in a dissipative XXZ spin chain

We study dynamical spin correlations in a dissipative XXZ spin chain subject to uniform local spin-loss and pumping. Starting from a mixed steady state that is featureless albeit possessing finite magnetization, rich dynamics emerges in time-dependent two-point correlators evaluated on top of it. For unitary evolution in which the reservoir is absent, the longitudinal correlators reproduce the established hierarchy of spin-transport universality classes - ballistic, Kardar-Parisi-Zhang (KPZ) superdiffusive, and diffusive - across the phase diagram. However, for finite magnetization, additional ballistic light cone propagation gets superimposed on the previous universality classes, arising from magnon propagation.The transverse correlator displays very fast, exponential decay of correlations without wavefront propagation in the easy-plane case. At the isotropic point, it follows KPZ scaling due to $SU(2)$ symmetry, while in the easy-axis regime, it is characterized by ballistic spreading of correlations. Under full Lindbladian dynamics, the universality classes are preserved at early times, while the correlations acquire an overall exponential damping in the long-time limit. In terms of methods, we have used vectorized TEBD for numerical simulations and exact analytical results obtained via a Pfaffian representation and the third-quantization framework for the noninteracting XX case.

cond-mat.str-el↗

arXiv2026

Two-parameter Family-Vicsek scaling in a dissipative XXZ spin chain

Family-Vicsek (FV) scaling provides an understanding for the growth and finite-size saturation of fluctuations in classical systems. Here, we extend the FV roughness to transferred segment magnetization after quantum quenches in a dissipative XXZ spin chain with homogeneous gain and loss, starting from a nonequilibrium steady state with finite magnetization. In the non-interacting limit, we derive a closed-form expression for the roughness in the presence of dissipation. It displays two-parameter FV scaling and smoothly interpolates between the clean ballistic behavior and the dissipation dominated scalings. For interacting chains, tensor-network simulations show that the non-dissipative ballistic growth at finite magnetization is robust, whereas the full Lindblad evolution is generically controlled by the dissipative relaxation time and exhibits a dissipation-dominated collapse.

quant-ph↗

arXiv2026

Nonstabilizerness generation in a multiparticle quantum walk

We investigate the generation of non-stabilizerness, or magic, in a multi-particle quantum walk by analyzing the time evolution of the stabilizer Rényi entropy $M_2$. Our study considers both single- and two-particle quantum walks in the framework of the XXZ Heisenberg model with varying interaction strengths. We demonstrate that the spread of magic follows the light-cone structure dictated by the system's dynamics, with distinct behaviors emerging in the easy-plane ($Δ< 1$) and easy-axis ($Δ> 1$) regimes. For $Δ< 1$, magic generation is primarily governed by single-particle dynamics, while for $Δ> 1$, doublon propagation dominates, resulting in a significantly slower growth of $M_2$. Furthermore, the magic exhibits logarithmic growth in time for both one and two-particle dynamics. Additionally, by examining the Pauli spectrum, we show that the statistical distribution of level spacings exhibits Poissonian behavior, independent of interaction strength or particle number. Our results shed light on the role of interactions on magic generation in a many-body system.

quant-ph↗

arXiv2026

Independent stabilizer Rényi entropy and entanglement fluctuations in random unitary circuits

We investigate numerically the joint distribution of magic ($M$) and entanglement ($S$) in $N$-qubit Haar-random quantum states. The distribution $P_N(M,S)$ as well as the marginals become exponentially localized, and centered around the values $\tilde{M_2} \to N-2$ and $\tilde{S} \to N/2$ as $N\to\infty$. Magic and entanglement fluctuations are, however, found to become exponentially uncorrelated. Although exponentially many states with magic $M_2=0$ and entropy $S\approx S_\text{Haar}$ exist, they represent an exponentially small fraction compared to typical quantum states, which are characterized by large magic and entanglement entropy, and uncorrelated magic and entanglement fluctuations.

quant-ph↗

arXiv2025

Nonstabilizerness in open XXZ spin chains: Universal scaling and dynamics

Magic, or nonstabilizerness, is a crucial quantum resource, yet its dynamics in open quantum systems remain largely unexplored. We investigate magic in the open XXZ spin chain under either boundary gain and loss, or bulk dephasing using the stabilizer Rényi entropy $M_2$. To enable scalable simulations of large systems, we develop a novel, highly efficient algorithm for computing $M_2$ within the matrix product states formalism while maintaining constant bond dimension--an advancement over existing methods. For boundary driving, we uncover universal scaling laws, $M_2(t) \sim t^{1/z}$, linked to the dynamical exponent $z$ for several distinct universality classes. We also disentangle classical and quantum contributions to magic by introducing a mean-field approximation for magic, thus emphasizing the prominent role of quantum critical fluctuations in nonstabilizerness. For bulk dephasing, dissipation can transiently enhance magic before suppressing it, and drive it to a nontrivial steady-state value. These findings position magic as a powerful diagnostic tool for probing universality and dynamics in open quantum systems.

quant-ph↗

arXiv2025

Non-stabilizerness as a Diagnostic of Criticality and Exceptional Points in Non-Hermitian Spin Chains

We investigate non-stabilizerness, also known as ``magic,'' to understand criticality and exceptional points in non-Hermitian quantum many-body systems. Our focus is on parity-time ($\mathcal{PT}$) symmetric spin chains, specifically the non-Hermitian transverse-field Ising and XX models. We calculate stabilizer Rényi entropies in their ground states using non-Hermitian matrix product state methods. Our findings show that magic exhibits unique and model-specific signs of phase transitions. In the Ising chain, it peaks along the regular Hermitian-like critical line but disappears across exceptional points. In contrast, in the XX chain, it reaches its maximum at the exceptional line where $\mathcal{PT}$ symmetry is broken. Finite-size scaling reveals that these effects become more pronounced with larger systems, highlighting non-stabilizerness as a sensitive marker for both quantum criticality and non-Hermitian spectral degeneracies. We also investigate magic in momentum space for the XX model analytically and find that is reaches a minimum around exceptional points. Our results indicate that magic takes extremal values at the exceptional points and serves as a valuable tool for examining complexity, criticality, and symmetry breaking in non-Hermitian quantum matter.

quant-ph↗

arXiv2025

Quantum Coherence in a Maximally Hot Hubbard Chain

We present a detailed study of the real-time dynamics and spectral properties of the one-dimensional fermionic Hubbard model at infinite temperature. Using tensor network simulations in Liouville space, we compute the single-particle Green's function and analyze its dynamics across a broad range of interaction strengths. To complement the time-domain approach, we develop a high-resolution Chebyshev expansion method within the density matrix formalism, enabling direct access to spectral functions in the frequency domain. In the non-interacting limit, we derive exact analytical expressions for the Green's function, providing a benchmark for our numerical methods. As interactions are introduced, we observe a transition in the spectral function from a sharp peak at the free dispersion to a broadened two-band structure associated with hole and doublon excitations. These features are well captured by a Hubbard-I mean-field approximation, even at intermediate coupling. At infinite interaction strength ($U = \infty$), we exploit a determinant representation of the Green's function to access both real-time and spectral properties. In this regime, the system retains a sharp, cosine-like momentum dispersion in frequency space, while the dynamics display nontrivial light-cone spreading with sub-ballistic scaling. Our results demonstrate that strong correlations and nontrivial quantum coherence can persist even at infinite temperature.

cond-mat.str-el↗

arXiv2025

Hot but Coherent: Doublons at Infinite Temperature in the Hubbard chain

We investigate the spectral properties and the dynamics of doublons in the one-dimensional Hubbard model at infinite temperature. Using a Chebyshev expansion approach formulated in the superfermionic representation, we compute the momentum- and frequency-resolved doublon spectral function across a wide range of interaction strengths $U$ and in the presence of an external electric field. Increasing the interaction, the spectrum gradually splits into separate bands associated with two-hole and two-particle excitations. Despite the presence of strong correlations, we find that doublons retain their coherence and undergo long-lived Bloch oscillations, as well as rich quantum walk dynamics characterized by light-cone spreadings at strong coupling, which we analyze through the time-and space-resolved Green's function.

cond-mat.str-el↗

arXiv2025

Dynamic scaling and Family-Vicsek universality in $SU(N)$ quantum spin chains

The Family-Vicsek scaling is a fundamental framework for understanding surface growth in non-equilibrium classical systems, providing a universal description of temporal surface roughness evolution. While universal scaling laws are well established in quantum systems, the applicability of Family-Vicsek scaling in quantum many-body dynamics remains largely unexplored. Motivated by this, we investigate the infinite-temperature dynamics of one-dimensional $SU(N)$ spin chains, focusing on the well-known $SU(2)$ XXZ model and the $SU(3)$ Izergin-Korepin model. We compute the quantum analogue of classical surface roughness using the second cumulant of spin fluctuations and demonstrate universal scaling with respect to time and subsystem size. By systematically breaking global $SU(N)$ symmetry and integrability, we identify distinct transport regimes characterized by the dynamical exponent $z$: (i) ballistic transport with $z=1$, (ii) superdiffusive transport with the Kardar-Parisi-Zhang exponent $z=3/2$, and (iii) diffusive transport with the Edwards-Wilkinson exponent $z=2$. Notably, breaking integrability always drives the system into the diffusive regime. Our results demonstrate that Family-Vicsek scaling extends beyond classical systems, holding universally across quantum many-body models with $SU(N)$ symmetry.

cond-mat.str-el↗

arXiv2025

Spectral Properties Versus Magic Generation in $T$-doped Random Clifford Circuits

We study the emergence of complexity in deep random $N$-qubit $T$-gate doped Clifford circuits, as reflected in their spectral properties and in magic generation, characterized by the stabilizer Rényi entropy distribution and the non-stabilizing power of the circuit. For pure (undoped) Clifford circuits, a unique periodic orbit structure in the space of Pauli strings implies peculiar spectral correlations and level statistics with large degeneracies. $T$-gate doping induces an exponentially fast transition to chaotic behavior, described by random matrix theory. We compare these complexity indicators with magic generation properties of the Clifford+$T$ ensemble, and determine the distribution of magic, as well as the average non-stabilizing power of the quantum circuit ensemble. In the dilute limit, $N_T \ll N$, magic generation is governed by single-qubit behavior. Magic is generated in approximate quanta, increases approximately linearly with the number of $T$-gates, $N_T$, and displays a discrete distribution for small $N_T$. At $N_T\approx N$, the distribution becomes quasi-continuous, and for $N_T\gg N$ it converges to that of Haar-random unitaries, and averages to a finite magic density, $m_2$, $\lim_{N\to\infty} \langle m_2 \rangle_\text{Haar} = 1$. This is in contrast to the spectral transition, where ${\cal O} (1)$ $T$-gates suffice to remove spectral degeneracies and to induce a transition to chaotic behavior in the thermodynamic limit. Magic is therefore a more sensitive indicator of complexity.

quant-ph↗

arXiv2024

Semiclassical analysis of spin dynamics in the non-Hermitian Hubbard model

We investigate a specific limit of the one-dimensional non-Hermitian Hubbard Hamiltonian with complex interactions. In this framework, fermions with different spin quantum numbers are mapped onto two distinct spin species, resulting in two $XY$ spin chains that are coupled through Ising $ZZ$ interaction. The spin ladder model is then examined within the semiclassical limit using a spin-coherent state basis, where the dynamics is governed by a set of coupled Landau-Lifshitz-Gilbert equations. The non-Hermitian interactions in this model generate a spin-transfer torque term. We analyze the system's evolution toward several potential steady states, including a state of decoupled chains that is accessible when each chain has uniform initial conditions. Other possible steady states involve dimerized configurations with decoupled rungs, where the rung spins are either ferromagnetically or antiferromagnetically coupled, depending on the sign of the imaginary interactions. The spin dynamics is then studied in the infinite-temperature limit, which favors dimerized steady states. Despite the decoupled rungs, we observe the formation of ferromagnetic domains along each chain in the steady state. Additionally, we investigate the spin correlation functions and identify signatures of anomalous spin dynamics.

cond-mat.stat-mech↗

arXiv2024

Spectral properties of fractionalized Shiba states

A magnetic impurity in a BCS superconductor induces the formation of a Shiba state and drives a local quantum phase transition. We generalize this concept to a one-dimensional superconductor with fractionalized excitations, where the dominant instability is superconducting. In this framework, conduction electrons fractionalize into gapless charge and gapped spin excitations. We show that magnetic impurity interacts exclusively with the spin degrees of freedom and induces a quantum phase transition. Furthermore, charge excitations influence dynamical observables, giving rise to the phenomenon we term the fractionalized Shiba state. At zero temperature, the tunneling spectrum exhibits universal power-law scaling with an exponent of $-1/2$ at half filling, stemming from the gapless charge modes that form a standard Luttinger liquid. Extending this analysis to finite temperatures reveals that the spectral features retain universal behavior at the critical point.

cond-mat.str-el↗

arXiv2024

Underscreened Kondo Compensation in a Superconductor

A magnetic impurity with a larger $S=1$ spin remains partially screened by the Kondo effect when embedded in a metal. However, when placed within an $s$-wave superconductor, the interplay between the superconducting energy gap $Δ$ and the Kondo temperature $T_K$ induces a quantum phase transition from an underscreened doublet Kondo to an unscreened triplet phase, typically occurring when $Δ/T_K\approx 1$. We investigate the Kondo compensation of the impurity spin resulting from this partial screening across the quantum phase transition, which together with the spin-spin correlation function serves as a measure of the Kondo cloud's integrity. Deep within the unscreened triplet phase, $Δ/T_K\gg 1$, the compensation vanishes, signifying complete decoupling of the impurity spin from the environment, while in the partially screened doublet phase, $Δ/T_K\ll 1$, it asymptotically approaches $1/2$, indicating that half of the spin is screened. Notably, there is a universal jump in the compensation precisely at the phase transition, which we accurately calculate. The spin-spin correlation function exhibits an oscillatory pattern with an envelope function decaying as $\sim 1/x$ at short distances. At larger distances, the superconducting gap induces an exponentially decaying behavior $\sim \exp(-x/ξ_Δ)$ governed by the superconducting correlation length $ξ_Δ$, irrespective of the phase, without any distinctive features across the transition. Furthermore, the spectral functions of some relevant operators are evaluated and discussed. In terms of the methods used, a consistent description is provided through the application of multiplicative, numerical and density matrix renormalization group techniques.

cond-mat.mes-hall↗

arXiv2024

Momentum space magic for the transverse field quantum Ising model

Stabilizer entropies and quantum magic have been extensively explored in real-space formulations of quantum systems within the framework of resource theory. However, interesting and transparent physics often emerges in momentum space, such as Cooper pairing. Motivated by this, we investigate the momentum-space structure of Pauli strings and stabilizer entropies in the one-dimensional transverse-field quantum Ising model. By mapping the Ising chain onto momentum-space qubits, where the stabilizer state corresponds to the paramagnetic state, we analyze the evolution of the Pauli string distribution. In the ferromagnetic phase, the distribution is broad, whereas in the paramagnetic phase, it develops a two-peaked structure. We demonstrate that all ferromagnetic states possess the same degree of magic in the thermodynamic limit, while stabilizer entropies are non-analytic at the critical point and vanish with increasing transverse field. The momentum-space approach to quantum magic not only complements its real-space counterpart but also provides advantages in terms of analyzing nonstabilizerness and classical simulability.

quant-ph↗