arXiv subjects

Alireza Akbari

Publications and source records attributed to Alireza Akbari.

At least 19 recordsLinked to original sources

arXiv2026

Signatures of spin-wave dynamics in quasiparticle interference

We investigate momentum-resolved quasiparticle interference (QPI) in a localized-itinerant antiferromagnet, where ordered local moments are exchange coupled to conduction electrons. Static antiferromagnetic order reconstructs the electronic bands, while one-magnon processes further dress the quasiparticles through a momentum- and frequency-dependent self-energy. Incorporating this dynamical renormalization directly into the Born impurity scattering, we find a characteristic crossover in the QPI spectrum. Below the lower magnon-emission edge, the modification is predominantly dispersive and governed by the real part of the self-energy, whereas above this scale its imaginary part produces pronounced broadening and redistribution of the scattering intensity. The dynamical response persists beyond the upper magnon energy and is strongly asymmetric in tunneling bias for a particle--hole-asymmetric band. These results show that Fourier-transform tunneling spectroscopy can distinguish static magnetic reconstruction from dynamical spin-wave renormalization.

cond-mat.str-el↗

arXiv2026

Kibble--Zurek Mechanism and Defect Freezing in Imbalanced-Pairing Kitaev Models

We investigate driven dynamics across critical and exceptional points in the one- and two-dimensional imbalanced-pairing Kitaev models using both the wave-function normalization approach and the biorthogonal framework. For a positive pairing imbalance parameter, the quasiparticle spectrum remains real, and a pairing imbalance neither shifts the equilibrium phase boundaries nor generates imaginary eigenenergies. In this regime, the defect density follows the conventional Kibble--Zurek scaling in one dimension and the extended Kibble--Zurek scaling, arising from a gapless manifold, in two dimensions within both frameworks. The corresponding scaling exponents are therefore governed by those of the Hermitian transition. For a negative pairing imbalance parameter, time-reversal symmetry is broken, the quasiparticle spectrum develops complex eigenvalues, and the gap closes at exceptional points. For ramps ending at an exceptional point, the defect density follows the modified Kibble--Zurek scaling in the wave-function normalization approach, whereas it obeys the conventional Kibble--Zurek scaling in the biorthogonal framework. When the ramp traverses the time-reversal-symmetry-broken region, a finite density of defects remains even in the adiabatic limit, leading to defect freezing in both frameworks. Although this frozen background indicates a breakdown of adiabaticity, the excess defects generated on top of this background continue to obey the conventional Kibble--Zurek scaling in one dimension and the extended Kibble--Zurek scaling in two dimensions.

cond-mat.stat-mech↗

arXiv2026

Dynamical Quantum Phase Transitions in a Pseudo-Hermitian Hamiltonian: The Imbalanced-Pairing Kitaev Model

Although parity-time (PT)-symmetric Hamiltonians are often associated with real energy spectra, PT symmetry is neither a sufficient nor a necessary condition for a real spectrum. More generally, real spectra are associated with the broader class of pseudo-Hermitian Hamiltonians, of which PT-symmetric Hamiltonians constitute a simple subclass. Here, we investigate the nonequilibrium dynamics of the imbalanced-pairing Kitaev model, a prototypical pseudo-Hermitian system, under a linearly time-dependent chemical potential. The dynamics are analyzed within the biorthogonal framework using the concept of dynamical quantum phase transitions (DQPTs). We show that, under a linear ramp protocol, DQPTs occur only when the post-ramp Hamiltonian possesses a real energy spectrum. For positive values of the non-Hermiticity parameter ($γ>0$), where the energy spectrum remains entirely real, a ramp crossing a single quantum critical point gives rise to a single family of critical times, analogous to the Hermitian case. Furthermore, for ramps crossing two critical or exceptional points, the critical sweep velocity above which DQPTs disappear decreases as the non-Hermiticity parameter is reduced and vanishes in the staggered-pairing limit, $γ=-1$.

quant-ph↗

arXiv2026

Local moment magnon spectrum in conduction electron tunnelling

The surface tunnelling spectrum of a dual system consisting of localised moments with antiferromagnetic order coupled to conduction electrons by on-site exchange interaction is investigated. In the static approximation of the local moment order it is known that magnetic band reconstruction leads to an anomalous tunnelling spectrum at the magnetic ordering vector of local moments although the latter cannot contribute directly. In this work we consider dynamic effects by including the scattering of conduction electrons from the local moment magnon excitations. They lead to self energies and renormalisation of conduction states which in turn appreciably modify the tunnelling spectrum beyond the influence of static order, interpreted as the appearance of magnon sidebands.

cond-mat.str-el↗

arXiv2026

Separation of the Kibble-Zurek Mechanism from Quantum Criticality

When a system is swept through a quantum critical point (QCP), the Kibble-Zurek mechanism predicts that the average number of topological defects follows a universal power-law scaling with the ramp time scale. This scaling behavior is determined by the equilibrium critical exponents of the underlying phase transition. We show that the correspondence between Kibble-Zurek scaling and quantum criticality does not hold generally. In particular, the defect density can exhibit a suppression faster than the Kibble-Zurek prediction even when the quench crosses a critical point, while conventional Kibble-Zurek scaling may persist for quenches through a non-critical point. Our results, based on models representative of a broad class of quasi-one-dimensional Fermi systems, identify the dynamical conditions under which universal defect scaling emerges and clarify the relation between defect generation and equilibrium criticality.

cond-mat.stat-mech↗

arXiv2026

Quantum Correlation Dynamics Subjected to Quantum Reset-Driven Environment

We study two central qubits interacting with a transverse-field Ising chain that serves as their environment. The environment is driven linearly in time across its quantum critical points (QCPs) and, during the evolution, is subjected to quantum reset (QR), where it is returned at random times to its initial state. We investigate how such QR of the environmental spin chain modifies the dynamics of entanglement and quantum discord between the qubits. Our results show that in the strong-coupling regime, entanglement and discord exhibit pronounced revivals within the interval bounded by the Ising QCPs, but these revivals diminish as the QR rate increases. In contrast, weak coupling leads to a monotonic reduction of quantum correlations. Numerically, we find that the revival peaks of concurrence decay and scale exponentially with the QR rate, while quantum discord shows no clear scaling behavior. In the weak-coupling regime without QR, the correlations decay monotonically as the driven field crosses the second QCP. When QR is applied, however, both entanglement and discord undergo oscillatory suppression, with the oscillation period increasing as either the QR rate or the ramp time scale is reduced.

quant-ph↗

arXiv2026

Scaling and Universality at Noise-Affected Non-Equilibrium Spin Correlation Functions

We investigate scaling and universality in nonequilibrium spin correlation functions in the presence of uncorrelated noise. In the absence of noise, spin correlation functions exhibit a crossover from monotonic decay at fast sweep velocities to oscillatory behavior at slow sweeps. We show that, under a stochastically driven field, the critical sweep velocity at which the spin correlation functions undergo an abrupt change decreases with increasing noise strength and scales linearly with the square of the noise intensity. Remarkably, when the noise intensity and sweep velocity are comparable, the excitation probability becomes locked to pk = 1/2 over a finite momentum window, signaling the emergence of noise-induced maximally mixed modes. This gives rise to a highly oscillatory region in the dynamical phase diagram, whose threshold sweep velocity increases with noise and likewise exhibits quadratic scaling with the noise strength. Finally, we identify a universal scaling function under which all boundary sweep-velocity curves collapse onto a single universal curve.

cond-mat.stat-mech↗

arXiv2025

Influence of Fermi Surface Geometry and Van Hove Singularities on the Optical Response of Sr$_2$RuO$_4$

Motivated by the sensitivity of Sr$_2$RuO$_4$ to Fermi surface reconstructions under strain, we investigate how Fermi surface geometry and Van Hove singularities influence the optical Hall response and polar Kerr effect. Within a three-orbital model, we explore the impact of chemical potential and interlayer hopping on superconducting pairing and response functions. We find that $d_{x^2-y^2}$ and $d_{x^2-y^2}+ig$ symmetries are the leading candidates for the quasi-2D orbital, while a chiral $p$-wave state in the quasi-1D orbitals is essential for generating an accessible Kerr angle. The Lifshitz transition is shown to affect coherence factors and density-of-states peaks, producing sharp signatures in $T_c$ and optical transport. Inter-orbital charge transfer further enhances these effects by modifying the balance between quasi-1D and quasi-2D contributions. These results provide a framework for interpreting Kerr effect experiments in multi-orbital superconductors.

cond-mat.supr-con↗

arXiv2025

Dynamics of quantum Fisher and Wigner-Yanase skew information following a noisy quench

We study the effect of noise on the dynamics of the transverse-field Ising model quenched across a quantum critical point. To quantify two-spin correlations, we employ the quantum Fisher information (QFI) and the Wigner-Yanase skew information (WYSI) as measures of quantum coherence. In the noiseless case, in contrast to the dynamics of entanglement in anisotropic XY chains, both QFI and WYSI increase monotonically with the ramp quench time, approaching their adiabatic limits without exhibiting any Kibble-Zurek type scaling with quench duration. In contrast, when noise is added to the quench protocol, the coherence dynamics change qualitatively: QFI and WYSI both decay exponentially with the time scale of a ramp quench, with an exponent determined by the noise intensity. Furthermore, the maximum ramp time, at which either of these measures reach their maximum, scales linearly with the noise variance, featuring the same exponent that determines the optimal annealing time for minimizing defect production in noisy quantum annealing.

quant-ph↗

arXiv2025

Noise-Affected Dynamical Quantum Phase Transitions

We investigate the effects of uncorrelated noise on dynamical quantum phase transitions (DQPTs) in fermionic two-band models following a quantum ramp across critical points. We consider a generalized Loschmidt echo for the noise-averaged density matrix $\barρ$, which is a mixed state in general, as well as the pure state Loschmidt echo calculated for each noise realization with the average performed over the corresponding return rates. $\barρ$ can be obtained from a master equation and we show that for two-band models noise destroys its coherences which typically drives $\barρ$ towards the completely mixed state which is an attractive fixed point. DQPTs are thus always smoothed out for finite noise. For single noise realizations, on the other hand, we find that DQPTs under certain conditions are always present irrespective of the noise level. This leads to remarkable stable though slightly broadened DQPT-like features in the averaged return rate. We illustrate our results for the XY model by considering a noisy ramp as well as noise in the energy levels of the final Hamiltonian.

cond-mat.stat-mech↗

arXiv2025

Scaling and Universality at Noisy Quench Dynamical Quantum Phase Transitions

Dynamical quantum phase transitions (DQPTs) have been studied in the extended XY model under both noiseless and noisy linear driven staggered field cases. In the time-independent staggered field case, the model exhibits a single critical point where the transition occurs from the spin-liquid phase to the antiferromagnetic phase. In the noiseless ramp case, unlike the transverse field XY model where DQPT always occurs for a quench crossing the single critical point, there is a critical sweep velocity above which the kinks corresponding to a DQPT are completely removed. Furthermore, in this case there are only two critical modes whose excitation probability is one-half. In the presence of a Gaussian white noise, we find that this critical sweep velocity decreases by increasing the noise strength, and scales linearly with the square of the noise intensity. A surprising result occurs when the noise intensity and sweep velocity are about the same order of magnitude, the number of critical modes is significantly increased, signalling a region with multiple critical modes. Furthermore, our findings indicate that the scaling of the dynamical free energy near the DQPTs time is the same for both noiseless and noisy ramp quenches.

cond-mat.stat-mech↗

arXiv2025

Dynamical Phase diagram of the Quantum Ising model with Cluster Interaction Under Noisy and Noiseless Driven field

In most lattice models, gap closing typically occurs at high-symmetry points in the Brillouin zone. In the transverse field Ising model with cluster interaction, besides the gap closing at high-symmetry points, the gap closing at the quantum phase transition between paramagnetic and cluster phases of the model can be moved by tuning the strength of the cluster interaction. We take advantage of this property to examine the nonequilibrium dynamics of the model in the framework of dynamical quantum phase transitions (DQPTs) after a noiseless and noisy ramp of the transverse magnetic field. The numerical results show that DQPTs always happen if the starting or ending point of the quench field is restricted between two critical points. In other ways, there is always critical sweep velocity above which DQPTs disappear. Our finding reveals that noise modifies drastically the dynamical phase diagram of the model. We find that the critical sweep velocity decreases by enhancing the noise intensity and scales linearly with the square of noise intensity for weak and strong noise. Moreover, the region with multi-critical modes induced in the dynamical phase diagram by noise. The sweep velocity under which the system enters the multi-critical modes (MCMs) region increases by enhancing the noise and scales linearly with the square of noise intensity

cond-mat.stat-mech↗

arXiv2025

Image of helical local moment magnetic order in the STM spectrum

The surface tunneling microscope (STM) method probes the itinerant conduction electron spectrum which is influenced by the presence of collective order parameters. It may in fact be used as a tool to obtain important information about their microscopic nature, for example the gap symmetry in unconventional superconductors. Surprisingly it has been found that the STM spectrum can also identify magnetic order of completely localised electrons, e.g., incommensurate helical structure of 4f electron moments, as observed in the compound GdRu$_2$Si$_2$. This is due to the fact that the exchange coupling of conduction states to the localised subsystem reconstructs the itinerant bands which then leaves an imprint on the STM spectrum. We develop a theory based on this idea that shows firstly the appearance of STM satellite peaks at the wave vector of localised moment helical order for the pure surface. Secondly we derive the quasiparticle interference spectrum in Born approximation due to the presence of surface impurities which contains information on the reconstruction process of itinerant states caused by the localised helical order. Furthermore we show that within full $t$-matrix approach impurity bound states are also influenced by the exchange coupling to helical magnetic order.

cond-mat.str-el↗

arXiv2025

Dynamics of decoherence in a noisy driven environment

We analyze the decoherence dynamics of a central spin coupled to a spin chain with a time-dependent noisy magnetic field, focusing on how noise influences the system's decoherence. Our results show that decoherence due to the nonequilibrium critical dynamics of the environment is amplified in the presence of uncorrelated and correlated Gaussian noise. We demonstrate that decoherence factor consistently signals the critical points, and exhibits exponential scaling with the system size, the square of noise intensity, and the noise correlation time at the critical points. We find that strong coupling between the qubit and the environment leads to partial revivals of decoherence, which diminish with increasing noise intensity or decreasing noise correlation time. In contrast, weak coupling leads to monotonic enhanced decoherence. The numerical results illustrate that, the revivals decay and scale exponentially with noise intensity. Moreover, the revivals increase and indicate linear or power law scaling with noise correlation time depends on how the correlated noise is fast or slow. Additionally, we explore the non-Markovianity of the dynamics, finding that it decays in the presence of noise but increases as the noise correlation time grows.

quant-ph↗

arXiv2025

Thermodynamics, elastic anomalies and excitations in the field induced phases of CeRh2As2

The tetragonal heavy fermion compound CeRh2As2 exhibits unconventional superconductivity accompanied by other broken symmetry phases that have been identified as presumably small moment intrinsic antiferromagnetism at low magnetic fields and induced quadrupolar order at higher in-plane fields. The latter may extend to very large pulsed-field range. The phase boundaries can be investigated by following thermodynamic anomalies like specific heat, magnetocaloric coefficient, thermal expansion and magnetostriction. We calculate their discontinuities and identify the influence of the field induced quadrupole on them. Furthermore we investigate the elastic constant anomalies which are determined by the static homogeneous quadrupolar RPA response functions. We present a calculation of these anomalies for the appropriate symmetry mode both in the disordered and ordered regime and investigate their change with applied field. In addition we consider the dynamical momentum dependent magnetic susceptibility and the associated dispersion of low energy magnetic modes and how their characteristics change across the phase boundary.

cond-mat.str-el↗

arXiv2025

Unprecedentedly large gap in HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ with the highest $T_c$ at ambient pressure

In cuprate superconductors, the highest $T_c$ is possessed by the HgBa$_2$Ca$_2$Cu$_3$O$_{8+δ}$ (Hg-1223) system at ambient pressure, but the reason remains elusive. Here we report the scanning tunneling measurements on the Hg-1223 single crystals with $T_c$ = 134 K. The observed gaps determined from the tunneling spectra (STS) can be categorized into two groups: the smaller gap $Δ_1$ ranges from about 45 to 70 meV, while the larger gap $Δ_2$ from about 65 to 98 meV. The STS was measured up to 200 K and the larger gap can persist well above $T_c$, indicating a pseudogap feature which may reflect the strong pairing energy in the inner layer. Interestingly, an extremely strong particle-hole asymmetry is observed in associating with a very robust coherence-like peak at the bias of the larger gap in the hole branch of the Bogoliubov dispersion. We argue that the observed asymmetry results may be from the interplay of a flat band (van Hove singularity) in the electronic spectrum and the large gap in the underdoped (inner) layer. A theoretical approach based on a trilayer model with an interlayer coupling can give a reasonable explanation. Our results provide deep insight into understanding the mechanism of superconductivity in cuprate superconductors.

cond-mat.supr-con↗

arXiv2024

Stacking of charge-density waves in 2H-NbSe$_2$ bilayers

We employ ab-initio electronic structure calculations to investigate the charge-density waves and periodic lattice distortions in bilayer 2H-NbSe$_2$. We demonstrate that the vertical stacking can give rise to a variety of patterns that may lower the symmetry of the charge-density waves exhibited separately by the two composing 1H-NbSe$_2$ monolayers. The general tendency to a spontaneous symmetry breaking observed in the ground state and the first excited states is shown to originate from a non-negligible inter-layer coupling. Simulated images for scanning tunnelling microscopy (STM) as well as diffraction/scattering patterns show signatures of the different stacking orders. This may not only be useful to reinterpret past experiments on surfaces and thin films, but may also be exploited to devise ad-hoc experiments for the investigation of the stacking order in 2H-NbSe$_2$. We anticipate that our analysis does not only apply to the 2H-NbSe$_2$ bilayer, but is also relevant for thin films and bulk, whose smallest centro-symmetric component is indeed the bilayer. Finally, our results illustrate clearly that the vertical stacking is not only important for 1T structures, as exemplified by the metal-to-insulator transition observed in 1T-TaS$_2$, but seems to be a general feature of metallic layered transition metal dichalcogenides as well.

cond-mat.str-el↗

arXiv2024

PANORAMIA: Privacy Auditing of Machine Learning Models without Retraining

We present PANORAMIA, a privacy leakage measurement framework for machine learning models that relies on membership inference attacks using generated data as non-members. By relying on generated non-member data, PANORAMIA eliminates the common dependency of privacy measurement tools on in-distribution non-member data. As a result, PANORAMIA does not modify the model, training data, or training process, and only requires access to a subset of the training data. We evaluate PANORAMIA on ML models for image and tabular data classification, as well as on large-scale language models.

cs.CR↗