In this study, we explore the influence of interlayer exchange coupling on magnetization reversal triggered by femtosecond laser pulses in ferromagnetic spin valves. Our experiments, focused on femtosecond laser-induced magnetization reversal, methodically vary the thickness of the copper (Cu) spacer layer. We identify a critical Cu thickness threshold at 2.32 nm. Above this threshold, a stable reversed magnetic domain is consistently generated upon exposure to a single laser pulse. Conversely, with a Cu spacer thinner than 2.32 nm, the observed magnetization reversal from parallel (P) to antiparallel states occurs only under continuous laser irradiation. Once the laser is stopped, the magnetic configuration relaxes back to its initial P state, influenced by ferromagnetic exchange coupling. This research enhances our understanding of the mechanisms that drive optically induced ultrafast magnetization reversal in ferromagnetic spin valves.

The quest for exotic quantum magnetic ground states, including the Kitaev spin liquid and quantum spin ices, has led to the discovery of several quantum materials where low-energy pseudospin-$1/2$ doublets arise from the splitting of spin-orbit entangled multiplets with higher degeneracy. Such systems include $d$-orbital and $f$-orbital Mott insulators. When the gap between the low-energy pseudospin-$1/2$ levels and the excited levels of the multiplet or “excitons” is not large, the effective low-energy exchange interactions between the low-energy pseudospin-$1/2$ moments can acquire significant corrections from coupling to the excitons. We extract these corrections using an extended perturbation theory that takes into account the most relevant higher-order perturbations. Such corrections can impact the exchange matrix for the low-energy pseudospin-$1/2$ levels by renormalizing the strength and the sign of Heisenberg exchange or Ising anisotropies, and potentially even inducing bond-anisotropic couplings such as Kitaev-$\mathrm{\Gamma }$ exchange interactions. We discuss recent experiments on various cobaltate and osmate materials, which hint at the ubiquity and importance of this physics. Our study suggests a distinct direction in the search for exotic spin liquids—quantum pseudospin-1/2 systems with low-energy spin-orbit excitons.

We investigate the intralayer correlations acting within the layers in a superfluid system of electron-hole spatially separated layers. In this system, superfluidity is predicted to be almost exclusively confined to the Bose-Einstein condensate (BEC) and crossover regimes where the electron-hole pairs are well localized. In this case, Hartree-Fock is an excellent approximation for the intralayer correlations. We find in the BEC regime that the effect of the intralayer correlations on superfluid properties is negligible but in the BCS-BEC crossover regime the superfluid gap is significantly weakened by the intralayer correlations. This is caused by the intralayer correlations boosting the number of low-energy particle-hole excitations that drive the screening. We further find that the intralayer correlations suppress the predicted phenomenon in which the average pair size passes through a minimum as the crossover regime is traversed.

In this work, we study the interaction between vortices and nematic domain walls within the framework of a Ginzburg-Landau approach. The free energy of the system is written in terms of a complex order parameter characteristic of $s$-wave superconductivity and a real (Ising-type) order parameter associated with nematicity. The interaction between both order parameters is described by a biquadratic and a trilinear derivative term. To study the effects of these interactions, we solve the time-dependent dissipative Ginzburg-Landau equations using a highly effective pseudospectral method by which we calculate the trajectories of a vortex that, for different coupling parameters, is either attracted or repelled by a wall, as well as of the wall dynamics. We show that despite its simplicity, this theory displays many phenomena observed experimentally in Fe-based superconductors. In particular, we find that the sign of the biquadratic term determines the attractive (pinning) or repulsive (antipinning) character of the interaction, as observed in FeSe and BaFeCoAs compounds, respectively. The trilinear term is responsible for the elliptical shape of vortex cores as well as the orientation of the axes of the ellipses and vortex trajectories with respect to the axes of the structural lattice. For the case of pinning, we show that the vortex core is well described by a heart-shaped structure in agreement with scanning tunneling microscopy experiments performed in FeSe.

We use magnetoresistance measurements at high magnetic field (${\mu }_{0}H\le 65$ T) and low temperature ($T\ge 500$ mK) to gain fresh insights into the behavior of the upper critical field $H_{c2}$ in superconducting ultrathin FeSe films of varying degrees of disorder, grown by molecular beam epitaxy on ${\mathrm{SrTiO}}_3$. Measurements of $H_{c2}$ across samples with a widely varying superconducting critical temperature (1.2 K $\le T_c\le 21$ K) generically show similar qualitative temperature dependence. We analyze the temperature dependence of $H_{c2}$ in the context of Werthamer-Helfand-Hohenberg (WHH) theory. The analysis yields parameters that indicate a strong Pauli paramagnetic pair-breaking mechanism which is also reflected by pseudoisotropic superconductivity in the limit of zero temperature. In the lower $T_c$ samples, we observe a spin-orbit scattering-driven enhancement of $H_{c2}$ above the strongly-coupled Pauli paramagnetic limit. We also observe clear deviations from WHH theory at low temperature, regardless of $T_c$. We attribute this to the multiband superconductivity of FeSe and possibly to the emergence of a low-temperature, high-field superconducting phase.

Silicon carbide, a semiconducting material, has gained importance in the fields of ceramics, electronics, and renewable energy due to its remarkable hardness and resistance. In this study, we delve into the impact of nuclear quantum motion, or vibrational mode quantization, on the structural and elastic properties of $3C\text{−}\mathrm{SiC}$. This aspect, elusive in conventional ab initio calculations, is explored through path-integral molecular dynamics (PIMD) simulations using an efficient tight-binding (TB) Hamiltonian. This investigation spans a wide range of temperatures and pressures, including tensile stress, adeptly addressing the quantization and anharmonicity inherent in solid-state vibrational modes. The accuracy of the TB model has been checked by comparison with density-functional-theory calculations at zero temperature. The magnitude of quantum effects is assessed by comparing PIMD outcomes with results obtained from classical molecular dynamics simulations. Our investigation uncovers notable reductions of 5%, 10%, and 4% in the elastic constants $C_{11}$, $C_{12}$, and $C_{44}$, respectively, attributed to atomic zero-point oscillations. Consequently, the bulk modulus and Poisson's ratio of $3C\text{−}\mathrm{SiC}$ exhibit reduced values by 7% and 5% at low temperatures. The persistence of these quantum effects in the material's structural and elastic attributes beyond room temperature underscores the necessity of incorporating nuclear quantum motion for an accurate description of these fundamental properties of SiC.

Cryogenic thermal cycling (CTC) treatment is a promising technique to rejuvenate metallic glasses (MGs), but whose effect is usually highly dependent on the original structure. Spanning shear bands and nonspanning shear bands in deformed MGs not only have different structures, but also are situated in different strain fields, so their responses to CTC are expected to be different. In the present work, the structure, energy state, and property evolutions of the spanning and nonspanning shear bands in deformed MGs during the CTC process were investigated by experiments and molecular dynamics simulations. It was found that CTC increased the proportion of atoms with inelastic strain and induced a rejuvenation in the immature shear band zones that nevertheless led a relaxation in the mature shear band zones. This work not only deepens the understanding of the rejuvenation mechanism of deformed MGs, but also provides a unique perspective for improving their mechanical properties.

We theoretically investigate a two-dimensional decorated honeycomb lattice framework to realize a second-order topological magnon insulator (SOTMI) phase featuring distinct corner-localized modes. Our study emphasizes the pivotal role of spin-magnon mapping in characterizing bosonic topological properties, which exhibit differences from their fermionic counterparts. We employ a symmetry indicator topological invariant to identify and characterize this SOTMI phase, particularly for systems respecting time-reversal and ${\mathsf{C}}_6$ rotational symmetry. Using a spin model defined on a honeycomb lattice geometry, we demonstrate that introducing “kekulé” type distortions yields a topological phase. In contrast, “antikekulé” distortions result in a nontopological magnonic phase. The presence of kekulé distortions manifests in two distinct topologically protected bosonic corner modes—an intrinsic and a pseudo, based on the specific edge terminations. On the other hand, antikekulé distortions give rise to Tamm/Shockley type bond-localized boundary modes, which are nontopological and reliant on particular edge termination. We further investigate the effects of random out-of-plane exchange anisotropy disorder on the robustness of these bosonic corner modes. The distinction between SOTMIs and their fermionic counterparts arises due to the system-specific magnonic onsite energies, a crucial feature often overlooked in prior literature. Our study unveils exciting prospects for engineering higher-order topological phases in magnon systems and enhances our understanding of their unique behavior within decorated honeycomb lattices.

The magnetization, magnetostriction, and electric polarization of ${\mathrm{CaBaCo}}_4{\mathrm{O}}_7$ single-crystal samples were investigated under static and pulsed magnetic fields. Experimental results show that the ferrimagnetic order and metamagnetic transition occur in a lower-temperature region. At 4.2 K, with the increase of the applied magnetic field, there is a transition from multidomain to single domain at about 2 T, and then with the further increase of the magnetic field, the spin of cobalt ions (Co1 and Co4 ions) changes from noncollinear to collinear. Along with this magnetization process, magnetostriction also exhibits two continuous processes: magnetostriction related to magnetic domain rotation and magnetostriction related to the increase of spin collinearity. Especially in pulsed magnetic fields, the magnetization, magnetostriction, and polarization transitions induced by magnetic order occur almost in the same critical magnetic field. We assume that in the system, the magnetic order can further polarize the orbital via spin-orbit interaction, resulting in strong magnetostriction.

${\mathrm{UTe}}_2$ is a strong candidate for a topological spin-triplet superconductor, and it is considered that the interplay of magnetic fluctuation and superconductivity is essential for the origin of the superconductivity. Despite various experiments suggesting ferromagnetic criticality, neutron scattering measurements observed only antiferromagnetic fluctuation and called for theories of spin-triplet superconductivity near the antiferromagnetic quantum critical point. We construct a periodic Anderson model with one-dimensional conduction electrons and two- or three-dimensional $f$ electrons, reminiscent of the band structure of ${\mathrm{UTe}}_2$, and show that ferromagnetic and antiferromagnetic fluctuations are reproduced depending on the Fermi surface of $f$ electrons. These magnetic fluctuations cooperatively stabilize spin-triplet $p$-wave superconductivity. We also study hybridization dependence as a possible origin of the pressure-induced superconducting phase transition and find that moderately large hybridization changes the antiferromagnetic wave vector and stabilizes $d$-wave superconductivity.

Measurements of the temperature-dependent ac magnetic susceptibility of superconducting films reveal reversible responses, i.e., irrespective of the magnetic and thermal history of the sample. This experimental fact is observed even in the presence of stochastic and certainly irreversible magnetic flux avalanches, which, in principle, should randomly affect the results. In this work, we explain such a paradoxical result by exploiting the spatial resolution of magneto-optical imaging. To achieve this, we successfully compare standard frequency-independent first-harmonic ac magnetic susceptibility results for a superconducting thin film with those obtained by ac-emulating magneto-optical imaging (acMOI). To demonstrate the possibilities of the acMOI technique, we further explore the experimental data. A quantitative analysis provides information regarding flux avalanches, reveals the presence of a vortex-antivortex annihilation zone in the region in which a smooth flux front interacts with preestablished avalanches, and demonstrates that the major impact on the flux distribution within the superconductor happens during the first ac cycle. Our results establish acMOI as a reliable approach for studying frequency-independent ac field effects in superconducting thin films while capturing local aspects of flux dynamics, otherwise inaccessible via global magnetometry techniques.

We study the finite-size effects on the BCS-BEC crossover in two dimensions, occurring in confined fermionic superfluids. We analyze several thermodynamic properties, such as the chemical potential, the energy gap, and the superfluid density, taking into account unavoidable quantum fluctuations, and, by means of the renormalization group procedure, we detect the putative Berezinskii-Kosterlitz-Thouless phase transition at finite size.

Inspired by the recent discovery of a successive evolutions of electronically ordered states in the vanadium-based kagome superconductors, we present a self-consistent theoretical analysis that treats the interactions responsible for the chiral charge order and superconductivity on an equal footing. It is revealed that the self-consistent theory captures the essential features of the successive temperature evolutions of the electronic states from the high-temperature “triple-$Q$” $2\times 2$ charge-density-wave state to the nematic charge-density-wave phase, and finally to the low-temperature superconducting state coexisting with the nematic charge density wave. We provide a comprehensive explanation for the temperature evolutions of the charge ordered states and discuss the consequences of the intertwining of the superconductivity with the nematic charge density wave. Our findings not only account for the successive temperature evolutions of the ordered electronic states discovered in experiments but also provide a natural explanation for the twofold rotational symmetry observed in both the charge-density-wave and superconducting states. Moreover, the intertwining of the superconductivity with the nematic charge-density-wave order may also be an advisable candidate to reconcile the divergent or seemingly contradictory experimental outcomes regarding the superconducting properties.

We investigate if and how the valence-bond-solid (VBS) state emerges in the Hubbard model on the honeycomb lattice when the Peierls-type electron-lattice coupling is introduced. We consider all possible lattice-distortion patterns allowed for this lattice model for graphene which preserve the reflection symmetry and determine the most stable configuration in the adiabatic limit by using an unbiased quantum Monte Carlo method. The VBS phase with Kekulé dimerization is found to appear as an intermediate phase between a semimetal and an antiferromagnetic Mott insulator for a moderately rigid lattice. This implies that the undistorted semimetallic graphene can be driven into the VBS phase by applying strain, accompanied by the single-particle excitation gap opening.

We study the logarithmic entanglement negativity of symmetry-protected topological (SPT) phases and quantum critical points (QCPs) of one-dimensional noninteracting fermions at finite temperatures. In particular, we consider a free fermion model that realizes not only quantum phase transitions between gapped phases but also an exotic topological phase transition between quantum critical states in the form of the fermionic Lifshitz transition. The bipartite entanglement negativity between adjacent fermion blocks reveals the crossover boundary of the quantum critical fan near the QCP between two gapped phases. Along the critical phase boundary between the gapped phases, the sudden decrease in the entanglement negativity signals the fermionic Lifshitz transition responsible for the change in the topological nature of the QCPs. In addition, the tripartite entanglement negativity between spatially separated fermion blocks counts the number of topologically protected boundary modes for both SPT phases and topologically nontrivial QCPs at zero temperature. However, the long-distance entanglement between the boundary modes vanishes at finite temperatures due to the instability of SPTs, the phases themselves.

The study of the interaction between superconductivity and charge ordering is helpful to resolve the pairing mechanism in high-temperature superconductors. Recently, several resistance oscillations studies triggered the speculation that an ultra-long-range charge ordering, with an enormous mesh size of several tens of nanometers (>$10a_0$), can possibly emerge in underdoped high-$T_c$ superconductors. However, spectroscopy studies have not traced this kind of ultra-long-range charge ordering. Here, we clarify the disagreement between the transport and spectroscopy studies on the mysterious ultra-long-range charge ordering by investigating the magneto-oscillations in underdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ flakes. Inspired by the observation that the oscillations evolve from periodic to aperiodic with decreasing doping level, we conclude that the magneto-oscillations can be generated by the interaction between vortices and superconducting loops that enclose randomly distributed underdoped puddles while an assumption of ultra-long-range charge ordering is not necessary.

We demonstrate how to devise a Matsubara-formalism-based one-loop approximation to the flow of the functional renormalization group (FRG) that reproduces identically the leading-logarithmic parquet approximation. This construction of a controlled fermionic FRG approximation in a regime not accessible by perturbation theory generalizes a previous study from the real-time zero-temperature formalism to the Matsubara formalism and thus to the de facto standard framework used for condensed-matter FRG studies. Our investigation is based on a simple model for the absorption of x rays in metals. It is a core part of our construction to exploit that in a suitable leading-logarithmic approximation the values of the particle-hole susceptibility on the real- and on the imaginary-frequency axes are identical.

Developing an accurate kinetic energy density functional (KEDF) remains a major hurdle in orbital-free density functional theory. We propose a machine-learning-based physical-constrained nonlocal (MPN) KEDF and implement it with the usage of the bulk-derived local pseudopotentials and plane wave basis sets in the abacus package. The MPN KEDF is designed to satisfy three exact physical constraints: the scaling law of electron kinetic energy, the free electron gas limit, and the non-negativity of Pauli energy density. The MPN KEDF is systematically tested for simple metals, including Li, Mg, Al, and 59 alloys. We conclude that incorporating nonlocal information for designing new KEDFs and obeying exact physical constraints are essential to improve the accuracy, transferability, and stability of ML-based KEDF. These results shed new light on the construction of ML-based functionals.

A detailed understanding of the full measurement statistics in experiments is the basis for thorough device characterization and advanced applications such as quantum metrology. In this spirit, we investigate the conditional counting statistics of an electron-spin-resonance pumped quantum dot (QD), which is tunnel-coupled to a side electrode and continuously monitored by a quantum point contact (QPC). For such a noninvasive detector, we find that the spin current through the QD and its related cumulants exhibit intriguing features and system parameter dependences when conditioned on the QPC current, which are in strong contrast to the unconditional cumulants. By turning the perspective around and considering the counting statistics of the QPC current conditioned on the QD current, we reveal an anomalous conditional QPC current noise, which is surprisingly suppressed for increasing coupling to the environment. Our results show that information backaction can substantially render statistical transport fluctuations. This unique advantage of the conditional counting statistics in probing the underlying correlations between system and detector, even for noninvasive measurements, can thus be employed for improving measurement protocols in quantum metrology.

We study cavity-mediated interactions that are generated in a two-dimensional two-band Hubbard model coupled to an optical cavity, when it is driven in-gap by a strong laser. Starting from a Floquet description of the driven system, we derive effective low-energy Hamiltonians by projecting out the high-energy degrees of freedom and treating intrinsic interactions on a mean-field level. We then investigate how the emergence of high-energy Frenkel excitons from the electronic interband coupling, which form near the upper electronic band, affects the interactions as well as the laser-induced Floquet renormalization of the electronic band structure. Cavity-mediated interactions are enhanced strongly when the light couples to an excitonic transition. Additionally, the interaction as well as the Floquet renormalization are strongly broadened in reciprocal space, which could further boost the impact of cavity-mediated interactions on the driven-dissipative steady state.