The generation of entanglement in mixed states is relevant to quantum systems coupled to an environment. The dissipative and mixing properties of the environment are unavoidable in physical platforms for quantum simulation and information processing, where entanglement can be a vital resource. In this work, we explore entanglement and heterogeneity in random Lindbladian dynamics describing open quantum systems. We propose a model of a one-dimensional chain of noninteracting, spinless fermions coupled to a local ensemble of baths. The jump operator mediating the interaction with the bath linked to each site has a power-law tail with an exponent $p$. We show that the system undergoes volume-to-area law entanglement phase transition in the mixed steady state by tuning $p$ which remains stable in the presence of coherent hopping. Unlike the entanglement transition in the pure-state quantum trajectories of open systems, this transition is exhibited by the averaged steady-state density matrix of the Lindbladian. The steady state in the area-law phase is characterized by a spatial heterogeneity in local population imbalance, while the jump operators exhibit a constant participation ratio of the sites they affect. Our work provides a theoretical description of an entanglement transition realized in long-ranged open quantum systems and provides an avenue to stabilize quantum correlations in mixed states.

The real mechanism of ferromagnetism in diluted magnetic semiconductors (DMSs), in particular in GaMnAs, is not yet fully understood. The well-known Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, mediated by itinerant charge carriers, is accepted to be responsible for the ferromagnetism in GaMnAs in its metallic state. At the same time, GaMnAs can be realized in an insulating state when there are no mobile carriers, and holes are localized at acceptors. At not too low Mn concentration, the overlap of acceptor-wave-function tails can be sufficient, and Mn $d$ shells can be aligned by means of indirect mechanism mediated by localized holes. An increase in the content of Mn ions leads to their incorporation into interstitial positions, which in turn leads to self-compensation, i.e., the number of localized holes per paramagnetic center decreases. This means that the pairwise interaction of $d$ shells can be mediated by two holes as well as single hole. In order to study the microscopic mechanism of indirect exchange, the energy and spin structure of molecule-like complexes with one and two charge carriers, localized by field of two paramagnetic ions, is investigated. It is shown that at short interionic distances (high concentration of magnetic component) the established mechanisms of indirect exchange resemble the well-known mechanisms of double- and superexchange with ferro- and antiferromagnetic alignment of magnetic moments, respectively. However, these results that can be found perturbatively, are significantly modified when the interionic distance $R$ increases. Our calculations show that the magnetic ordering in insulating DMS crystal significantly depends not only on the concentration of magnetic component but on the compensation degree as well: cases of low or total compensation correspond to the absence of any magnetic ordering, whereas, the pronounced ferromagnetism is realized at a compensation degree close to a half. The simple estimate for the Curie temperature is derived in the limit of half-compensation.

Antiferromagnets show potential as materials for generating perpendicular magnetic anisotropy (PMA) in neighboring ferromagnetic (FM) thin films. In this study, we investigate the behaviors of PMA induction and the underlying mechanisms in a series of epitaxially grown face-centered-cubic-like ${\mathrm{Fe}}_x{\mathrm{Mn}}_{1-x}/\mathrm{Co}/\mathrm{Fe}/\mathrm{Cu}\left(001\right)$ antiferromagnetic (AFM)/FM films. Our research reveals that decreasing the Fe composition ratio enhances the long-range AFM ordering of ${\mathrm{Fe}}_x{\mathrm{Mn}}_{1-x}$ films. When the Fe composition ratio is lower than approximately 0.3, the impact of the AFM ${\mathrm{Fe}}_x{\mathrm{Mn}}_{1-x}$ films on inducing PMA in the adjacent FM layer shifts from enhancement to suppression, providing important clues to a transformation in the spin structures of AFM ${\mathrm{Fe}}_x{\mathrm{Mn}}_{1-x}$ films from a FeMn-like three-dimensional quadratic structure to an Mn-like two-dimensional layered structure. Our findings elucidate the compositional effects of AFM ${\mathrm{Fe}}_x{\mathrm{Mn}}_{1-x}$ films on inducing PMA in the adjacent FM layer, as well as their relationship with the AFM spin structure and long-range AFM ordering, thereby showing the potential of ${\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}$ ultrathin films as promising candidates for effectively generating PMA. Such insights are crucial for understanding the AFM spin configuration of ${\mathrm{Fe}}_x{\mathrm{Mn}}_{1-x}$ alloy films and for furthering the control of PMA using AFM layers.

A phase of matter in which fermion quartets form a superconducting condensate, rather than the paradigmatic Cooper pairs, is a recurrent subject of experimental and theoretical studies. However, a comprehensive microscopic understanding of charge-$4e$ superconductivity as a quantum phase is lacking. Here, we study a two-orbital tight-binding model with attractive Hubbard-type interactions. Such a model naturally provides the Bose-Einstein condensate as a limit for electron quartets and supports charge-$4e$ superconductivity, as we show by mapping it to a spin-1/2 chain in this perturbative limit. Using density matrix renormalization group calculations for the one-dimensional case, we further establish that the ground state is indeed a superfluid phase of $4e$ charge carriers and that this phase can be stabilized well beyond the perturbative regime. Importantly, we demonstrate that $4e$ condensation dominates over $2e$ condensation even for nearly decoupled orbitals, which is a more likely scenario in electronic materials. Our model paves the way for both experimental and theoretical exploration of $4e$ superconductivity and provides a natural starting point for future studies beyond one dimension or more intricate $4e$ states.

We study the superconducting Josephson junction diode operating via the magnetic field of skyrmions. Inspired by the near-field optical microscopy, we propose to partially screen the magnetic field and analyze part by part the magnetic texture of the skyrmion. The detected asymmetric supercurrent is influenced by the skyrmionic magnetic field and magnetic texture. This enables the Josephson junction diode to function as a hyperfine sensor and to read out the information about the morphology of the complex magnetic textures. The proposed setup opens another avenue in magnetometry and represents an alternative to the technologies based on the nitrogen-vacancy centers.

We study cross-relaxation features near zero magnetic field with ensembles of nitrogen-vacancy (NV) centers in diamond and examine their properties in samples with a range of 0.9–16.0 ppm of NV concentrations. The observed NV-NV cross-relaxation features between differently oriented NV centers in high ($\gtrsim 0.9$ ppm)-NV-density samples hold promise for a variety of magnetometry applications where microwave fields (or any bias field) disturb the system under study. We theoretically determine the values of the bias magnetic fields corresponding to cross relaxations between different axes and experimentally validate them. The behavior of zero-field cross-relaxation features as a function of temperature is also investigated.

We consider a quantum many-body lattice system that is coupled to ancillary degrees of freedom (“detectors”), which are periodically measured by means of strong projective measurements. The concentration ${\rho }_a$ of ancillae and their coupling $M$ to the main system are considered as parameters. We explore the dynamics of density and of entanglement entropy in the chain, for various values of ${\rho }_a$ and $M$ for two models of the detector-chain interaction that couple the local density in the chain to a detector degree of freedom. It is found that, for the density-density ($S_z{s}_z$-type in spin language) coupling, the critical values $M_c$ for the measurement-induced entanglement transition depends sensitively on ${\rho }_a$. Moreover, our results indicate that for a sufficiently small ${\rho }_a$ the transition in this model disappears; i.e., a finite density of detectors is needed to reach a disentangling phase. The behavior is qualitatively different for the second model, with density-hopping ($S_z{s}_x$-type) coupling. Specifically, the dynamics is much less sensitive to the concentration ${\rho }_a$ of detectors than in the first model. Furthermore, the dependence of entanglement on the coupling strength $M$ is strongly nonmonotonic, indicating reentrance of the entangling phase at large $M$.

Investigating the time evolution of complexity in quantum systems entails evaluating the spreading of the system's state across a defined basis in its corresponding Hilbert space. Recently, the Krylov basis has been identified as the one that minimizes this spreading. In this study, we develop a numerical exploration of the Krylov complexity in quantum states following a quench in the Lipkin-Meshkov-Glick model. Our results reveal that the long-term averaged Krylov complexity acts as an order parameter for this model. It effectively discriminates between the two dynamic phases induced by the quench, sharing a critical point with the conventional order parameter. Additionally, we examine the inverse participation ratio and the Shannon entropy in both the Krylov basis and the energy basis. A matching dynamic behavior is observed in both bases when the initial state possesses a specific symmetry. This behavior is analytically explained by establishing the equivalence between the Krylov basis and the prequench energy eigenbasis.

We study the stability of the antiferromagnetic all-in–all-out state observed in dipolar-octupolar pyrochlores that have neodymium as the magnetic species. Different types of disorder are considered, either affecting the immediate environment of the ${\mathrm{Nd}}^{3+}$ ion or substituting it with a nonmagnetic ion. Starting from the well-studied ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ compound, Ti substitution on the Zr site and dilution on the Nd magnetic site with La substitution are investigated. The recently discovered entropy stabilized compound NdMox, which exhibits a high degree of disorder on the nonmagnetic site is also studied. Using a range of experimental techniques, especially very low-temperature magnetization and neutron scattering, we show that the all-in–all-out state is very robust and withstands substitutional disorder up to large rates. From these measurements, we estimate the Hamiltonian parameters and discuss their evolution in the framework of the phase diagram of dipolar-octupolar pyrochlore magnets.

It has been recently shown that under pressure trilayer Ruddlesden-Popper nickelate ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ (LNO) becomes superconducting below a critical temperature $\approx 20$ K, in addition to the infinite-layer and bilayer systems. Motivated by this observation, we explore the effects of electron correlations on its electronic structure and magnetic properties using the advanced density functional theory plus dynamical mean-field theory approach. Our results for the normal-state electronic structure and correlation effects in LNO show much in common with the infinite-layer and bilayer nickelates, with remarkable site- and orbital-dependent renormalizations of the Ni $3d$ bands and notable incoherence of the Ni $d_{3z^2-r^2}$ states, caused by correlation effects. Our analysis of the Fermi surface and magnetic correlations suggests the emergence of competing spin and charge stripe states, implying the importance of in-plane spin fluctuations to explain superconductivity in this material.

A bang-bang (BB) algorithm prepares the ground state of a two-dimensional (2D) quantum many-body Hamiltonian $H=H_1+H_2$ by evolving an initial product state alternating between $H_1$ and $H_2$. We use the neighborhood tensor update to simulate the BB evolution with an infinite pair-entangled projected state (iPEPS). The alternating sequence is optimized with the final energy as a cost function. The energy is calculated with a tangent space power method for the sake of its stability. The method is benchmarked in the 2D transverse field quantum Ising model near its quantum critical point against a ground state obtained with variational optimization of the iPEPS. The optimal BB sequence differs nonperturbatively from a sequence simulating quantum annealing or adiabatic preparation (AP) of the ground state. The optimal BB energy converges with the number of bangs much faster than the optimal AP energy.

We develop a theory of Coulomb interaction-related contribution to the photogalvanic current of the carriers of charge in two-dimensional noncentrosymmetric Dirac materials possessing a nontrivial structure of valleys and exposed to an external electromagnetic field. The valley photogalvanic effect occurs here due to the trigonal warping of electrons and holes' dispersions in a given valley of the monolayer. We study the low-frequency limit of the external field: The field frequency is smaller than the temperature $T$, and the electron-electron and electron-hole scattering times are much larger than the electron-impurity and hole-impurity scattering times. In this regime, we employ the Boltzmann transport equations and show that electron-hole scattering dominates electron-electron scattering in intrinsic semiconductors. A Coulomb electron-hole interaction-related contribution to the valley photogalvanic current can reduce the value of the bare photogalvanic current as electron and hole currents flow in opposite directions.

Experiments in Rydberg atoms have recently found unusually slow decay from a small number of special initial states. We investigate the robustness of such long-lived states (LLS) by studying an ensemble of locally constrained random systems with tunable range $\mu$. Upon varying $\mu$, we find a transition between thermal and weakly nonergodic (supporting a finite number of LLS) phases. Furthermore, we demonstrate that the LLS observed in the experiments disappear upon the addition of small perturbations so that the transition reported here is distinct from known ones. We then show that the LLS dynamics explores only part of the accessible Hilbert space, thus corresponding to localization in Hilbert space.

From first-principles calculations we reveal that beryllium has the highest lattice thermal conductivity (${\kappa }_{\mathtt{ph}}$) among all elemental metals at room temperature. Specifically, the calculated ${\kappa }_{\mathtt{ph}}$ is 104(125) ${\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$, contributing $\sim 50$% (60%) to the total thermal conductivity along the $a(c$) axis, contrary to the common belief that ${\kappa }_{\mathtt{ph}}$ is negligible in metals. ${\kappa }_{\mathtt{ph}}$ reach the maxima with values of $\sim 210 {\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$ for both axes at 125 K. The unusually high ${\kappa }_{\mathtt{ph}}$ is related to the weak three-phonon scattering with a dip in the intermediate-frequency region, which arises from its high Debye temperature and bunched phonon dispersions. Another consequence of the weak three-phonon scattering is the strong effect of higher-order (fourth-order) anharmonicity and electron-phonon coupling on ${\kappa }_{\mathtt{ph}}$. We also predict that ${\kappa }_{\mathtt{ph}}$ increases significantly with pressure, mainly due to the weakening of four-phonon scattering, and consequently exceeds the electronic contribution ${\kappa }_{\mathtt{e}}$ by more than one third in both axes at 20 GPa. Our work deepens the understanding of thermal transport in metals, and can benefit the search of metals with high thermal conductivity.

Understanding the impact of electronic correlations and disorder is essential for an accurate description of solids. Here, we study the role of correlations, disorder, and multimagnon processes in THz spin dynamics. We reveal that a significant part of the electron self-energy, which goes beyond the adiabatic local spin density approximation, arises from the interaction between electrons and a virtual magnon gas. This interaction leads to a substantial modification of the exchange splitting and a renormalization of magnon energies, in agreement with the experimental data. Finally, we establish a quantitative hierarchy of magnon relaxation processes based on first principles.

We analyze the transition temperature $T_c$ of bipolaronic superconductivity in a bond Su-Schrieffer-Heeger (bond-SSH) model—also known as a bond Peierls model—where bond phonons modulate the electron hoppings. Using a semiclassical instanton approximation justifiable in the adiabatic limit of slow phonons, we find that the bipolaron mass is only weakly enhanced in contrast to the typically large mass enhancement found in models where the phonons couple to electron density. Strikingly, in the “atomic” limit, we find a bond bipolaron can freely slide within a continuously degenerate energy manifold, explaining the origin of the weak mass enhancement in the strong coupling limit. A gas of these bipolarons can undergo a superfluid transition at a critical temperature for which we obtain an upper bound. This bound is exponentially larger than that in the Holstein model. Our study provides an analytical understanding of the mechanism behind the high-$T_c$ bipolaronic superconductivity numerically observed in [Phys. Rev. X 13, 011010 (2023)].

We theoretically demonstrate nonreciprocity in signal transmission within a three-mode system by exploiting the nonlinear magnetostrictive interaction within a ferrimagnetic material enclosed in a magnomechanical cavity system. This setup encompasses two cavity modes: a magnonic mode and phononic modes. Externally, it is driven by two classical microwave fields: a strong driving magnetic field and a weak probe field. In the cavity setup, a yttrium iron garnet sphere is subjected to a strong magnetic field, significantly affecting the spins within the sphere and leading to deformations in its geometry. These deformations, in turn, induce phonon modes within the sphere. Moreover, three distinct types of nonlinearities are triggered: magnetostriction, magnon self-Kerr, and magnon-phonon cross-Kerr nonlinearities. Additionally, magnons interact with cavity microwave photons through a magnetic dipole interaction. The nonlinear magnetostrictive interaction induces a phase shift in the cavity's photons, causing the breaking of time-reversal symmetry and ultimately resulting in the observed nonreciprocal phenomenon in our hybrid magnomechanical cavity system. Furthermore, by adjusting the magnetostrictive interaction, we can achieve maximal nonreciprocal signal transmission at either port. These results also encompass the significance of the magnon dissipation rate while realizing its nonreciprocity. The detunings of all three modes, that is, two microwave photonic modes and a magnonic mode, are found to be critical for fine tuning the process of nonreciprocal transmission and modulating its characteristics. Since the proposed setup is quite simple, tunable, and experimentally realizable, the proposed configuration and related outcomes could be utilized for applications in quantum communication networks and quantum computing, boost up the sensitivity of microwave detectors, and contribute to advancements in circulators, switching, and other quantum information processing tasks.