Systems with embedded magnetic ions that exhibit a competition between magnetic order and disorder down to absolute zero can display unusual low-temperature behaviors of the resistivity, susceptibility, and specific heat. Moreover, the dynamic response of such a system can display hyperscaling behavior in which the relaxation back to equilibrium when an amount of energy $E$ is given to the system at temperature $T$ only depends on the ratio $E/T$. $\mathrm{Ce}{\left({\mathrm{Fe}}_{0.755}{\mathrm{Ru}}_{0.245}\right)}_2{\mathrm{Ge}}_2$ is a system that displays these behaviors. We show that these complex behaviors are rooted in a fragmentation of the magnetic lattice upon cooling caused by a distribution of local Kondo screening temperatures, and that the hyperscaling behavior can be attributed to the flipping of the total magnetic moment of magnetic clusters that spontaneously form and order upon cooling. We present our arguments based on the review of two-decades worth of neutron scattering and transport data on this system, augmented with new polarized and unpolarized neutron scattering experiments.

We study transport across a magnetic impurity by means of a recently developed slave-spin technique that does not require any constraint. Within a conserving mean-field approximation we find a conductance that displays both the known zero-bias anomaly and also the expected peak at a bias of order $U$. We extend the slave-spin mean-field approximation to study the out-of-equilibrium transient evolution of a quantum dot. We apply the method to investigate the time evolution of a quantum dot induced by a time-dependent electrochemical potential applied to the contacts. Similar to the time-dependent Gutzwiller approximation, the mean-field slave-spin dynamics is able to capture dissipation in the leads, so that a steady state is reached after a characteristic relaxation time.

Kondo physics in doped monolayer graphene is predicted to exhibit unusual features due to the linear vanishing of the pristine material's density of states at the Dirac point. Despite several attempts, conclusive experimental observation of the phenomenon remains elusive. One likely obstacle to identification is a very small Kondo temperature scale $T_K$ in situations where the chemical potential lies near the Dirac point. We propose tailored mechanical deformations of monolayer graphene as a means of revealing unique fingerprints of the Kondo effect. Inhomogeneous strains are known to produce specific alternating changes in the local density of states (LDOS) away from the Dirac point that signal sublattice symmetry-breaking effects. Small LDOS changes can be amplified in an exponential increase or decrease of $T_K$ for magnetic impurities attached at different locations. We illustrate this behavior in two deformation geometries: a circular “bubble” and a long fold, both described by Gaussian displacement profiles. We calculate the LDOS changes for modest strains and analyze the relevant Anderson impurity model describing a magnetic atom adsorbed in either a “top-site” or a “hollow-site” configuration. Numerical renormalization-group solutions of the impurity model suggest that higher expected $T_K$ values, combined with distinctive spatial patterns under variation of the point of graphene attachment, make the top-site configuration the more promising for experimental observation of signatures of the Kondo effect. The strong strain sensitivity of $T_K$ may lift top-site Kondo physics into the range experimentally accessible using local probes such as scanning tunneling microscopy.

Quantum confinement at complex oxide interfaces establishes an intricate hierarchy of the strongly correlated $d$ orbitals which is widely recognized as a source of emergent physics. The most prominent example is the (001) ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (LAO/STO) interface, which features a dome-shaped phase diagram of superconducting critical temperature and spin-orbit coupling (SOC) as a function of electrostatic doping, arising from a selective occupancy of $t_{2g}$ orbitals of different character. Here we study (111)-oriented LAO/STO interfaces, where the three $t_{2g}$ orbitals contribute equally to the subband states caused by confinement, and investigate the impact of this unique feature on electronic transport. We show that transport occurs through two sets of electronlike subbands, and the carrier density of one of the sets shows a nonmonotonic dependence on the sample conductance. Using tight-binding modeling, we demonstrate that this behavior stems from a band inversion driven by on-site Coulomb interactions. The balanced contribution of all $t_{2g}$ orbitals to electronic transport is shown to result in strong SOC with reduced electrostatic modulation.

Dirac materials respond to lattice deformations as if the electrons were coupled to gauge fields. We derive the elastic gauge fields in the hyperhoneycomb lattice, a three-dimensional (3D) structure with trigonally connected sites. In its semimetallic form, this lattice is a nodal-line semimetal with a closed loop of Dirac nodes. Using strain engineering, we find a whole family of strain deformations that create uniform nearly flat Landau levels in 3D. We propose that those Landau levels can be created and tuned in metamaterials with the application of a simple uniaxial temperature gradient. In the 3D quantum anomalous Hall phase, which is topological, we show that the components of the elastic Hall viscosity tensor are multiples of ${\eta }_H={\beta }^2\sqrt{3}/\left(16\pi a^3\right)$, where $\beta$ is an elastic parameter and $a$ is the lattice constant.

Utilizing femtosecond time-resolved pump probe spectroscopy, we investigated the ultrafast quasiparticle dynamics of the ferromagnetic Kondo lattice compound ${\mathrm{CeAgSb}}_2$. The photoinduced reflectivity demonstrates a positive relaxation process connected with simple metal physics at room temperature, while an additional negative decay channel associated with Kondo screening effect can be recognized immediately with temperature decreasing. The temperature-dependent amplitude of the latter exhibits a kink at $T^{*}\approx$ 45 K, which is close to the energy scale of the first excited crystal field splitting Ce $4f$ level. We propose that an orbital crossover is responsible for this intriguing feature. The dominating $4f$ orbitals that participated in the Kodno $c-f$ hybridization varies from the ground state below $T^{*}$ to multiple crystal field splitting levels. In the meantime, the hybridization gap changes from 12.6 to 220.4 K correspondingly, which is also in agreement with the fluence-dependent measurements.

In high-resolution core-valence-valence (CVV) Auger electron spectroscopy from the surface of a solid at thermal equilibrium, the main correlation satellite, visible in the case of strong valence-electron correlations, corresponds to a bound state of the two holes in the final state of the CVV Auger process. We discuss the physical significance of this satellite in nonequilibrium pump-probe Auger spectroscopy by numerical analysis of a single-band Hubbard-type model system, including core states and a continuum of high-energy scattering states. It turns out that the spectrum of the photo-doped system, due to the increased double occupancy, shares features with the equilibrium spectrum at higher fillings. The pumping of doublons can be watched when working with overlapping pulses at short $\mathrm{\Delta }t$. For larger pump-probe delays $\mathrm{\Delta }t$ and on the typical femtosecond timescale for electronic relaxation processes, spectra are hardly $\mathrm{\Delta }t$ dependent, reflecting the high stability of bound two-hole states for strong Hubbard $U$. We argue that taking into account the spatial expansion of single-particle orbitals when these are doubly occupied, as described by the dynamical Hubbard model, produces an oscillation of the barycenter of the satellite as a function of $\mathrm{\Delta }t$. Pump-probe Auger-electron spectroscopy is thus highly sensitive to dynamical screening of the Coulomb interaction.

${\mathrm{Si}}_2\mathrm{BN}$ has been recently predicted theoretically as a new entirely planar two-dimensional material with a honeycomblike structure (like graphene), which is stable even at $T>1000\mathrm{K}$. In the present paper, we study the structural deformations and mechanical properties of ${\mathrm{Si}}_2\mathrm{BN}$ and graphene under both uniaxial (along the direction of the arm chair and zigzag chains) and uniform biaxial tensile strain till the fracture limit and we compare those properties of the two structures with each other. According to our findings, in the ${\mathrm{Si}}_2\mathrm{BN}$ structure, Si–Si and Si–B bonds are weaker than B–N and Si–N bonds, respectively, contrary to graphene bonds, which all have the same strength. In particular, B–N bond lengths of ${\mathrm{Si}}_2\mathrm{BN}$ remain almost unchanged under the strain conditions we studied, not exceeding $\approx 6\%$ of their initial length. ${\mathrm{Si}}_2\mathrm{BN}$ was found to be anisotropic, exhibiting large Young's and biaxial modulus values of the order of 1/3 and 2/5 of that of graphene, respectively. The different bond strengths in ${\mathrm{Si}}_2\mathrm{BN}$ explains its anisotropy and makes it behave very differently under strain when compared to graphene.

We calculate the interference correction to the conductivity of doped transition metal dichalcogenide (TMDC) monolayers. Because of the interplay between valley structure and intrinsic spin-orbit coupling (SOC), these materials exhibit a rich weak localization (WL) behavior that is qualitatively different from conventional metals or similar two-dimensional materials such as graphene. Our results can also be used to describe graphene/TMDC heterostructures, where the SOC is induced in the graphene sheet. We discuss parameter regimes that go beyond existing theories, and can be used to interpret recent experiments in order to assess the strength of SOC and disorder. Furthermore, we show that an in-plane Zeeman field can be used to distinguish the contributions of different kinds of SOC to the WL magnetoconductance.

We investigate the dynamics of the XXZ spin chain after a geometric quench, which is realized by connecting two half chains prepared in their ground states with zero and maximum magnetizations, respectively. The profiles of magnetization after the subsequent time evolution are studied numerically by density-matrix renormalization group methods, and a comparison to the predictions of generalized hydrodynamics yields a very good agreement. We also calculate the profiles of entanglement entropy and propose an ansatz for the noninteracting XX case, based on arguments from conformal field theory. In the general interacting case, the propagation of the entropy front is studied numerically both before and after the reflection from the chain boundaries. Finally, our results for the magnetization fluctuations indicate a leading order proportionality relation to the entanglement entropy.

We study the layered $J_1-J_2$ classical Heisenberg model on the square lattice using a self-consistent bond theory. We derive the phase diagram for fixed $J_1$ as a function of temperature $T, J_2$, and interplane coupling $J_z$. Broad regions of (anti)ferromagnetic and stripe order are found, and are separated by a first-order transition near $J_2\approx 0.5$ (in units of $|J_1|$). Within the stripe phase the magnetic and vestigial nematic transitions occur simultaneously in first-order fashion for strong $J_z$. For weaker $J_z$, there is in addition, for $J_2^{*}<J_2<J_2^{**}$, an intermediate regime of split transitions implying a finite temperature region with nematic order but no long-range stripe magnetic order. In this split regime, the order of the transitions depends sensitively on the deviation from $J_2^{*}$ and $J_2^{**}$, with split second-order transitions predominating for $J_2^{*}\ll J_2\ll J_2^{**}$. We find that the value of $J_2^{*}$ depends weakly on the interplane coupling and is just slightly larger than 0.5 for $|J_z|\lesssim 0.01$. In contrast, the value of $J_2^{**}$ increases quickly from $J_2^{*}$ at $|J_z|\lesssim 0.01$ as the interplane coupling is further reduced. In addition, the magnetic correlation length is shown to directly depend on the nematic order parameter and thus exhibits a sharp increase (or jump) upon entering the nematic phase. Our results are broadly consistent with the predictions based on itinerant electron models of the iron-based superconductors in the normal state and, thus, help substantiate a classical spin framework for providing a phenomenological description of their magnetic properties.

We investigate the dynamical effects of pairing interaction on superconductivity in BCS-BEC crossover by studying the Holstein model at half filling where the electron-phonon coupling $g$ controls the crossover. The dynamical mean-field theory was employed in combination with the numerical renormalization group technique to study the frequency dependence of phonon mediated interaction between electrons in a superconductive state. The frequency dependence induces distinct features such as the absence of the dispersion back bending of Bogoliubov quasiparticles, nonmonotonous coupling dependence of the pairing gap, and the critical temperature $T_c$ being maximum at $g_{c1}$, the lower critical coupling of the normal state insulator to metal transition. Some of these features have intriguing similarities with the recent observations in the ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ and ${\mathrm{Fe}}_{1+y}{\mathrm{Se}}_x{\mathrm{Te}}_{1-x}$ iron chalcogenides in the BCS-BEC crossover.

In the momentum space of two-dimensional (2D) periodic photonic systems, the polarization states of radiating Bloch modes on a single band can form vortex polarization singularities. Here, we study vortex polarization singularities at the band degeneracies. We design a quadratic degeneracy in a plasmonic crystal with $C_{4v}$ symmetry which, by symmetry breaking, splits into a pair of Dirac points with half vortices. The integer vortex singularity at the quadratic degeneracy and the half one at the Dirac degeneracy are measured using polarization-resolved momentum-space imaging spectroscopy. The complete evolution of the splitting from the integer vortex to a pair of half vortices is observed in experiments.

Nanoscale ${\mathrm{CeO}}_2$ (nanoceria) is a prototypical system that presents $d^0$ ferromagnetism. Using a combination of x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and modeling, we show that the nanostructure, defects and disorder, and nonstoichiometry create magnetically polarized Ce $4f$ and O $2p$ hybridized states captured by the vacancy orbitals ($V_{\text{orb}}$) that are vital to ferromagnetism. Further, we demonstrate that foreign ions (Fe and Co) enhance the moment at Ce $4f$ sites while the number of $V_{\text{orb}}$ is unchanged, pointing clearly to the mechanism of orbital hybridization being the key missing ingredient to understanding the unexpected ferromagnetism in many nanoscale dilute magnetic oxides and semiconductors.

The switching of magnetization by current-induced spin-orbit torque (SOT) has potential applications for energy-efficient spintronic devices. In the past, most conventional works have been focused on SOT in heavy metals. Here the SOT from a ferromagnetic metal is investigated, and two mechanisms of the field-free SOT induced magnetization switching are demonstrated to be from the interlayer exchange coupling and the tilted perpendicular magnetic anisotropy. We exclude the spin torque contribution from the anomalous Hall effect and the interfacial Rashba effect combined with spin precession. A spin Hall angle ${\theta }_{\mathrm{SH}}=-0.022$ of CoFeB is obtained by the current-induced hysteresis loop shift method, and the obtained ${\theta }_{\mathrm{SH}}$ is comparable with heavy metals. This work demonstrates that a considerable SOT can come from a ferromagnetic metal, and indicates the unconventional origin of spin-orbit coupling.

We study the anomalous proximity effect in diffusive normal metal (DN)/unconventional superconductor junctions, where the local density of states (LDOS) in the DN has a zero-energy peak due to the penetration of the odd-frequency spin-triplet $s$-wave pairing. In this study, we consider a two-dimensional unconventional superconductor on the substrate in the presence of a Rashba spin-orbit coupling (RSOC) $\lambda$, where the Rashba vector is parallel to the $z$ direction. The anomalous proximity effect, originally predicted in spin-triplet $p$-wave superconductor junctions, is sensitive to the RSOC. It disappears with the increase of $\lambda$. On the other hand, the anomalous proximity effect can be switched on by the large $\lambda$ values in the spin-singlet $d_{xy}$-wave superconductor junctions. The resulting zero-energy LDOS and the magnitude of the odd-frequency spin-triplet $s$-wave pair amplitude increase with the increase of $\lambda$.

Tin sulfide (SnS) single crystals were synthesized using a melt and slow-cooling method. The obtained crystals were characterized by x-ray diffraction, energy-dispersive spectrometry, and by x-ray photoelectron spectroscopy. Electrical transport properties of SnS were investigated under high pressure using a diamond anvil cell with boron-doped metallic diamond electrodes and an undoped diamond insulating layer. We successfully observed an insulator to metal transition between 8.1 and 12.5 GPa and pressure-induced superconductivity with a transition temperature ($T_c$) of 5.8 K under 47.8 GPa as predicted by theoretical calculations. Comparing with the pressure-induced superconductivity in related SnSe compound, a possible $T_c$ enhancement in SnS by an increase of density of states at the Fermi level is expected.

There is a dichotomy in the nonequilibrium dynamics of quantum many-body systems. In the presence of integrability, expectation values of local operators equilibrate to values described by a generalized Gibbs ensemble, which retains extensive memory about the initial state of the system. On the other hand, in generic systems such expectation values relax to stationary values described by the thermal ensemble, fixed solely by the energy of the state. At the heart of understanding, this dichotomy is the eigenstate thermalization hypothesis (ETH): individual eigenstates in nonintegrable systems are thermal, in the sense that expectation values agree with the thermal prediction at a temperature set by the energy of the eigenstate. In systems where ETH is violated, thermalization can be avoided. Thus, establishing the range of validity of ETH is crucial in understanding whether a given quantum system thermalizes. Here, we study a simple model with confinement, the quantum Ising chain with a longitudinal field, in which ETH is violated. Despite an absence of integrability, there exist rare (nonthermal) states that persist far into the spectrum. These arise as a direct consequence of confinement: pairs of particles are confined, forming new “meson” excitations whose energy can be extensive in the system size. We show that such states are nonthermal in both the continuum and in the low-energy spectrum of the corresponding lattice model. We highlight that the presence of such states within the spectrum has important consequences, with certain quenches leading to an absence of thermalization and local observables evolving anomalously.