The non-Hermitian skin effect, namely, that the eigenvalues and eigenstates of a non-Hermitian tight-binding Hamiltonian have significant differences under open or periodic boundary conditions, is a remarkable phenomenon of non-Hermitian systems. Inspired by the presence of the non-Hermitian skin effect, we study the evolution of wave packets in non-Hermitian systems, which can be determined using the single-particle Green's function. Surprisingly, we find that in the thermodynamic limit, the Green's function does not depend on boundary conditions, despite the presence of skin effect. We provide a general proof for this statement in arbitrary dimension with finite hopping range, with an explicit illustration in the non-Hermitian Su-Schrieffer-Heeger model. We also explore its applications in noninteracting open quantum systems described by the master equation. We demonstrate that the evolution of the density matrix is independent of the boundary condition.

Using dynamical Hartree-Fock mean-field theory, we study the high harmonic generation (HHG) in the fullerene molecules ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$ under strong pump wave driving. We consider a strong-field regime and show that the output harmonic radiation exhibits multiple plateaus, whose borders are defined by the molecular excitonic lines and cutoff energies within each plateau scale linearly with the field strength amplitude. In contrast to atomic cases for the fullerene molecule, with the increase of the pump wave photon energy the cutoff harmonic energy is increased. We also show that with the increase of the electron-electron interaction energy overall the HHG rate is suppressed. We demonstrate that the ${\mathrm{C}}_{70}$ molecule shows richer HHG spectra and a stronger high harmonic intensity than the ${\mathrm{C}}_{60}$.

The atomic structure of well-ordered single steps on the $\mathrm{Si}\left(111\right)-\sqrt{3}\times \sqrt{3}-\mathrm{Sn}$ surface and the dynamics of Sn adatoms in the vicinity of these steps was studied. The work was performed using low-temperature scanning tunneling microscopy (STM) and ab initio calculations based on the density-functional theory. The STM tip was used to record the tunneling current versus time on top of oscillating adatoms, keeping the feedback loop turned off. The dynamics of adatoms, detected as the telegraph noise present in the tunneling current, was registered near the steps at 80 K. The atomic structure model of the single steps consisting of Sn atomic chains along the steps was developed. This structure leads to the formation of potential double wells near the steps acting as traps for Sn atoms and explains the fluctuating current recorded in these areas.

In this study, we use first-principles calculations to investigate the electronic and structural properties of $\text{Mo}{X}_2$ $(X=\text{S}, \mathrm{Se}, \mathrm{Te})$ monolayers doped with substitutional Sb atoms, with a central focus on the Sb(Mo) substitution. In ${\mathrm{MoS}}_2$, we observe that this substitution is energetically favored under S-rich conditions, where the ${\mathrm{S}}_2$ gaseous phase is likely to be present. This result is compatible with a recent experimental observation in Sb-doped ${\mathrm{MoS}}_2$ nanosheets grown by chemical vapor deposition. A similar behavior is found in ${\mathrm{MoSe}}_2$, but in ${\mathrm{MoTe}}_2$ the Sb(Mo) substitution is less likely to occur due to the possible absence of gaseous Te phases in experimental setups. In all cases, several impurity-induced states are found inside the band gap, with energies that span the entire gap. The Fermi energy is pinned a few tenths of eV above the top of the valence band, suggesting a predominant $p$-type behavior, and gap energies are slightly increased in comparison to the pristine systems. The orbital nature of these states is further investigated with projected and local density of states calculations, which reveal similarities to defect states induced by single Mo vacancies as well as their rehybridization with the $5s$ orbital from Sb. Additionally, we find that the band gap of the doped systems is increased in comparison with the pristine materials, in contrast with a previous calculation in Sb-doped ${\mathrm{MoS}}_2$ that predicts a gap reduction with a different assignment of valence band and impurity levels. We discuss the similarities, discrepancies, and the limitations of both calculations. We also speculate possible reasons for the experimentally observed redshifts of the $A$ and $B$ excitons in the presence of the Sb dopants in ${\mathrm{MoS}}_2$. We hope that these results spark future investigations on other aspects of the problem, particularly those concerning the effects of disorder and electron-hole interaction, and continue to reveal the potential of doped transition-metal dichalcogenides for applications in optoelectronic devices.

$\mathrm{Zr}{\mathrm{Te}}_5$ is a layered material that exhibits intricate topological effects. Intensive theoretical and experimental efforts have been devoted to try to understand the physics in this material. In this paper the temperature dependent magnetotransport properties of $\mathrm{Zr}{\mathrm{Te}}_5$ thin flakes are investigated. A characteristic temperature $T^{*}$ is observed in the temperature dependence of three different types of magnetoresistance simultaneously, which are the saturated Hall anomaly, the chiral anomaly, and the longitudinal magnetoresistance. Furthermore, the value of $T^{*}$ decreases monotonically from 200 to 160 K with increasing thickness of the $\mathrm{Zr}{\mathrm{Te}}_5$ thin flakes from 42 to 89 nm. Temperature induced topological phase transitions are attributed to the cause of such anomaly in the three types of magnetoresistance at $T^{*}$. Our findings provide a multiparameter indicator for the emergence of topological phase transition in $\mathrm{Zr}{\mathrm{Te}}_5$ and could be extended to the study of other topological materials. The temperature dependence of the three types of magnetoresistance also sheds light on the role of anomalous Hall effect in the transport properties of $\mathrm{Zr}{\mathrm{Te}}_5$.

When two solids at different temperatures are separated by a vacuum gap they relax toward their equilibrium state by exchanging heat either by radiation or by phonon or electron tunneling, depending on their separation distance and on the nature of materials. The interplay between this exchange of energy and its spreading through each solid entirely drives the relaxation dynamics. Here we highlight a significant slowing-down of this process in the extreme near-field regime at distances where the heat flux exchanged between the two solids is comparable or even dominates over the flux carried by conduction inside each solid. This mechanism, leading to a strong effective increase in the system thermal inertia, should play an important role in the temporal evolution of the thermal state of interacting solid systems at nanometric and subnanometric scales.

We experimentally demonstrate the formation of room-temperature skyrmions with radii of about 25 nm in easy-plane anisotropy multilayers with an interfacial Dzyaloshinskii-Moriya interaction (DMI). We detect the formation of individual magnetic skyrmions by magnetic force microscopy and find that the skyrmions are stable in out-of-plane fields up to about 200 mT. We determine the interlayer exchange coupling as well as the strength of the interfacial DMI. Additionally, we investigate the dynamic microwave spin excitations by broadband magnetic resonance spectroscopy. From the uniform Kittel mode we determine the magnetic anisotropy and low damping ${\alpha }_{\mathrm{G}}<0.04$. We also find clear magnetic resonance signatures in the nonuniform (skyrmion) state. Our findings demonstrate that skyrmions in easy-plane multilayers are promising for spin-dynamical applications.

Doping of quantum spin liquid (QSL) insulators by electron or hole leads to intriguing phase transitions to metallic and superconducting states. The barlowite family with geometrically perfect $S=1/2$ kagome planes and tunable interkagome coupling is an emerging platform to realize spin-ordered, valence bond crystal, QSL states. Theoretical investigations on electron doping revealed localized states in the band gap of barlowite unlike metallicity in cuprate (${\mathrm{Nd}}_2\mathrm{Cu}{\mathrm{O}}_4$). We present successful anchoring of phase-pure barlowite crystallites onto reduced graphene oxide (rGO). The resulting barlowite-rGO system was found to be an electrical semiconductor with Arrhenius activation energy of 0.07 eV. Semiconducting properties of the barlowite-rGO system were further modulated with retention of structural integrity. We have attributed such a transformation of electrical transport response to plausible electron doping thereby making charge-doping experiments on barlowite and its analogs propitious.

We theoretically investigate how the spin susceptibility of a planar Josephson junction is affected when the system transits into the topological superconducting state. We show that the magnetic flux and magnetic field dependence of the spin susceptibility closely maps the phase diagram of the system. In the absence of an external magnetic flux the system can self-tune into the topological superconducting state by minimizing its free energy. Self-tuned topological transitions are accompanied by sharp peaks in the spin susceptibility, which can therefore be used as measurable fingerprints for detecting the topological superconducting state. Away from the peaks, the amplitude of the spin susceptibility can provide qualitative information about the relative size of the topological gap. The signatures in the spin susceptibility are robust, even in junctions with narrow superconducting leads, where critical current minima may no longer serve as an indication of topological phase transitions. The predicted results could be particularly relevant for future experiments on realization and detection of the topological superconducting state in planar Josephson junctions.

We study the symmetries of twisted trilayer graphene's band structure under various extrinsic perturbations, and analyze the role of long-range electron-electron interactions near the first magic angle. The electronic structure is modified by these interactions in a similar way to twisted bilayer graphene. We analyze electron pairing due to long-wavelength charge fluctuations, which are coupled among themselves via the Coulomb interaction and additionally mediated by longitudinal acoustic phonons. We find superconducting phases with either spin-singlet/valley-triplet or spin-triplet/valley-singlet symmetry, with critical temperatures up to a few Kelvin for realistic choices of parameters.

As condensed-matter manifestations of the chiral anomaly, the coexistence of positive longitudinal magnetoconductivity (LMC) and the planar Hall effect (PHE) is regarded as a characteristic transport signal for the Weyl semimetal (WSM) phase. By including a finite Newtonian mass into the general linearized WSM model, we derive an analytical expression for the chiral chemical potential, which includes a topological term of the chiral anomaly and a nontopological term. The nontopological term stems from the Zeeman-field-induced tilt of the Weyl cones, which breaks the symmetry of the fermion filling between the Weyl valleys of opposite chiralities and further leads to a chiral chemical potential $\propto \mathit{E}·\mathit{B}$, much as the effect of the chiral anomaly. We demonstrate that the resulting LMC is positive and can coexist with the PHE without invoking the mechanism of the chiral anomaly. Hence, the chiral chemical potential might not be exclusively respected to the chiral anomaly. Experimentally, one can distinguish the chiral anomaly from the proposed tilt mechanism by inspecting the dependence of magnetoconductivity on the Fermi energy.

The field-intensity dependence of high-harmonic generation in bulk gallium arsenide is studied. We experimentally find the oscillatory behavior at high fields where a perturbative scaling law no longer holds. By constructing a theoretical framework based on the Luttinger-Kohn model, we succeed in reproducing the observed oscillatory behavior. The qualitative agreement between the experiment and theory indicates that field-induced dynamic band modification is crucial in the nonperturbative regime. We show that the oscillatory behavior is naturally understood by dynamic localization that is based on the the Floquet subband picture.

A Ginzburg-Landau model embedded into a vibrational model is used to study the flexocaloric effect in a beam near a ferroelastic transition. The caloric response upon bending is characterized by the isothermal entropy change and the adiabatic temperature change of the beam. We obtain a larger response relative to the strength of the applied forces at temperatures slightly above the transition temperature. It is also obtained that the maximum caloric response is almost linear with the bending angle of the beam, whereas the relation between the bending angle and the applied forces is highly nonlinear. Small hysteresis associated with the phase transition is obtained for sufficiently large bending forces due to the existence of a critical point in the temperature-stress phase diagram of the ferroelastic material. Finally, the microstructure changes with bending in the beam are consistent with previous experimental observations.

We combine diamond anvil cell techniques, synchrotron-based infrared and Raman scattering spectroscopies, a symmetry analysis, and complementary lattice dynamics calculations to explore the pressure-driven phase transition in multiferroic $h\text{−}{\mathrm{Lu}}_{0.6}{\mathrm{Sc}}_{0.4}{\mathrm{FeO}}_3$. Comparison of the measured and predicted phonon patterns reveals a $P{6}_{3}cm\to P\overline{3}c1$ structural transition at 15 GPa. Symmetry breaking across the polar $\to$ antipolar transition takes place via changes in the bipyramidal tilting direction and Lu/Sc displacement pattern, analogous to the strain-driven $K_3$ distortion pathway in $h\text{−}{\mathrm{LuFeO}}_3$ and temperature-induced transitions in the rare-earth manganites.

Magnetic impurities in diamond influence the relaxation properties and thus limit the sensitivity of magnetic, electric, strain, and temperature sensors based on nitrogen-vacancy color centers. Diamond samples may exhibit significant spatial variations in the impurity concentrations hindering the quantitative analysis of relaxation pathways. Here, we present a local measurement technique which can be used to determine the concentration of various species of defects by utilizing double electron-electron resonance. This method will help to improve the understanding of the physics underlying spin relaxation and guide the development of diamond samples, as well as offering protocols for optimized sensing.

We evaluated the self-induced spin-orbit torque (SOT) in a single ferromagnetic layer. Spin-torque ferromagnetic resonance (ST-FMR) was measured for the very thin Ni-Fe (permalloy, Py) layers with and without structural inversion symmetry: asymmetric and symmetric Py layers. For both structures, the fieldlike component coming from the Oersted field clearly appeared. On the other hand, the dampinglike component was observed only for asymmetric Py with thickness $\le 3\mathrm{nm}$. This dampinglike torque is attributable to the significant spatial change in the properties of Py. We propose a toy model to analyze the self-induced SOT.

The Anderson transition driven by non-Hermitian (NH) disorder has been extensively studied in recent years. In this paper, we present in-depth transfer matrix analyses of the Anderson transition in three NH systems, NH Anderson, U(1), and Peierls models in three-dimensional systems. The first model belongs to NH class ${\mathrm{AI}}^{†}$, whereas the second and the third ones to NH class A. We first argue a general validity of the transfer matrix analysis in NH systems, and clarify the symmetry properties of the Lyapunov exponents, scattering ($S$) matrix and two-terminal conductance in these NH models. The unitarity of the $S$ matrix is violated in NH systems, where the two-terminal conductance can take arbitrarily large values. Nonetheless, we show that the transposition symmetry of a Hamiltonian leads to the symmetric nature of the $S$ matrix as well as the reciprocal symmetries of the Lyapunov exponents and conductance in certain ways in these NH models. Using the transfer matrix method, we construct a phase diagram of the NH Anderson model for various complex single-particle energy $E$. At $E=0$, the phase diagram as well as critical properties become completely symmetric with respect to an exchange of real and imaginary parts of on-site NH random potentials. We show that the symmetric nature at $E=0$ is a general feature for any NH bipartite-lattice models with the on-site NH random potentials. Finite size scaling data are fitted by polynomial functions, from which we determine the critical exponent $\nu$ at different single-particle energies and system parameters of the NH models. We conclude that the critical exponents of the NH class ${\mathrm{AI}}^{†}$ and the NH class A are $\nu =1.19\pm 0.01$ and $\nu =1.00\pm 0.04$, respectively. In the NH models, a distribution of the two-terminal conductance is not Gaussian. Instead, it contains small fractions of huge conductance values, which come from rare-event states with huge transmissions amplified by on-site NH disorders. Nonetheless, a geometric mean of the conductance enables the finite-size scaling analysis. We show that the critical exponents obtained from the conductance analysis are consistent with those from the localization length in these three NH models.

$\delta -\mathrm{AlOOH}$ is of significant crystallochemical interest due to a subtle structural transition near 10 GPa from a $P{2}_{1}nm$ to a $Pnnm$ structure, the nature and origin of hydrogen disorder, the symmetrization of the O-$\mathrm{H}\cdots \mathrm{O}$ hydrogen bond and their interplay. We perform a series of density functional theory-based simulations in combination with high-pressure nuclear magnetic resonance (NMR) experiments on $\delta -\mathrm{AlOOH}$ up to 40 GPa with the goal to better characterize the hydrogen potential and therefore the nature of hydrogen disorder. Simulations predict a phase transition in agreement with our NMR experiments at $10-11\mathrm{GPa}$ and hydrogen bond symmetrization at $14.7\mathrm{GPa}$. Calculated hydrogen potentials do not show any double-well character and there is no evidence for proton tunneling in our NMR data.

We demonstrate that a three-dimensional time-periodically driven (Floquet) lattice can exhibit chiral hinge states and describe their interplay with Weyl physics. A peculiar type of the hinge states is enforced by the repeated boundary reflections with lateral Goos-Hänchen–type shifts occurring at the second-order boundaries of our system. Such chiral hinge modes coexist in a wide range of parameter regimes with Fermi-arc surface states connecting a pair of Weyl points in a two-band model. We find numerically that these modes still preserve their locality along the hinge and their chiral nature in the presence of local defects and other parameter changes. We trace the robustness of such chiral hinge modes to special band structure unique in a Floquet system allowing all the eigenstates to be localized in quasi-one-dimensional regions parallel to each other when open hinge boundaries are introduced. The implementation of a model featuring both the second-order Floquet skin effect and the Weyl physics is straightforward with ultracold atoms in optical superlattices.

We study the energy-density dynamics at finite momentum of the two-dimensional Kitaev spin model on the honeycomb lattice. Due to fractionalization of magnetic moments, the energy relaxation occurs through mobile Majorana matter, coupled to a static ${\mathbb{Z}}_2$ gauge field. At finite temperatures, the ${\mathbb{Z}}_2$ flux excitations act as a thermally induced disorder, which strongly affects the energy dynamics. We show that sufficiently far above the flux proliferation temperature, but not yet in the high-temperature classical regime, disordered gauge configurations modify the coherent low-temperature energy-density dynamics into a form which is almost diffusive, with a diffusion kernel that is nearly momentum independent, but which remains retarded, primarily due to the presence of two distinct relaxation channels of particle-hole and particle-particle nature. Relations to the thermal conductivity are clarified. Our analysis is based on complementary calculations in the low-temperature homogeneous gauge configuration and a mean-field treatment of thermal gauge fluctuations, valid above the flux proliferation temperature.