We present a microscopic theory for quantum thermoelectric and heat transport in the Schwarzian regime of the Sachdev-Ye-Kitaev (SYK) model. As a charged fermion realization of the SYK model in nanostructures, we assume a setup based on a quantum dot connected to the charge reservoirs through weak tunnel barriers. We analyze particle-hole symmetry breaking effects crucial for both Seebeck and Peltier coefficients. We show that the quantum charge and heat transport at low temperatures are defined by the interplay between elastic and inelastic processes such that the inelastic processes provide a leading contribution to the transport coefficients at the temperatures that are smaller compared to the charging energy. We demonstrate that both electric and thermal conductance obey a power law in temperature behavior, while thermoelectric, Seebeck, and Peltier coefficients are exponentially suppressed. This represents selective suppression of only nondiagonal transport coefficients. We discuss the validity of the Kelvin formula in the presence of a strong Coulomb blockade.

The hexagonal rare-earth ferrite $R\mathrm{Fe}{\mathrm{O}}_3$ family represents a unique class of multiferroics exhibiting weak ferromagnetism, and a strong coupling between magnetism and structural trimerization is predicted. However, the hexagonal structure for $R\mathrm{Fe}{\mathrm{O}}_3$ remains metastable in conventional conditions. We have succeeded in stabilizing the hexagonal structure of polycrystalline $\mathrm{Yb}\mathrm{Fe}{\mathrm{O}}_3$ by partial Sc substitution of Yb. Using bulk magnetometry and neutron diffraction, we find that ${\mathrm{Yb}}_{0.42}{\mathrm{Sc}}_{0.58}\mathrm{Fe}{\mathrm{O}}_3$ orders into a canted antiferromagnetic state with the Néel temperature $T_N\sim 165\mathrm{K}$, below which the ${\mathrm{Fe}}^{3+}$ moments form the triangular configuration in the $ab$ plane and their in-plane projections are parallel to the [100] axis, consistent with magnetic space group $P{6}_3{c}^{\prime }{m}^{\prime }$. It is determined that the spin canting is aligned along the $c$ axis, giving rise to the weak ferromagnetism. Furthermore, the ${\mathrm{Fe}}^{3+}$ moments reorient toward a new direction below reorientation temperature $T_R\sim 40\mathrm{K}$, satisfying magnetic subgroup $P{6}_3$, while the ${\mathrm{Yb}}^{3+}$ moments order independently and ferrimagnetically along the $c$ axis at the characteristic temperature $T_{\mathrm{Yb}}\sim 15\mathrm{K}$. Interestingly, reproducible modulation of electric polarization induced by magnetic field at low temperature is achieved, suggesting that the delicate structural distortion associated with two-up/one-down buckling of the Yb/Sc planes and tilting of the $\mathrm{Fe}{\mathrm{O}}_5$ bipyramids may mediate the coupling between ferroelectric and magnetic orders under magnetic field. The present work represents substantial progress to search for high-temperature multiferroics in hexagonal ferrites and related materials.

Kinetics in amorphous $\mathrm{Ti}{\mathrm{O}}_2$ driven by ion irradiation are discussed in the framework computer simulations. Thermal spike like regions along the path of incident ions are identified and their effects on the glassy system are described. Benefiting from the complementary approaches of purely ballistic TRIM (transport in matter) simulations and molecular dynamics simulations, a distinction between characteristic scales for short-range diffusive relocations ($\lesssim 4.5\text{Å}$) and long-range ballistic relocations is achieved. It is found that diffusion dynamics within thermally activated cascade zones sets in at the critical temperature $T_c$, identified as the critical temperature of the mode coupling theory, while the mean relocation limit for thermal diffusion in the glassy system is attributed to an overlap between the energy regime of purely ballistic and thermally activated relocation processes.

We show that in some trigonal and hexagonal crystals the Zeeman coupling of band electrons with an external magnetic field is strongly anisotropic and necessarily vanishes along the main symmetry axis. This leads to qualitative changes in the temperature dependence of the electron spin susceptibility in the superconducting state. In particular, the power-law exponents at low temperatures due to the contribution of the nodal quasiparticles are considerably modified compared to their textbook values.

We use ab initio molecular dynamics simulations to investigate the properties of the dry surface of pure silica and sodium silicate glasses. The surface layers are defined based on the atomic distributions along the direction $\left(z\mathrm{direction}\right)$ perpendicular to the surfaces. We show that these surfaces have a higher concentration of dangling bonds as well as two-membered (2M) rings than the bulk samples. Increasing the concentration of ${\mathrm{Na}}_2\mathrm{O}$ reduces the proportion of structural defects. From the vibrational density of states, one concludes that 2M rings have a unique vibrational signature at a frequency $\approx 850{\mathrm{cm}}^{-1}$, compatible with experimental findings. We also find that, due to the presence of surfaces, the atomic vibration in the $z\mathrm{direction}$ is softer than for the two other directions. The electronic density of states shows clearly the differences between the surface and interior and we can attribute these to specific structural units. Finally, the analysis of the electron localization function allows to get insight on the influence of local structure and the presence of Na on the nature of chemical bonding in the glasses.

In this work we propose a topological valley phononic crystal plate and we extensively investigate the refraction of valley modes into the surrounding homogeneous medium. This phononic crystal includes two sublattices of resonators (A and B) modeled by mass-spring systems. We show that two edge states confined at the AB/BA and BA/AB type domain walls exhibit different symmetries in physical space and energy peaks in the Fourier space. As a result, distinct refraction behaviors, especially through an armchair cut edge, are observed. On the other hand, the decay depth of these localized topological modes, which is found to be solely determined by the relative resonant strength between the scatterers, significantly affects the refraction patterns. More interestingly, the outgoing traveling wave through a zigzag interface becomes evanescent when operating at deep subwavelength scale. This is realized by tuning the average resonant strength. We show that the evanescent modes only exist along a particular type of outlet edge and that they can couple with both topological interface states. We also present a near-ideal monopole and a dipole emitter based on our phononic structure.

Magnetodielectric (MD) properties of as-prepared (AP) and air-annealed ${\mathrm{Bi}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Fe}}_{1-y}{\mathrm{Ti}}_y{\mathrm{O}}_{3-\delta }$ nanoparticle ceramics made by spark plasma sintering process are investigated as a function of temperature. Aliovalent ${\mathrm{Ca}}^{2+}$ substitution at ${\mathrm{Bi}}^{3+}$ site creates oxygen vacancies (${\mathrm{V}}_{\mathrm{O}}$) in the lattice disrupting the intrinsic spin cycloid of ${\mathrm{BiFeO}}_3$, which are suppressed when the charge compensating ${\mathrm{Ti}}^{4+}$ is co-substituted. In addition, cation substitution reduces the grain size and increases surface oxygen vacancies. These lattice and surface ${\mathrm{V}}_{\mathrm{O}}$ defects play a significant role in enhancing the magnetic properties. Zero-field-cooled magnetization curves of all AP samples show a sharp Verwey-like transition at ∼120 K, which weakens on air-annealing. A coexistence of positive and negative MD [MD = $\frac{\mathrm{\Delta }\varepsilon \left(H\right)}{\varepsilon \left(H=0\right)}$; $\mathrm{\Delta }\varepsilon \left(H\right)=\varepsilon \left(H\right)-\varepsilon \left(H=0\right)$] response is observed, with the former dominating at 300 K and the latter at 10 K. As-prepared 5 at.% (10 at.%) Ca and Ca-Ti substituted ${\mathrm{BiFeO}}_3$ ceramics exhibit a maximum MD response of –10% (∼+3%) at 10 K (300 K). Negative MD response diminishes for air-annealed ${\mathrm{Bi}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Fe}}_{1-y}{\mathrm{Ti}}_y{\mathrm{O}}_{3-\delta }$ ceramics due to the reduction in ${\mathrm{V}}_{\mathrm{O}}$ concentration. Samples exhibiting dominant positive MD response show a similar trend for MD $vs$ H and $M^2$ vs H plots. This agreement between $M^2$ and $\mathrm{\Delta }\varepsilon \left(H\right)$ demonstrates a strong inherent MD coupling. On the contrary, negative MD does not follow this trend yet shows a linear relationship of MD vs $M^2$, suggesting a strong coupling between the magnetic and dielectric properties. Temperature-dependent MD studies carried out at 5 T show a gradual change from negative to positive values. Negative MD at low temperatures could be activated by the spin-lattice coupling, which dominates even at high frequency (1 MHz) under the applied field. Other contributions, including Verwey-like transition, magnetoresistance, and Maxwell-Wagner effects, do not influence the observed MD response. A prominent role of oxygen vacancies in altering the MD behavior of ${\mathrm{BiFeO}}_3$ is discussed in detail.

The treatment of intershell interactions remains a major challenge in the theoretical description of strongly correlated materials. Most previous approaches considered the influence of intershell interactions at best in a static fashion, neglecting dynamic effects. In this paper, we propose a slave-rotor method that goes beyond this approximation by incorporating the effect of intershell interactions in a dynamic manner. Our method is derived and implemented as a quantum impurity solver in the context of dynamical mean field theory and benchmarked on a two-orbital model system. The results from our slave-rotor technique are found to be in good agreement with our reference calculations that include intershell interactions explicitly. We identify and analyze qualitative features emerging from the dynamic treatment. Our results thus provide qualitative new insights, revealing the ambivalent effect of intershell interactions in strongly correlated materials.

The coupling between the oxygen stoichiometry and strain-induced lattice distortions has huge bearings on the electronic and magnetic properties of $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_3$ (SMO). In this paper we have observed oxygen stoichiometry-mediated epitaxial tetragonal to nonepitaxial-oriented hexagonal topotactic phase transition of $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_3$ thin film on $\mathrm{SrTi}{\mathrm{O}}_3$ (100) substrate. The oxygen stoichiometric hexagonal SMO films are oriented in nature, while the oxygen-deficient epitaxially strained $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3-\delta }$ films are tetragonal in nature. It is observed that oxygen vacancy is obligatory to hold on the epitaxial strain in $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3-\delta }$ thin film, while oxygenated stoichiometric cubic $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_3$ thin films are observed to be strain free. The structural phase and stoichiometry are found to govern the nature and mechanism of magnetic interactions in these films. The emergence of unusual weak ferromagnetism in oxygen stoichiometric hexagonal SMO films established a root towards the local structural distortion as confirmed through the Raman spectroscopy and extended x-ray absorption fine structure measurements. This work proves for us the idea that the oxygen deficiency is not the necessary effect of strain; rather, it is the cause of strain for topotactic SMO film.

We study the transport properties for a family of geometrically frustrated models on the triangular lattice with an interaction scale far exceeding the single-particle bandwidth. Starting from the interaction-only limit, which can be solved exactly, we analyze the transport and thermodynamic behavior as a function of filling and temperature at the leading nontrivial order in the single-particle hopping. Over a broad range of intermediate temperatures, we find evidence of a DC resistivity scaling linearly with temperature and with typical values far exceeding the quantum of resistance $h/e^2$. At a sequence of commensurate fillings, the bad metallic regime eventually crosses over into interaction induced insulating phases in the limit of low temperatures. We discuss the relevance of our results to experiments in cold-atom and moiré heterostructure based platforms.

We study single crystals of the magnetic superconductor $\mathrm{Eu}\mathrm{Rb}{\mathrm{Fe}}_4{\mathrm{As}}_4$ by magnetization, electron spin resonance (ESR), angle-resolved photoemission spectroscopy, and electrical resistance in pulsed magnetic fields up to 63 T. The superconducting state below 36.5 K is almost isotropic and is only weakly affected by the development of ${\mathrm{Eu}}^{2+}$ magnetic order at 15 K. On the other hand, for the external magnetic field applied along the $c$ axis the temperature dependence of the ESR linewidth reveals a Berezinskii-Kosterlitz-Thouless topological transition below 15 K. This indicates that ${\mathrm{Eu}}^{2+}$ planes are a good realization of a two-dimensional $\mathit{XY}$ magnet, which reflects the decoupling of the ${\mathrm{Eu}}^{2+}$ magnetic moments from superconducting FeAs layers.

In this work, we identify a two-dimensional layered nitridosilicate ${\mathrm{SnSi}}_2{\mathrm{N}}_4$ in hexagonal structure via first-principles calculations. From the total energy calculations, ${\mathrm{SnSi}}_2{\mathrm{N}}_4$ is thermodynamically stable. The dynamic stabilities are demonstrated by the phonon spectrum and molecular-dynamics simulations. The ${\mathrm{SnSi}}_2{\mathrm{N}}_4$ exhibits semiconducting behavior with a direct band gap of 3.70 eV $\left({\mathrm{GW}}_0\right)$, Young's modulus of 478 GPa and breaking strength of 47 GPa, and electron mobility of $2736\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{\text{–}1}{\mathrm{s}}^{\text{–}1}$ (Perdew-Burke-Ernzerhof). According to the absorption spectra, ${\mathrm{SnSi}}_2{\mathrm{N}}_4$ presents good absorption of ultraviolet light with high energy. The chemical stability of aqueous solution is verified by the molecular-dynamics simulations.

We propose a quantized Majorana pump (QMP) in semiconductor-superconductor heterostructures. The pump consists of a Majorana nanowire, i.e., a Rashba wire in proximity to an $s$-wave superconductor and in a magnetic field, and two ferromagnetic leads. When the orientation of the magnetic field is rotated by $2\pi$, an elementary charge $e$ is transferred between two leads. The number of the pumped charge in a cycle is related to the winding number of the Andreev reflection amplitude. In the topologically trivial phase without Majorana zero mode (MZM), the pumped charge decreases rapidly to zero. More importantly, the pump works at zero bias and provides a more effective way to distinguish MZMs from trivial partially separated Andreev bound states. Therefore, this QMP provides not only a smoking-gun signature of MZMs, but also an ideal platform to realize topological single-electron pumps with the potential for realizing novel current standards in electrical metrology.

The anomalous charge transport observed in some strongly correlated metals raises questions as to the universal applicability of Landau Fermi-liquid theory. The coherence temperature $T_{\text{FL}}$ for normal metals is usually taken to be the temperature below which $T^2$ is observed in the resistivity. Below this temperature, a Fermi liquid with well-defined quasiparticles is expected. However, metallic ruthenates in the Ruddlesden-Popper family frequently show non-Drude low-energy optical conductivity and unusual $\omega /T$ scaling, despite the frequent observation of $T^2$ dc resistivity. Herein we report time-domain THz spectroscopy measurements of several different high-quality metallic ruthenate thin films and show that the optical conductivity can be interpreted in more conventional terms. In all materials, the conductivity has a two Lorentzian line shape at low temperature and a crossover to a one Drude peak line shape at higher temperatures. The two component low-temperature conductivity is indicative of two well-separated current relaxation rates for different conduction channels. In ${\mathrm{SrRuO}}_3$ and ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, both relaxation rates scale as $T^2$, while in ${\mathrm{CaRuO}}_3$ the slow relaxation rate shows $T^2$, and the fast relaxation rate generates a constant background in conductivity. We discuss three particular possibilities for the separation of rates: (a) strongly energy-dependent inelastic scattering; (b) an almost conserved pseudomomentum operator that overlaps with the current, giving rise to the narrower Drude peak; and (c) the presence of multiple conduction channels that undergoes a crossover to stronger interband scattering at higher temperatures. None of these scenarios requires the existence of exotic quasiparticles. However, the interpretation in terms of multiple conduction channels in particular is consistent with the existence of multiple Fermi surfaces in these compounds and with the expected relative weakness of ${\omega }^2$ dependent effects in the scattering as compared to $T^2$ dependent effects in the usual Fermi-liquid treatment. The results may give insight into the possible significance of Hund's coupling in determining interband coupling in these materials. Our results also show a route towards understanding the violation of Matthiessen's rule in this class of materials and deviations from the “Gurzhi” scaling relations in Fermi liquids.

The interplay between non-Hermiticity and topology opens an exciting avenue for engineering novel topological matter with unprecedented properties. While previous studies have mainly focused on one-dimensional systems or Chern insulators, here we investigate topological phase transitions to and from quantum spin Hall (QSH) insulators driven by non-Hermiticity. We show that a trivial to QSH insulator phase transition can be induced by solely varying non-Hermitian terms, and there exists exceptional edge arcs in QSH phases. We establish two topological invariants for characterizing the non-Hermitian phase transitions: (i) with time-reversal symmetry, the biorthogonal ${\mathbb{Z}}_2$ invariant based on non-Hermitian Wilson loops, and (ii) without time-reversal symmetry, a biorthogonal spin Chern number through biorthogonal decompositions of the Bloch bundle of the occupied bands. These topological invariants can be applied to a wide class of non-Hermitian topological phases beyond Chern classes, and provides a powerful tool for exploring novel non-Hermitian topological matter and their device applications.

Massless chiral fermions emergent in inversion symmetry-breaking Weyl semimetals (WSMs) reside in the vicinity of multiple low-symmetry nodes and thus acquire strongly anisotropic dispersion. We investigate the longitudinal electromagnetic modes of two-component degenerate Weyl plasma relevant to the realistic band structure of type-I WSM. We show that the actual spectrum of three-dimensional collective density excitations in the TaAs family of WSM is gapless due to emergence of acoustic plasmons corresponding to out-of-phase oscillations of the plasma components. These modes exist around the [001] crystallographic direction and are weakly damped, due to large difference in the Weyl velocities of the $W_1$ and $W_2$ quasiparticles propagating along [001]. We show that acoustic plasmons can manifest themselves as slow beatings of electric potential superimposed on fast plasmonic oscillations upon charge relaxation. The revealed acoustic modes can stimulate purely electronic superconductivity, collisionless plasmon instabilities, and formation of Weyl soundarons.

We theoretically investigate the piezo-optic effect of high-harmonic generation (HHG) in shear-strained semiconductors. By focusing on a typical semiconductor, GaAs, we show that there is optical activity, meaning different responses to right-handed and left-handed elliptically polarized electric fields. We also show that this optical activity is more pronounced for higher harmonics whose perturbative order exceeds the band-gap energy. These findings point to a useful pathway for strain engineering of nonlinear optics to control the reciprocity of HHG.

Rydberg excitons in ${\mathrm{Cu}}_2\mathrm{O}$ can be an emergent platform for solid-state quantum information processing by utilizing the exaggerated properties of high-lying excited states within the material. To develop practical quantum systems, high-temperature operation is desirable. Here, we study the temperature dependence of the yellow and green Rydberg exciton resonances in a thin ${\mathrm{Cu}}_2\mathrm{O}$ crystal via broad-band phonon-assisted absorption spectra between 4 K and 100 K. At 4 K, we can identify the principal quantum number $n=11$ yellow and $n=4$ green Rydberg exciton states, beyond which we are limited by the spectral resolution of standard absorption techniques. Above liquid nitrogen boiling temperature ($\sim 80\mathrm{K}$), the $n=6$ yellow and $n=4$ green Rydberg exciton states are readily captured and higher-temperature yellow Rydberg exciton optical properties still exhibit the standard scaling laws seen at low temperatures. This promising result lays the groundwork for a new route to build a high-temperature Rydberg quantum information processing architecture with solid-state ${\mathrm{Cu}}_2\mathrm{O}$.

One of the long-standing goals of quantum transport is to use the noise, rather than the average current, for information processing. However, achieving this requires on-demand control of quantum fluctuations in the electric current. In this paper, we demonstrate theoretically that transport through a molecular spin valve provides access to many different statistics of electron tunneling events. Simply by changing highly tunable parameters, such as electrode spin polarization, magnetization angle, and voltage, one is able to switch between Poisson behavior, bunching and antibunching of electron tunnelings, and positive and negative temporal correlations. The molecular spin valve is modeled by a single spin-degenerate molecular orbital with local electronic repulsion coupled to two ferromagnetic leads with magnetization orientations allowed to rotate relative to each other. The electron transport is described via Born-Markov master equation and fluctuations are studied with higher-order waiting time distributions. For highly magnetized parallel-aligned electrodes, we find that strong positive temporal correlations emerge in the voltage range where the second transport channel is partially open. These are caused by a spin-induced electron-bunching, which does not manifest in the stationary current alone.