We classify interacting topological insulators and superconductors with order-two crystal symmetries (reflection or twofold rotation), focusing on the case where interactions reduce the noninteracting classification. We find that the free-fermion ${\mathbb{Z}}_2$ classifications are stable against quartic contact interactions, whereas the $\mathbb{Z}$ classifications reduce to ${\mathbb{Z}}_N$, where $N$ depends on the symmetry class and the dimension $d$. These results are derived using a quantum nonlinear $\sigma$ model (QNLSM) that describes the effects of the quartic interactions on the boundary modes of the crystalline topological phases. We use Clifford algebra extensions to derive the target spaces of these QNLSMs in a unified way. The reduction pattern of the free-fermion classification then follows from the presence or absence of topological terms in the QNLSMs, which is determined by the homotopy group of the target spaces. We show that this derivation can be performed using either a complex fermion or a real Majorana representation of the crystalline topological phases and demonstrate that these two representations give consistent results. To illustrate the breakdown of the noninteracting classification, we present examples of crystalline topological insulators and superconductors in one, two, and three dimensions whose surface modes are unstable against interactions. For the three-dimensional example, we show that the reduction pattern obtained by the QNLSM method agrees with the one inferred from the stability analysis of the boundary modes using bosonization.

We have studied the topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$(111) by means of helium atom scattering. The average electron-phonon coupling $\lambda$ of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$(111) is determined by adapting a recently developed quantum-theoretical derivation of the helium scattering probabilities to the case of degenerate semiconductors. Based on the Debye-Waller attenuation of the elastic diffraction peaks of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$(111), measured at surface temperatures between 110 and $355\text{K}$, we find $\lambda$ to be in the range of $0.04\text{–}0.11$. This method allows us to extract a correctly averaged $\lambda$ and to address the discrepancy between previous studies. The relatively modest value of $\lambda$ is not surprising even though some individual phonons may provide a larger electron-phonon interaction. Furthermore, the surface Debye temperature of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$(111) is determined as ${\mathrm{\Theta }}_D=(81\pm 6)\text{K}$. The electronic surface corrugation was analyzed based on close-coupling calculations. By using a corrugated Morse potential a peak-to-peak corrugation of 9% of the lattice constant is obtained.

In graphene, out-of-plane (flexural) vibrations and static ripples imposed by the substrate relax the electron spin, intrinsically protected by mirror symmetry. We calculate the relaxation times in different scenarios, accounting for all the possible spin-phonon couplings allowed by the hexagonal symmetry of the lattice. Scattering by flexural phonons imposes the ultimate bound to the spin lifetimes, in the ballpark of hundreds of nanoseconds at room temperature. This estimate and the behavior as a function of the carrier concentration are substantially altered by the presence of tensions or pinning with the substrate. Static ripples also influence the spin transport in the diffusive regime, dominated by motional narrowing. We find that the D'yakonov-Perel' mechanism saturates when the mean free path is comparable to the correlation length of the height profile. In this regime, the spin-relaxation times are exclusively determined by the geometry of the corrugations. Simple models for typical corrugations lead to lifetimes of the order of tens of microseconds.

We study theoretically the dynamics of a Shiba state forming around precessing classical spin in an $s$-wave superconductor. Utilizing a rotating wave description for the precessing magnetic impurity, we find the resulting Shiba bound state quasienergy and the spatial extension of the Shiba wave function. We show that such a precession pertains to dc charge and spin currents flowing through a normal STM tip tunnel coupled to the superconductor in the vicinity of the impurity. We calculate these currents and find that they strongly depend on the magnetic impurity precession frequency, precession angle, and on the position of the Shiba energy level in the superconducting gap. The resulting charge current is found to be proportional to the difference between the electron and hole wave functions of the Shiba state, being a direct measure for such an asymmetry. By dynamically driving the impurity one can infer the spin dependence of the Shiba states in the absence of a spin-polarized STM tip.

We study the topological classification of parafermionic chains in the presence of a modified time-reversal symmetry that satisfies ${\mathcal{T}}^2=1$. Such chains can be realized in one-dimensional structures embedded in fractionalized two-dimensional states of matter, e.g., at the edges of a fractional quantum spin Hall system, where counterpropagating modes may be gapped either by backscattering or by coupling to a superconductor. In the absence of any additional symmetries, a chain of ${\mathbb{Z}}_m$ parafermions can belong to one of several distinct phases. We find that when the modified time-reversal symmetry is imposed, the classification becomes richer. If $m$ is odd, each of the phases splits into two subclasses. We identify the symmetry-protected phase as a Haldane phase that carries a Kramers doublet at each end. When $m$ is even, each phase splits into four subclasses. The origin of this split is in the emergent Majorana fermions associated with even values of $m$. We demonstrate the appearance of such emergent Majorana zero modes in a system where the constituent particles are either fermions or bosons.

We study a series of perturbations on the Sachdev-Ye-Kitaev (SYK) model. We show that the maximal chaotic non-Fermi-liquid phase described by the ordinary $q=4$ SYK model has marginally relevant or irrelevant (depending on the sign of the coupling constants) four-fermion perturbations allowed by symmetry. Changing the sign of one of these four-fermion perturbations leads to a continuous chaotic-nonchaotic quantum phase transition of the system accompanied by a spontaneous time-reversal symmetry breaking. Starting with the ${\mathrm{SYK}}_q$ model with a $q$-fermion interaction, similar perturbations can lead to a series of new fixed points with continuously varying exponents.

We prepared superconducting and nonsuperconducting FeSe films on $\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ substrates (FeSe/STO) and investigated the superconducting transition induced by charge transfer between organic molecules and FeSe layers by low temperature scanning tunneling microscopy and spectroscopy. At low coverage, donor- and acceptor-type molecules adsorbed preferentially on the nonsuperconducting and superconducting FeSe layers, respectively. Superconductivity was induced by donor molecules on nonsuperconducting FeSe layer, while the superconductivity was suppressed near acceptor molecules. The corresponding evolutions of electronic states and work function were also resolved by scanning tunneling microscopy. These results illustrate the important role played by local electron concentration in the superconducting transition of FeSe/STO.

We consider a semi-infinite dielectric with multiple spatially dispersive resonances in the susceptibility. The effect of the boundary is described by an arbitrary reflection coefficient for polarization waves in the material at the surface, with specific values corresponding to various additional boundary conditions (ABCs) for Maxwell's equations. We derive exact expressions for the electromagnetic reflection and transmission coefficients and present the results for a variety of materials with multiple exciton bands. We find an improved single-band approximation for heavy/light exciton bands and extend our model to exciton dispersion relations with linear $k$ terms which occur in uniaxial crystals. Finally, we calculate the spectral energy density of thermal and zero-point radiation for a variety of multiresonance models and ABCs.

In the presence of a magnetic field frustrated spin systems may exhibit plateaus at fractional values of saturation magnetization. Such plateau states are stabilized by classical and quantum mechanisms including order by disorder, triplon crystallization, and various competing order effects. In the case of electrically conducting systems, free electrons represent an incisive probe for the plateau states. Here we study the electrical transport of Ising-type rare-earth tetraborides $R{\mathrm{B}}_4$ ($R=\mathrm{Er}$, Tm), a metallic Shastry-Sutherland lattice showing magnetization plateaus. We find that the longitudinal and transverse resistivities reflect scattering with both the static and the dynamic plateau structure. We model these results consistently with the expected strong uniaxial anisotropy on a quantitative level, providing a framework for the study of plateau states in metallic frustrated systems.

We demonstrate that the instability of the Sb vacancy in GaSb leads to a further increase in the acceptor-type defect concentration in proton irradiated undoped, $p$-type GaSb. Using positron annihilation spectroscopy in situ with 10 MeV proton irradiation at 35 K, we find that the irradiation produces both native vacancy defects in GaSb. However, the Sb vacancy is unstable above temperatures of 150 K and undergoes a transition resulting in a Ga vacancy and a Ga antisite. The activation energy of this transition is determined to be $0.6\mathrm{eV}\pm 0.1$ eV. Our results are in line with the established amphoteric defect model and prove that the instability of the Sb vacancy in GaSb has a profound role on the native defect concentration in GaSb.

We report the results of an exhaustive study of ‘zero-field’ electrical resistivity $\rho \left(T\right)$ and magnetoresistance (MR) (in magnetic fields up to 90 kOe) over the temperature range 1.8–300 K in nanocrystalline (nc-) Ni with average crystallite size $d$ ranging from 10 nm to 40 nm. A quantitative comparison of our results with the predictions of the recent self-consistent calculations permits us to unambiguously identify the scattering mechanisms responsible for $\rho \left(T\right)$ and MR in different temperature ranges and accurately determine their relative magnitudes in nc-Ni samples of different $d$. Like in bulk $3d$ transition metal ferromagnets, $\rho$ varies with temperature as $T^2$ at $T\lesssim 15$ K. Contrary to the widely-held view that the $T^2$ variation of $\rho$ at low temperatures arises from the electron-magnon ($e\text{−}m$) scattering, this contribution to $\rho \left(T\right)$ is shown to originate from the electron-electron (Baber) scattering. In the temperature range $15\text{K}\le T\le 300$ K, the phonon-induced non-spin-flip (NSF) intraband [i.e., $s^{\uparrow \downarrow }\text{−}{s}^{\uparrow \downarrow }$, $d^{\uparrow \downarrow }\text{−}{d}^{\uparrow \downarrow }$ electron-phonon ($e\text{−}p$)] scattering and magnon-induced spin-flip (SF) interband (i.e., $s^{\uparrow \downarrow }\text{−}{d}^{\downarrow \uparrow }e\text{−}m$) scattering contributions completely account for the intrinsic resistivity. The former contribution dominates over the latter at $T>T^{☆}$ whereas the reverse is true at temperatures 15 K $\le T<T^{☆}$. $T^{☆}$, the temperature at which the SF interband $e\text{−}m$ and NSF intraband $e\text{−}p$ scattering contributions become equal, increases from $\sim 30$ K for $d=10\mathrm{nm}$ to $\sim 65\mathrm{K}$ for bulk Ni. At $T>T^{☆}$, ${\rho }_{e\text{−}m}(T,H=0)$ becomes comparable in magnitude to ${\rho }_{e\text{−}p}(T,H=0)$ for $d\ge 25$ nm. By contrast, the MR in nc-Ni is entirely due to the $s^{\uparrow \downarrow }\text{−}{d}^{\downarrow \uparrow }e\text{−}m$ scattering. The present work clearly brings out the importance of the thermal renormalization of magnon mass (caused mainly by the magnon-magnon interactions) over the temperature range $15\text{K}\le T\le 300$ K. Irrespective of the value of $d$ (including the bulk, $d=\infty$), phonon-induced $s^{\uparrow \downarrow }\text{−}{s}^{\uparrow \downarrow }$, $d^{\uparrow \downarrow }\text{−}{d}^{\uparrow \downarrow }$ transitions are more frequent than the $e\text{−}p{s}^{\uparrow \downarrow }\text{−}{d}^{\uparrow \downarrow }$ transitions over the entire temperature range $1.8\text{K}\le T\le 300$ K. The saturation magnetization at 0 K, spin wave stiffness at 0 K, Debye temperature as well as the parameters that are a direct measure of the strength of $e\text{−}p$ coupling, $e\text{−}m$ coupling, and level of suppression of $e\text{−}m$ scattering by external magnetic field, all exhibit power law variations with $d$. The power law behavior asserts that the average crystallite size is the dominant length scale so far as the magnetism, electrical transport, and magnetotransport in nc-Ni are concerned.

We have investigated the magnetotransport above the upper critical field ($H_{c2}$) in ${\mathrm{Fe}}_{1.14}{\mathrm{Te}}_{0.7}{\mathrm{Se}}_{0.3}, {\mathrm{Fe}}_{1.02}{\mathrm{Te}}_{0.61}{\mathrm{Se}}_{0.39}, {\mathrm{Fe}}_{1.05}{\mathrm{Te}}_{0.89}{\mathrm{Se}}_{0.11}$, and ${\mathrm{Fe}}_{1.06}{\mathrm{Te}}_{0.86}{\mathrm{S}}_{0.14}$. The $\mu \mathrm{SR}$ measurements confirm electronic phase separation in ${\mathrm{Fe}}_{1.06}{\mathrm{Te}}_{0.86}{\mathrm{S}}_{0.14}$, similar to ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$. Superconductivity is suppressed in high magnetic fields above 60 T, allowing us to gain insight into the normal-state properties below the zero-field superconducting transition temperature ($T_c$). We show that the resistivity of ${\mathrm{Fe}}_{1.14}{\mathrm{Te}}_{0.7}{\mathrm{Se}}_{0.3}$ and ${\mathrm{Fe}}_{1.02}{\mathrm{Te}}_{0.61}{\mathrm{Se}}_{0.39}$ above $H_{c2}$ is metallic as $T\to 0$, just like the normal-state resistivity above $T_c$. On the other hand, the normal-state resistivity in ${\mathrm{Fe}}_{1.05}{\mathrm{Te}}_{0.89}{\mathrm{Se}}_{0.11}$ and ${\mathrm{Fe}}_{1.06}{\mathrm{Te}}_{0.86}{\mathrm{S}}_{0.14}$ is nonmetallic down to lowest temperatures, reflecting the superconductor-insulator transition due to electronic phase separation.

We used Raman scattering to study the lattice and magnetic excitations in the hexagonal ${\mathrm{HoMnO}}_3$ single crystals. The $E_2$ phonon mode at 237 ${\mathrm{cm}}^{-1}$ is affected by the magnetic order. This mode is related to the displacement of Mn and O ions in the $a–b$ plane and modulates the Mn-O-Mn bond angles in the $a–b$ plane and in the in-plane Mn-Mn superexchange interaction. The mode at 269 ${\mathrm{cm}}^{-1}$ associated to the displacement of the apical ${\mathrm{Ho}}^{3+}$ ions along the $c$ direction presents an abrupt change of slope at $T_N$, showing that the role of the rare-earth ions cannot be neglected in the magnetic transition. We have identified magnon and crystal-field excitations. The temperature dependence of the magnetic excitations has been compared to the Mn and Ho moment and indicates that the exchange-interaction pattern between Mn and Ho atoms drives the uniaxial anisotropy gap above the Mn-spin-rotation transition.

The bilayered member of the Ruddesden-Popper family of ruthenates, ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, has received increasing attention due to its interesting properties and phases. By using first principle calculations we find that the ground state is characterized by a ferromagnetic (FM) half-metallic state. This state strongly competes with an antiferromagnetic metallic phase, which indicates the possible presence of a particular state characterized by the existence of different magnetic domains. To drive the system towards a phase transition we studied the electronic and magnetic properties as a function of ${\mathrm{RuO}}_6$ octahedra rotations and found that the magnetic phase does not couple with the rotation angle. Our results provide accurate electronic, structure, and magnetic ground-state properties of ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ and stimulate the investigation of other types of octahedra rotations and distortions in the search of phase transitions.

With infrared (IR) ellipsometry and dc resistance measurements, we investigated the photodoping at the (001) and (110) surfaces of $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) single crystals and at the corresponding interfaces of $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (LAO/STO) heterostructures. In the bare STO crystals, we find that the photogenerated charge carriers, which accumulate near the (001) surface, have a similar depth profile and sheet carrier concentration as the confined electrons that were previously observed in LAO/STO (001) heterostructures. A large fraction of these photogenerated charge carriers persist at low temperature at the STO (001) surface even after the ultraviolet light has been switched off again. These persistent charge carriers seem to originate from oxygen vacancies that are trapped at the structural domain boundaries, which develop below the so-called antiferrodistortive transition at $T^{*}=105\mathrm{K}$. This is most evident from a corresponding photodoping study of the dc transport in STO (110) crystals for which the concentration of these domain boundaries can be modified by applying a weak uniaxial stress. The oxygen vacancies and their trapping by defects are also the source of the electrons that are confined to the interface of LAO/STO (110) heterostructures, which likely do not have a polar discontinuity as in LAO/STO (001). In the former, the trapping and clustering of the oxygen vacancies also has a strong influence on the anisotropy of the charge carrier mobility. We show that this anisotropy can be readily varied and even inverted by various means, such as a gentle thermal treatment, UV irradiation, or even a weak uniaxial stress. Our experiments suggest that extended defects, which develop over long time periods (of weeks to months), can strongly influence the response of the confined charge carriers at the LAO/STO (110) interface.

We study the thermal conductance across solid-solid interfaces as the composition of an intermediate matching layer is varied. In the absence of phonon-phonon interactions, an added layer can make the interfacial conductance increase or decrease depending on the interplay between (1) an increase in phonon transmission due to better bridging between the contacts and (2) a decrease in the number of available conduction channels that must conserve their momenta transverse to the interface. When phonon-phonon interactions are included, the added layer is seen to aid conductance when the decrease in resistances at the contact-layer boundaries compensate for the additional layer resistance. For the particular systems explored in this work, the maximum conductance happens when the layer mass is close to the geometric mean of the contact masses. The surprising result, usually associated with coherent antireflection coatings, follows from a monotonic increase in the boundary resistance with the interface mass ratio. This geometric mean condition readily extends to a compositionally graded interfacial layer with an exponentially varying mass that generates the thermal equivalent of a broadband impedance matching network.

Millimeter wave radiation-induced magnetoresistance oscillations are examined in the GaAs/AlGaAs 2D electron system under bichromatic excitation in order to study the evolution of the oscillatory diagonal magnetoresistance, $R_{xx}$ as the millimeter wave intensity is changed systematically for various frequency combinations. The results indicate that at low magnetic fields, the lower frequency millimeter wave excitation sets the observed $R_{xx}$ response, as the higher frequency millimeter wave component determines the $R_{xx}$ response at higher magnetic fields. The observations are qualitatively explained in terms of the order of the involved transitions. The results are also modeled using the radiation-driven electron orbit theory.

Based on density functional theory (DFT) calculations, we predict that a monolayer of ${\mathrm{OsCl}}_3$ (which is a layered material whose interlayer coupling is weaker than in graphite) possesses a quantum anomalous Hall (QAH) insulating phase generated by the combination of honeycomb lattice of osmium atoms, their strong spin-orbit coupling (SOC), and ferromagnetic ground state with in-plane easy axis. The band gap opened by SOC is $E_g\simeq 67$ meV (or $\simeq 191$ meV if the easy axis can be tilted out of the plane by an external electric field), and the estimated Curie temperature of such an anisotropic planar rotator ferromagnet is $T_{\mathrm{C}}\lesssim 350$ K. The Chern number $\mathcal{C}=-1$, generated by the manifold of Os $t_{2g}$ bands crossing the Fermi energy, signifies the presence of a single chiral edge state in nanoribbons of finite width, where we further show that edge states are spatially narrower for zigzag than armchair edges and investigate edge-state transport in the presence of vacancies at Os sites. Since $5d$ electrons of Os exhibit both strong SOC and moderate correlation effects, we employ DFT+$U$ calculations to show how increasing on-site Coulomb repulsion $U$: gradually reduces $E_g$ while maintaining $\mathcal{C}=-1$ for $0<U<U_c$, leads to a metallic phase with $E_g=0$ at $U_c$, and opens a gap of topologically trivial Mott insulating phase with $\mathcal{C}=0$ for $U>U_c$.

Aurivillius-phase oxides exhibit the coexistence of ferroelectricity (FE) and ferromagnetism (FM) above room temperature, and the FE and FM orders can be modified by various dopants. However, detailed examinations of their electronic structures in response to the dopants are still lacking. Here, the $\mathrm{B}{\mathrm{i}}_6\mathrm{F}{\mathrm{e}}_2\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_{18}, \mathrm{B}{\mathrm{i}}_6\mathrm{FeCoT}{\mathrm{i}}_3{\mathrm{O}}_{18}$, and $\mathrm{LaB}{\mathrm{i}}_5\mathrm{FeCoT}{\mathrm{i}}_3{\mathrm{O}}_{18}$ epitaxial thin films were prepared and the systematic experimental and theoretical resonant inelastic x-ray scattering (RIXS) studies were performed at both Fe and Co $L$ edges. The RIXS measurements reveal the modified electronic structures induced by the doping of Co and La. The RIXS simulations demonstrate that the electronic structure variations are the outcome of modified crystal-field parameters of $\mathrm{Fe}{\mathrm{O}}_6$ and $\mathrm{Co}{\mathrm{O}}_6$ octahedra.