We study odd-parity nematic superconductivity in doped topological insulators in the presence of surface magnetic impurities. The peculiar surface subgap spectrum, characterized by a Majorana flat band, nodal cones, and the surface states of the parent topological insulator, gives rise to overall ferromagnetic RKKY interactions between the surface impurities. An additional coupling between the impurities and a preemptive chiral order parameter promote a surface time-reversal symmetry breaking solution at the surface of the system. We discuss the relevant scenarios and suggest to engineer surface chiral superconductivity by properly choosing magnetic adatoms with highly anisotropic exchange coupling.

We describe a new approach to compute the electron-phonon self-energy and carrier mobilities in semiconductors. Our implementation does not require a localized basis set to interpolate the electron-phonon matrix elements, with the advantage that computations can be easily automated. Scattering potentials are interpolated on dense $\mathbf{q}$ meshes using Fourier transforms and ab initio models to describe the long-range potentials generated by dipoles and quadrupoles. To reduce significantly the computational cost, we take advantage of crystal symmetries and employ the linear tetrahedron method and double-grid integration schemes, in conjunction with filtering techniques in the Brillouin zone. We report results for the electron mobility in Si, GaAs, and GaP obtained with this new methodology.

We demonstrate the control of vortical motion of neutral classical particles in driven superlattices. Our superlattice consists of a superposition of individual lattices whose potential depths are modulated periodically in time but with different phases. This driving scheme breaks the spatial reflection symmetries and allows an ensemble of particles to rotate with an average angular velocity. An analysis of the underlying dynamical attractors provides an efficient method to control the angular velocities of the particles by changing the driving amplitude. As a result, spatially periodic patterns of particles showing different vortical motions can be created. Possible experimental realizations include holographic optical lattice based setups for colloids or cold atoms.

We consider a spin-fermion model consisting of free electrons coupled to classical spins where the latter are embedded in a quasi-one-dimensional superlattice structure consisting of spin blocks separated by spinless buffers. Using a spiral ansatz for the spins, we study the effect of the electron mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction on the $T=0$ ground state of the system. We find that the RKKY interaction can lead to ferromagnetic, antiferromagnetic, or intermediate spiral phases for different system parameters. When the width is much larger than the length of the individual blocks, the spiral phases are suppressed, and the ground state oscillates between ferromagnetic and antiferromagnetic orders as the size of the buffer regions is varied. This is accompanied by a corresponding oscillation in the Drude weight reflecting an increased conductivity in the ferromagnetic state compared to the antiferromagnetic one. These results are reminiscent of classic giant magnetoresistance phenomena observed in a similar geometry of thin sandwiched magnetic and nonmagnetic layers. Our analysis provides a robust framework for understanding the role of the RKKY interaction on the ground-state order and corresponding transport properties of such systems, extending beyond the conventional perturbative regime.

We report field-induced magnetic phase transitions and transport properties of antiferromagnetic DyPtBi. We show that DyPtBi hosts a delicate balance between two different magnetic ground states, which can be controlled by a moderate magnetic field. Furthermore, it exhibits giant anomalous Hall effect $\left({\sigma }^A=1540{\mathrm{\Omega }}^{-1}\mathrm{c}{\mathrm{m}}^{-1},{\theta }_{\mathrm{AHE}}=24\%\right)$ in a field-induced type-I spin structure, presumably attributed to the enhanced Berry curvature associated with avoided band crossings near the Fermi energy and/or nonzero spin chirality. The latter mechanism points DyPtBi towards a rare potential realization of anomalous Hall effect in an antiferromagnet with face-centered-cubic lattice that was proposed in [Physical Review Letters 87, 116801 (2001)].

Recent experiments on twisted bilayer graphene show an anomalous quantum Hall (AQH) effect at filling of three electrons per moiré unit cell. The AQH effect arises in an insulating state with both valley and ferromagnetic order. We argue that weak doping of such a system leads to the formation of a novel topological spin texture, a “double-tetarton lattice.” The building block of this lattice, the “double-tetarton,” is a spin configuration which covers $1/4$ of the unit sphere twice. In contrast to skyrmion lattices, the net magnetization of this magnetic texture vanishes. Only at large magnetic fields are more conventional skyrmion lattices recovered. But even for large fields the addition of a single charge to the ferromagnetic AQH state flips hundreds of spins. Our analysis is based on the investigation of an effective nonlinear sigma model which includes the effects of long-ranged Coulomb interactions.

A microscopic theory of the interaction of Cr spins with electric and magnetic fields in ${\mathrm{DyCrO}}_4$ is described. The derived effective energy operator allows calculating the electric polarization vector using information about the magnetic structure. The calculated linear electric polarization in electric and magnetic fields at $H<3\text{T}$ is consistent with the available experimental data. The electric field is induced by an odd crystalline field on chromium ions and the charge transfer process from the nearest oxygen ions. The enhancement of the magnetic field is associated with magnetic susceptibility, mainly due to dysprosium ions.

Elastic properties and sound velocities of superionic ice X and ice XVIII are investigated using ab initio molecular dynamics at 200 GPa and temperatures up to 4500 K. The dislocation of protons from their lattice sites leads to the significant elastic softening in ice X with increasing temperature. The phase transition from ice X to ice XVIII leads to an increasing in proton diffusion and elastic softening at 2000 K. We also observed the elastic stiffening with increasing temperature from 2000 to 3000 K in ice XVIII, which is similar to the behavior of some liquids. Above 3000 K, all the elastic constants decrease dramatically. Our study suggests proton diffusion has significant influence on the elastic properties, and the elastic property of superionic ice with increasing temperature is different from ordinary solids.

We study the short-imaginary-time quantum critical dynamics (SITQCD) in the $J-Q_3$ spin chain, which hosts a quasi-long-range-order phase to a valence bond solid transition. By using the scaling form of the SITQCD with a saturated ordered phase, we are able to locate the critical point at $q_{\mathrm{c}}=0.170\left(14\right)$. We also obtain the critical initial slip exponent $\theta =-0.507\left(3\right)$ and the static exponent $\beta /\nu =0.498\left(2\right)$. More strikingly, we find that the scaling dimension of the initial order parameter $x_0$ is close to zero, which suggests that the initial order parameter is a marginal operator. As a result, there is no initial increase behavior of the order parameter in the short-imaginary-time relaxation process for this model, which is very different from the relaxation dynamics in the Ising-type phase transitions. Our numerical results are realized by the projector quantum Monte Carlo algorithm.

The presence of ordered oxygen vacancies in perovskites governs magnetic phase stability owing to changes in crystal-field splitting with different anion geometries, polyhedral arrangements, and electronic configurations of the transition-metal cations. Here we use density functional theory calculations to assess the magnetic phase stability of ${\mathrm{Sr}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$ (with a $d^5$ electronic configuration) and ${\mathrm{Sr}}_2{\mathrm{Mn}}_2{\mathrm{O}}_5$ ($d^4$ configuration), exhibiting the ${\mathrm{Ca}}_2{\mathrm{Mn}}_2{\mathrm{O}}_5$-type oxygen-deficient perovskite structure, with hydrostatic pressure. The ${\mathrm{Ca}}_2{\mathrm{Mn}}_2{\mathrm{O}}_5$-type structure is composed of square pyramidal units, the crystal-field splitting and polyhedral connectivities of which support different ground-state magnetic orders depending on $d$-orbital filling: E-type antiferromagnetic (AFM-E) for ${\mathrm{Sr}}_2{\mathrm{Mn}}_2{\mathrm{O}}_5$ ($d^4$) and G-type antiferromagnetic (AFM-G) for ${\mathrm{Sr}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$ ($d^5$). We show that hydrostatic pressure enhances the crystal-field splitting and affects the magnetic stability. We find that the AFM-E order exhibited by ${\mathrm{Sr}}_2{\mathrm{Mn}}_2{\mathrm{O}}_5$ is robust over the surveyed ranges of applied pressures, whereas ${\mathrm{Sr}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$ shows a magnetic transition from AFM-G to ferromagnetic spin order at $\approx 24.5$ GPa. We also discuss the effect of correlation strength, treated using the Hubbard $U$ correction, which we find suppresses a spin crossover transition in ${\mathrm{Sr}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$ and shifts it to higher pressures.

Graphene may exhibit different topological phases as a result of proximity to different substrates. We study the effect of superconductivity in such systems using the effective Bogolyubov-de Gennes Hamiltonian with different superconducting pairing order parameters. We analyze the topological phase transition and symmetry class of the system in different parameter regimes. A particularly interesting situation occurs when nearest-neighbor spin-singlet superconducting pairing is present in phases of proximitized graphene that exhibit either inverted band or quantum spin Hall behavior. Both superconducting phases show similar characteristics in the low-energy range, including the appearance of robust edge states, and are neighboring phases across a transition that closes the quasiparticle gap as the chemical potential changes. Detailed construction and analysis of the existence and nature of edge states are presented in different system regimes.

Hexagonal boron nitride (hBN) is attracting tremendous interest as an essential component in van der Waals heterostructures due to its ability to provide weakly interacting interfaces and because of its large bandgap. Although most of theoretical calculations yield the standard $A{A}^{\prime }$ stacking for few-layer hBN, the exact determination of its structural and electronic properties remains unrevealed to date. Here, we provide the direct observation of structural and electronic transitions in few layers of isotopically pure exfoliated ${\mathrm{h}}^{11}\mathrm{BN}$ flakes. Our nanoscopic angle-resolved photoemission spectroscopy measurements combined with density-functional theory calculations indicate that the stacking and the band structure can be strongly affected by the thickness of ${\mathrm{h}}^{11}\mathrm{BN}$. Hence, we show that hBN presents an $A{A}^{\prime }$ stacking in its bulk form and another more exotic stacking for three and four layers. Our findings open perspectives in understanding and controlling the stackings in hBN, which could be of great interest for optoelectronic applications.

High-temperature bad-metal transport has been recently studied both theoretically and in experiments as one of the key signatures of strong electronic correlations. Here we use the dynamical mean field theory and its cluster extensions, as well as the finite-temperature Lanczos method to explore the influence of lattice frustration on the thermodynamic and transport properties of the Hubbard model at high temperatures. We consider the triangular and the square lattices at half-filling and at 15% hole doping. We find that for $T\gtrsim 1.5t$ the self-energy becomes practically local, while the finite-size effects become small at lattice size $4\times 4$ for both lattice types and doping levels. The vertex corrections to optical conductivity, which are significant on the square lattice even at high temperatures, contribute less on the triangular lattice. We find approximately linear temperature dependence of dc resistivity in doped Mott insulator for both types of lattices.

Recent optical conductivity measurements reveal the presence of Hubbard excitons in certain Mott insulators. In light of these results, it is important to revisit the dynamics of these materials to account for excitonic correlations. We investigate time-resolved excitation and relaxation dynamics as a function of temperature in perovskite-type ${\mathrm{LaVO}}_3$ thin films using ultrafast optical pump-probe spectroscopy. ${\mathrm{LaVO}}_3$ undergoes a series of phase transitions at roughly the same critical temperature $T_C\cong 140$ K, including a second-order magnetic phase transition ($\mathrm{PM}\stackrel{}{\to }\mathrm{AFM}$) and a first-order structural phase transition, accompanied by $C$-type spin order and $G$-type orbital order. Ultrafast optical pump-probe spectroscopy at 1.6 eV monitors changes in the spectral weight of the Hubbard exciton resonance which serves as a sensitive reporter of spin and orbital fluctuation dynamics. We observe dramatic slowing down of the spin, and orbital dynamics in the vicinity of $T_C\cong 140$ K, reminiscent of a second-order phase transition, despite the (weakly) first-order nature of the transition. We emphasize that since it is spectral weight changes that are probed, the measured dynamics are not reflective of conventional exciton generation and recombination, but are related to the dynamics of Hubbard exciton formation in the presence of a fluctuating many-body environment.

We study a flux qubit consisting of a symmetrical pair of superconducting loops, with two Josephson junctions in each, joined by a common Josephson junction—a ‘twin’ flux qubit. The qubit is capacitively coupled to a transmission line, which allows us to characterize the spectrum of the device by measuring the scattering of propagated electromagnetic waves. We perform a detailed analytical analysis of the double-loop system, revealing its properties, and compare experimental results with numerical simulations. At half-flux quantum bias of both loops, the qubit is protected against global and local magnetic field fluctuations with much less sensitivity to the global field in the second order. The system selection rules allow even-odd transitions and prohibit transitions between even-even or odd-odd levels due to the symmetry of the device.

The violation of Wiedemann-Franz (WF) law has been widely discussed in quantum transport experiments as an indication of deviation from Fermi-liquid behavior. The conventional form of WF law is only concerned with the transmission function at the Fermi level, which, however, vanishes in many practical situations. We reinvestigate the WF law in noninteracting quantum systems with vanishing zero-energy transmission and report a universal number $21/5$ as an upper bound of the Lorenz ratio $\mathcal{R}$ in weakly energy-dependent scattering theory. We provide different experimental realizations for the observation of $\mathcal{R}=21/5$, namely, the transport setups with graphene, the multilevel quantum dot and double quantum dot. The reported universal Lorenz ratio paves an efficient way of experimentally obtaining the information about the associated quantum interferences in the system. Our work also provides enough evidence which concludes that the violation of WF law does not necessarily imply the non-Fermi-liquid nature of underlying transport processes; equally, the Fermi-liquid transport characteristics cannot be concluded by an observed validation of WF law.

Excitons, quasiparticles composed of an electron and a hole, play an important role in optical responses in low-dimensional nanostructures. In this work, we have investigated exciton diffusion in monolayer $\mathrm{Mo}{\mathrm{Se}}_2$ encapsulated between flakes of hexagonal boron nitride ($h\mathrm{BN}/\mathrm{MoS}{\mathrm{e}}_2/h\mathrm{BN}$). Through photoluminescence imaging and numerical solution of the two-dimensional diffusion equation, we revealed that temperature dependence of exciton mobility, ${\mu }_{\mathrm{ex}}$, in $h\mathrm{BN}/\mathrm{MoS}{\mathrm{e}}_2/h\mathrm{BN}$ shows a nonsaturating increase at low temperature, which is qualitatively different from those of quantum wells composed of compound semiconductors. The ultraflat structure of monolayer $\mathrm{Mo}{\mathrm{Se}}_2$ in $h\mathrm{BN}/\mathrm{MoS}{\mathrm{e}}_2/h\mathrm{BN}$ probably leads to the suppression of charged-impurity scattering and surface-roughness scattering.

Helical molecules have recently attracted interest due to their capability for robust spin polarization of transmitted electrons. By means of mechanically controlled break Au junctions, we analyze the transport properties of single lysine-doped and cysteine-terminated polyalanine (PA) molecules of various lengths (2.4–5.4 nm). The conductance varies exponentially with the (effective) length of the molecules and does not depend on the temperature (90–300 K), thus electron tunneling is the dominant transport mechanism. The decay constant for the PA molecule is found to be $3.5{\mathrm{nm}}^{-1}$, significantly smaller compared to those of other organic molecules, emphasizing the high conductivity along the helical polypeptides and reflecting a low tunneling barrier, which decreases further for fields exceeding $5\times {10}^5\mathrm{V}/\mathrm{cm}$. The conductance histograms of all PA molecules investigated reveal characteristic satellite peaks, which correlate with the apparent molecule length in multiples of characteristic peptide sequences. We attribute this effect to a racheting of interdigitated molecules adsorbed on each side of the electrodes during the opening/closing cycles.

In a recent work [Jiang et al., Phys. Rev. B 101, 035151 (2020)], the authors solved a model with a Cu impurity in an O-$2p$ band as an approximation to the local electronic structure of a hole doped cuprate. One of their conclusions is that the ground state has only $\approx 50\%$ overlap with a Zhang-Rice singlet (ZRS). This claim is based on the definition of the ZRS in a different representation, in which the charge fluctuations at the Cu site have been eliminated by a canonical transformation. The correct interpretation of the results, based on known low-energy reduction procedures for a multiband model including $3d^8$ and $3d^{10}$ configurations of Cu, indicates that this overlap is near 94%.