${\mathrm{URu}}_2{\mathrm{Si}}_2$ exhibits an anomalous peak in the nonlinear magnetic susceptibility at the hidden order transition. In order to investigate this anomaly, we conducted direct magnetization measurements and investigated the detailed angular dependence of the ${}^{29}\mathrm{Si}$ nuclear magnetic resonance Knight shift tensor. We find that the nonlinear magnetization is smaller than previously reported, and the analogous nonlinear Knight shift tensor is below the detection limit. Our results suggest that the magnitude of the anomalous peak is sample dependent.

We present a method which combines plane waves (PW) and numerical atomic orbitals (NAO) to efficiently evaluate nonlocal operators in density functional theory with periodic boundary conditions. Nonlocal operators are first expanded using PW and then transformed to NAO so that the problem of distance-truncation is avoided. The general formalism is implemented using the hybrid functional HSE06 where the nonlocal operator is the exact exchange. Comparison of electronic structures of a wide range of semiconductors to a pure PW scheme validates the accuracy of our method. Due to the locality of NAO, thus sparsity of matrix representations of the operators, the computational complexity of the method is asymptotically quadratic in the number of electrons. Finally, we apply the technique to investigate the electronic structure of the interface between a single-layer black phosphorous and the high-$\kappa$ dielectric material $c-{\mathrm{HfO}}_2$. We predict that the band offset between the two materials is 1.29 eV and 2.18 eV for valence and conduction band edges, respectively, and such offsets are suitable for 2D field-effect transistor applications.

By means of Shubnikov–de Haas and de Haas–van Alphen oscillations, and ab initio calculations, we have studied the Fermi surface of high-quality ${\mathrm{BaNiS}}_2$ single crystals, with mean free path $l\sim 400\text{Å}$. The angle and temperature dependence of quantum oscillations indicates a quasi-two-dimensional Fermi surface, made of an electronlike tube centered at $\mathrm{\Gamma }$, and of four holelike cones, generated by Dirac bands, weakly dispersive in the out-of-plane direction. Ab initio electronic structure calculations, in the density functional theory framework, show that the inclusion of screened exchange is necessary to account for the experimental Fermi pockets. Therefore, the choice of the functional becomes crucial. A modified HSE hybrid functional with 7% of exact exchange outperforms both GGA and $\mathrm{GGA}+U$ density functionals, signaling the importance of nonlocal screened-exchange interactions in ${\mathrm{BaNiS}}_2$, and, more generally, in $3d$ compensated semimetals.

Engineering effective $p$-wave superconductors hosting Majorana quasiparticles (MQPs) is nowadays of particular interest, also in view of the possible utilization of MQPs in fault-tolerant topological quantum computation. In quasi-one-dimensional systems, the parameter space for topological superconductivity is significantly reduced by the coupling between transverse modes. Together with the requirement of achieving the topological phase under experimentally feasible conditions, this strongly restricts in practice the choice of systems which can host MQPs. Here, we demonstrate that semiconducting carbon nanotubes (CNTs) in proximity with ultrathin $s$-wave superconductors, e.g., exfoliated ${\mathrm{NbSe}}_2$, satisfy these needs. By precise numerical tight-binding calculations in the real space, we show the emergence of localized zero-energy states at the CNT ends above a critical value of the applied magnetic field, of which we show the spatial evolution. Knowing the microscopic wave functions, we unequivocally demonstrate the Majorana nature of the localized states. An effective four-band model in the $k$-space, with parameters determined from the numerical spectrum, is used to calculate the topological phase diagram and its phase boundaries in analytic form. Finally, the impact of symmetry breaking contributions, like disorder and an axial component of the magnetic field, is investigated.

The gate bias dependence of excited-state relaxation dynamics in metallic single-walled carbon nanotubes (MCNTs) was investigated using pump-probe transient absorption spectroscopy coupled with electrochemical doping through an ionic liquid. The transient transmittance decayed exponentially with the pump-probe delay time, whose value could be tuned via the Fermi-level modulation of Dirac electrons under a bias voltage. The obtained relaxation time was the shortest when the Fermi level was at the Dirac point of the MCNTs, and exhibited a U-shaped dependence on the bias voltage. Because optical dipole transitions between the Dirac bands are forbidden in MCNTs, the observed dynamics were attributed to carrier relaxation from the $E_{11}$ band to the Dirac band. Using a model that considers the suppression of electron-electron scattering (impact ionization) due to Pauli blocking, we could qualitatively explain the obtained bias dependence of the relaxation time.

Weyl semimetals (WSMs), a new type of topological condensed matter, are currently attracting great interest due to their unusual electronic states and intriguing transport properties such as chiral anomaly induced negative magnetoresistance, a semiquantized anomalous Hall effect, and the debated chiral magnetic effect. These systems are close cousins of topological insulators (TIs) which are known for their disorder-tolerant surface states. Similarly, WSMs exhibit unique topologically protected Fermi-arc surface states. Here, we analyze electron-phonon scattering, a primary source of resistivity in metals at finite temperatures, as a function of the shape of the Fermi arc where we find that the impact on surface transport is significantly dependent on the arc curvature and disappears in the limit of a straight arc. Next, we discuss the effect of strong surface disorder on the resistivity by numerically simulating a tight-binding model with the presence of quenched surface vacancies using the coherent potential approximation and Kubo-Greenwood formalism. We find that the limit of a straight arc geometry is remarkably disorder tolerant, producing surface conductivity that is one to two orders of magnitude larger than a comparable setup with surface states of TI. This is primarily attributed to a significantly different hybridization strength of the surface states with the remaining electrons in two systems. Finally, a simulation of the effects of surface vacancies on TaAs is presented, illustrating the disorder tolerance of the topological surface states in a recently discovered WSM material.

Acceptor and donor doping is a standard for tailoring semiconductors. More recently, doping was adapted to optimize the behavior at ferroelectric domain walls. In contrast to more than a century of research on semiconductors, the impact of chemical substitutions on the local electronic response at domain walls is largely unexplored. Here, the hexagonal manganite ${\mathrm{ErMnO}}_3$ is donor doped with ${\mathrm{Ti}}^{4+}$. Density functional theory calculations show that ${\mathrm{Ti}}^{4+}$ goes to the $B$ site, replacing ${\mathrm{Mn}}^{3+}$. Scanning probe microscopy measurements confirm the robustness of the ferroelectric domain template. The electronic transport at both macroscopic and nanoscopic length scales is characterized. The measurements demonstrate the intrinsic nature of emergent domain wall currents and point towards Poole-Frenkel conductance as the dominant transport mechanism. Aside from the new insight into the electronic properties of hexagonal manganites, $B$-site doping adds an additional degree of freedom for tuning the domain wall functionality.

A unique feature of the hybrid quantum Monte Carlo (HQMC) method is the potential to simulate negative sign free lattice fermion models with subcubic scaling in system size. Here we will revisit the algorithm for various models. We will show that for the Hubbard model the HQMC suffers from ergodicity issues and unbounded forces in the effective action. Solutions to these issues can be found in terms of a complexification of the auxiliary fields. This implementation of the HQMC that does not attempt to regularize the fermionic matrix so as to circumvent the aforementioned singularities does not outperform single spin flip determinantal methods with cubic scaling. On the other hand we will argue that there is a set of models for which the HQMC is very efficient. This class is characterized by effective actions free of singularities. Using the Majorana representation, we show that models such as the Su-Schrieffer-Heeger Hamiltonian at half filling and on a bipartite lattice belong to this class. For this specific model subcubic scaling is achieved.

A theory describing how ferroic properties can emerge transiently in the ultrafast regime by breaking symmetry with light through three-phonon coupling is presented. Particular emphasis is placed on the special case when two exactly degenerate midinfrared or THz phonons are resonantly pumped, since this situation can give rise to an exactly rectified ferroic response with damping envelopes of $\sim 1$ ps or less. Light-induced ferroelectricity and ferromagnetism are discussed in this context and a number of candidate materials that could display these phenomena are proposed. The same analysis is also applied to the interpretation of previous femtomagnetism experiments, performed in different frequency ranges (visible and near infrared), but sharing similar symmetry characteristics.

Inelastic neutron scattering was used to study the vibrational dynamics of fused silica and its mixed binary glasses that were doped with either ${\text{TiO}}_2$ or ${\text{K}}_2\text{O}$. The energy transfer was measured from zero to $180\mathrm{meV}$ where the so-called Boson peaks (BP) at low energy and molecular vibrations at high energy are included. Although most of the vibrational spectra at the high energy resemble those reported in earlier literature, a defect-mode-like peak is observed for the doped binary systems near $120\mathrm{meV}$. At very low temperature, the BP intensity increases rapidly with temperature and then, at higher temperature, the peak intensity decreases. As a result, a maximum is observed in the temperature dependence of the BP intensity. This maximum was shown in all four samples, but the pure ${\text{SiO}}_2$ sample shows the highest intensity peak and the lowest temperature for peak position. Broadband energy spectra reveal a shift of intensity from BP to the more localized modes at higher energy. Temperature evolution of BP and its relationship with heat conduction and thermal expansion are discussed.

The exchange coupling between ferromagnetic (FM) and antiferromagnetic materials has been intensely studied for fundamental physics and technological applications in various devices. However, the experimental reported magnitudes of exchange coupling are often smaller than that predicted theoretically, and for which the formation of springlike spin structure in the FM layer has been suggested as the cause. However, investigating the spin structure around the interface of exchange-coupled systems is challenging. Here we report the direct imaging of the cross-sectional magnetization reversal and spin structure at the interface of a model exchange-biased CoFeB/IrMn bilayer by a Lorentz transmission electron microscope with electron holography techniques. Through imaging of in situ magnetization reversal and spin structure at the remanent state, the springlike spin structure (either Bloch-wall-like or Néel-wall-like) in the CoFeB layer has been deduced within subnanometer region of the interface. This result puts a strong constraint on the theories of exchange coupling in inhomogeneous magnetic systems.

Currents can induce spin excitations in antiferromagnets, even when they are insulating. We investigate how spin transfer can cause antiferromagnetic resonance in bilayers and trilayers that consist of one antiferromagnetic insulator and one or two metals. An ac voltage applied to the metal generates a spin Hall current that drives the magnetic moments in the antiferromagnet. We consider excitation of a sublattice macrospin mode and of transverse standing-spin-wave modes. By solving the Landau-Lifshitz-Gilbert equation in the antiferromagnetic insulator and the spin-diffusion equation in the normal metal, we derive analytical expressions for the spin-Hall-magnetoresistance and spin-pumping inverse-spin-Hall dc voltages. In bilayers, the two contributions compensate each other and cannot easily be distinguished. We present numerical results for a ${\mathrm{MnF}}_2|\mathrm{Pt}$ bilayer. Trilayers facilitate separation of the spin-Hall-magnetoresistance and spin-pumping voltages, thereby revealing more information about the spin excitations. We also compute the decay of the pumped spin current through the antiferromagnetic layer as a function of frequency and the thickness of the antiferromagnetic layer.

We use electrical transport, magnetoresistance, and heat capacity measurements on high quality single crystals of the recently discovered superconducting type-II Dirac semimetal ${\mathrm{PdTe}}_2$, to probe the nature of its superconducting phase. The magnitude of the electronic heat-capacity anomaly at $T_c$, the low temperature exponential $T$ dependence of the heat capacity, the linear $H$ dependence of the $T=0$ electronic Sommerfeld coefficient, and a conventional $H-T$ phase diagram establish that the superconductivity in ${\mathrm{PdTe}}_2$ is conventional in nature despite the presence of a topologically nontrivial Fermi surface band, which contributes to the electrical conduction.

We predict a magnetic Weyl semimetal in the inverse Heusler ${\mathrm{Ti}}_2\mathrm{MnAl}$, a compensated ferrimagnet with a vanishing net magnetic moment and a Curie temperature of over 650 K. Despite the vanishing net magnetic moment, we calculate a large intrinsic anomalous Hall effect (AHE) of about 300 S/cm. It derives from the Berry curvature distribution of the Weyl points, which are only 14 meV away from the Fermi level and isolated from trivial bands. Different from antiferromagnets ${\mathrm{Mn}}_{3}X$ ($X=\mathrm{Ge}$, Sn, Ga, Ir, Rh, and Pt), where the AHE originates from the noncollinear magnetic structure, the AHE in ${\mathrm{Ti}}_2\mathrm{MnAl}$ stems directly from the Weyl points and is topologically protected. The large anomalous Hall conductivity (AHC) together with a low charge carrier concentration should give rise to a large anomalous Hall angle. In contrast to the Co-based  ferromagnetic Heusler compounds, the Weyl nodes in ${\mathrm{Ti}}_2\mathrm{MnAl}$ do not derive from nodal lines due to the lack of mirror symmetries in the inverse Heusler structure. Since the magnetic structure breaks spin-rotation symmetry, the Weyl nodes are stable without SOC. Moreover, because of the large separation between Weyl points of opposite topological charge, the Fermi arcs extent up to $75\%$ of the reciprocal lattice vectors in length. This makes ${\mathrm{Ti}}_2\mathrm{MnAl}$ an excellent candidate for the comprehensive study of magnetic Weyl semimetals. It is the first example of a material with Weyl points, large anomalous Hall effect, and angle despite a vanishing net magnetic moment.

We use transport and inelastic neutron-scattering measurements to investigate single crystals of iron pnictide ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x{\mathrm{As}}_2(x=0,0.03)$, which exhibit a tetragonal-to-orthorhombic structural transition at $T_s$ and stripe antiferromagnetic order at $T_N(T_s\ge T_N)$. Using a tunable uniaxial pressure device, we detwin the crystals and study their transport and spin excitation properties at antiferromagnetic wave-vector $S_1(1,0)$ and its ${90}^{\circ }$ rotated wave-vector $S_2(0,1)$ under different pressure conditions. We find that uniaxial pressure necessary to detwin and maintain the single domain orthorhombic antiferromagnetic phase of ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x{\mathrm{As}}_2$ induces resistivity and spin excitation anisotropy at temperatures above zero pressure $T_s$. In the uniaxial pressure-free detwinned sample, spin excitation anisotropy between $S_1(1,0)$ and $S_2(0,1)$ first appears in the paramagnetic orthorhombic phase below $T_s$. These results are consistent with predictions of spin nematic theory, suggesting the absence of structural or nematic phase transition above $T_s$ in iron pnictides.

Coherent operation of gate-voltage-controlled hybrid transmon qubits (gatemons) based on semiconductor nanowires was recently demonstrated. Here we experimentally investigate the anharmonicity in epitaxial InAs-Al Josephson junctions, a key parameter for their use as a qubit. Anharmonicity is found to be reduced by roughly a factor of 2 compared to conventional metallic junctions and dependent on gate voltage. Experimental results are consistent with a theoretical model, indicating that Josephson coupling is mediated by a small number of highly transmitting modes in the semiconductor junction.

We study azimuthal spin-wave (SW) excitations in a circular ferromagnetic nanodot in different inhomogeneous, topologically nontrivial magnetization states, specifically, vortex, Bloch-type skyrmion, and Néel-type skyrmion states. A continuous transition between these states is realized by gradually changing the out-of-plane magnetic anisotropy and the Dzyaloshinskii-Moriya exchange interaction (DMI), and the corresponding SW spectra are calculated for each state. We observe the lifting of degeneracy of SW mode frequencies and a change in the systematics of frequency levels. The latter effect is induced by the geometric Berry phase, which occurs in SWs localized at the edge of the dot in the vortex state, and vanishes in the skyrmion states. Furthermore, channeling of edge-localized azimuthal SWs and a related large frequency splitting are observed in the skyrmion states. This is attributed to DMI-induced nonreciprocity, while the coupling of the breathing and gyrotropic modes is related to the skyrmion motion. Finally, we demonstrate efficient coupling of the dynamic magnetization to a uniform magnetic field in nanodots of noncircular symmetry in the skyrmion states.

We study the nonlinear conductance $\mathcal{G}\sim {\partial }^{2}I/\partial V^2{|}_{V=0}$ in coherent quasi-one-dimensional weakly disordered metallic wires. Our analysis is based on the scattering approach and includes the effect of Coulomb interaction. The nonlinear conductance correlations can be related to integrals of two fundamental correlation functions: the correlator of functional derivatives of the conductance and the correlator of injectivities (the injectivity is the contribution to the local density of states of eigenstates incoming from one contact). These correlators are obtained explicitly by using diagrammatic techniques for weakly disordered metals. In a coherent wire of length $L$, we obtain $\mathrm{rms}\left(\mathcal{G}\right)\simeq 0.006E_{\mathrm{Th}}^{-1}$ (and $〈\mathcal{G}〉=0)$, where $E_{\mathrm{Th}}=\hslash D/L^2$ is the Thouless energy of the wire and $D$ the diffusion constant; the small dimensionless factor results from screening, i.e., cannot be obtained within a simple theory for noninteracting electrons. Electronic interactions are also responsible for an asymmetry under magnetic field reversal; the antisymmetric part of the nonlinear conductance (at high magnetic field) being much smaller than the symmetric one, $\mathrm{rms}\left({\mathcal{G}}_a\right)\simeq 0.001{\left(g{E}_{\mathrm{Th}}\right)}^{-1}$, where $g\gg 1$ is the dimensionless (linear) conductance of the wire. In a weakly coherent wire (i.e., $L_{\phi }\ll L$, where $L_{\phi }$ is the phase coherence length), the nonlinear conductance is of the same order as the result ${\mathcal{G}}_0$ of a free electron calculation (although screening again strongly reduces the dimensionless prefactor); we get $\mathcal{G}\sim {\mathcal{G}}_0\sim {(L_{\phi }/L)}^{7/2}{E}_{\mathrm{Th}}^{-1}$, while the antisymmetric part (at high magnetic field) now behaves as ${\mathcal{G}}_a\sim {(L_{\phi }/L)}^{11/2}{\left(g{E}_{\mathrm{Th}}\right)}^{-1}\ll \mathcal{G}$. The effect of thermal fluctuations is studied: when the thermal length $L_T=\sqrt{\hslash D/k_{B}T}$ is the smallest length scale, $L_T\ll L_{\phi }\ll L$, the free electron result ${\mathcal{G}}_0\sim {(L_T/L)}^3{(L_{\phi }/L)}^{1/2}{E}_{\mathrm{Th}}^{-1}$ is negligible and the dominant contribution is provided by screening, $\mathcal{G}\sim (L_T/L){(L_{\phi }/L)}^{7/2}{E}_{\mathrm{Th}}^{-1}$; in this regime, the antisymmetric part is ${\mathcal{G}}_a\sim {(L_T/L)}^2{(L_{\phi }/L)}^{7/2}{\left(g{E}_{\mathrm{Th}}\right)}^{-1}$. All the precise dimensionless prefactors are obtained. Crossovers from zero to strong magnetic field regimes are also analyzed.