We report the synthesis of single-crystal NbC, a transition metal carbide with various unusual properties. Transport, magnetic susceptibility, and specific heat measurements demonstrate that NbC is a conventional superconductor with a superconducting transition temperature $\left(T_c\right)$ of 11.5 K. Our theoretical calculations show that NbC is a type-II Dirac semimetal with strong Fermi-surface nesting, which is supported by our angle-resolved photoemission spectroscopy (ARPES) measurement results. We also observed the superconducting gaps of NbC using ARPES and found some unconventional behaviors. These intriguing superconducting and topological properties, combined with the high corrosion resistance, make NbC an ideal platform for both fundamental research and device applications.

Electron-optical phonon interaction is the dominant energy-loss mechanism in low-dimensional Ge/SiGe heterostructures and represents a key parameter for the design and realization of electronic and optoelectronic devices based on this material system compatible with the mainstream Si complementary metal-oxide semiconductor technology. Here we investigate the intersubband relaxation dynamics of $n$-type Ge/SiGe multiquantum wells with different symmetry and design by means of single-color pump-probe spectroscopy. By comparing the experimental differential transmittance data as a function of the pump-probe delay with numerical calculations based on an energy-balance rate-equation model, we could quantify an effective value for the optical phonon deformation potential describing the electron-phonon coupling in two-dimensional Ge-based systems. We found nonradiative relaxation times longer than 20 ps even in samples having intersubband energy separations larger than the optical phonon energy, evidencing the presence of a less effective electron-phonon coupling with respect to that estimated in bulk Ge.

Terahertz scanning tunneling microscopy (THz-STM) has enabled studies of ultrafast dynamics in materials down to the atomic scale. However, despite recent advances, more work is needed to better understand and quantify the subpicosecond THz pulse-induced tunnel currents and corresponding THz-STM images of nanoscale features on surfaces. Here, we perform THz-STM on a metal surface and fully characterize the observed THz pulse-induced tunnel current and nanoscale imaging at atomic steps and defects using a Bardeen tunneling model in a three-dimensional (3D) tip geometry. We show that the measured steady-state STM current-voltage curves can be used in our model to accurately map the observed ultrafast THz-induced tunnel currents and calibrate the magnitude of the near-field peak transient THz voltage bias in the tunnel junction. Peak THz voltage bias transients greater than 10 V across the STM junction are achieved leading to field emission of subpicosecond tunnel currents with current densities exceeding ${10}^9\mathrm{A}/\mathrm{c}{\mathrm{m}}^2$ in THz-STM imaging of a Cu(111) surface. Our results establish an important benchmark for future studies in THz-STM by quantifying the ultrafast THz-induced currents and bias voltages in the tunnel junction and providing a 3D tunneling model for understanding and accurately simulating THz-STM images of nanoscale features on metal surfaces.

We have scrutinized the growth of germanium (Ge) on molybdenum disulfide (${\mathrm{MoS}}_2$) using scanning tunneling microscopy and density functional theory calculations in order to resolve the still outstanding question whether Ge atoms prefer to intercalate between the ${\mathrm{MoS}}_2$ layers or rather form germanene islands on top of the ${\mathrm{MoS}}_2$ substrate. We found that, at a fixed growth temperature, germanene islands are formed on top of the ${\mathrm{MoS}}_2$ substrate at high deposition rates, whereas at low deposition rates the Ge intercalates between the ${\mathrm{MoS}}_2$ layers. Scanning tunneling spectra recorded on the germanene islands reveal a $V$-shaped density of states, which is one of the hallmarks of a two-dimensional Dirac material. The intercalated Ge clusters have a band gap of 0.5–0.6 eV. Density functional theory calculations have been conducted in order to study the stability and electronic band structure of several intercalated Ge cluster configurations. Based on these calculations we are able to identify two promising stable configurations that have a band gap that compares favorably well with the experimental observations. Scanning tunneling spectroscopy measurement recorded on the intercalated Ge clusters reveals signatures of Coulomb blockade.

A class of Aubry-André-Harper models of spin-orbit coupled electrons exhibits a topological phase diagram where two regions belonging to the same phase are split up by a multicritical point. The critical lines which meet at this point each defines a topological quantum phase transition with a second-order nonanalyticity of the ground-state energy, accompanied by a linear closing of the spectral gap with respect to the control parameter; except at the multicritical point which supports fourth-order transitions with parabolic gap-closing. Here both types of criticality are characterized through a scaling analysis of the curvature function defined from the topological invariant of the model. We extract the critical exponents of the diverging curvature function at the non–high symmetry points in the Brillouin zone where the gap closes, and also apply a renormalization group approach to the flattening curvature function at high symmetry points. We also derive a basis-independent correlation function between Wannier states to characterize the transition. Intriguingly, we find that the critical exponents and scaling law defined with respect to the spectral gap remain the same regardless of the order of the transition.

Microscopic description of Raman spectra in nanopowders of nonpolar crystals is accomplished by developing the theory of disorder-induced broadening of optical vibrational eigenmodes. Analytical treatment of this problem is performed, and line shape and width are determined as functions of phonon quantum numbers, nanoparticle shape, size, and the strength of disorder. The results are found to be strongly dependent on whether the broadened line is separated from or overlaps other lines of the spectrum. Three models of disorder, i.e., weak pointlike impurities, weak smooth random potential, and strong rare impurities, are investigated in detail. The possibility of forming the phonon-impurity bound state is also studied.

Disorder-induced broadening of optical vibrational eigenmodes in nanoparticles of nonpolar crystals is studied numerically. The methods previously used to treat the phonons in defectless particles are adjusted for numerical evaluation of the disordered problem. Imperfections in the forms of Gaussian and binary disorders as well as surface irregularities are investigated thoroughly in a wide range of impurity concentrations and disorder strengths. For dilute and weak pointlike impurities the regimes of separated and overlapped phonon levels are obtained and the behavior of the linewidth predicted analytically is confirmed; the crossover scale falls into the actual range of several nanometers. These notions survive for strong dilute impurities, as well. Regimes and crossovers predicted by the analytical approach are checked and identified, and the minor discrepancies are discussed. We mention a few of them: slower than in analytics increasing of the linewidth with the phonon quantum number for weak disorder and only a qualitative agreement between analytics and numerics for the resonant broadening in strong dilute disorder. The novel phenomena discovered numerically are the “mesoscopic smearing” of the distribution function in the ensemble of identical disordered particles, an inflection of the linewidth dependence on the impurity concentration for light “dense” binary impurities, and a position-dependent capability of a strong impurity to catch the phonon. It is shown that surface irregularities contribute to the phonon linewidth less than the volume disorder, and their rates reveal faster decay with increasing of the particle size. It is argued that the results of the present research are applicable also for quantum dots and short quantum wires.

The competing magnetic ground states of the itinerant magnet EuCuSb, which has a hexagonal layered structure, were studied via magnetization, resistivity, and neutron-diffraction measurements on single-crystal samples. EuCuSb has a three-dimensional semimetallic band structure as confirmed by band calculation and angle-resolved photoelectron spectroscopy, consistent with the nearly isotropic metallic conductivity in the paramagnetic state. However, below the antiferromagnetic transition temperature of $T_{\mathrm{N}1}$ (8.5 K), the resistivity, especially along the hexagonal axis, increases significantly. This implies the emergence of anisotropic magnetic ordering coupled to the conducting electrons. Neutron-diffraction measurements show that the Eu spins, which order ferromagnetically within each layer, are collinearly modulated (up-up-down-down) along the hexagonal axis below $T_{\mathrm{N}1}$, followed by the partial emergence of helical spin modulation below $T_{\mathrm{N}2}$ (6 K). Based on the observation of anomalous magnetoresistance with hysteretic behavior, we discuss the competing nature of the ground state inherent in a frustrated Heisenberg-like spin system with a centrosymmetric structure.

The coupling of a spin to an underlying substrate is the basis for a plethora of phenomena. In the case of a metallic substrate, Kondo screening of the adatom magnetic moment can occur. As the substrate turns superconducting, an intriguing situation emerges where pair breaking due to the adatom spins leads to Yu-Shiba-Rusinov bound states, but also intertwines with Kondo phenomena. Through scanning tunneling spectroscopy, we analyze the interdependence of Kondo screening and superconductivity. Our data obtained on single Fe adatoms on Nb(110) show that the coupling and the resulting YSR states are strongly adsorption site-dependent and reveal a quantum phase transition at a Kondo temperature comparable to the superconducting gap. The experimental signatures are rationalized by combined density-functional theory and continuous-time quantum Monte Carlo calculations to rigorously treat magnetic and hybridization effects on equal footing.

Point defects significantly change electronic properties of alkali halides and thereby enhance their reactivity. However, both the experimental and theoretical description of defects such as the $F$ center and the $M$ center are still far from complete, in particular for the less common bromides. A self-consistent dielectric-dependent global hybrid and plane-wave approach is employed for a comparative theoretical study of the electronic properties of NaCl, KCl, NaBr, and KBr bulk and (100) surface, both perfect and defective. For these systems, a zero-point renormalization was calculated to account for electron-phonon interaction and enhance comparability with the experiment. We focus on anion vacancy defects, the so-called $F$ and $M$ centers. The methodology employed is capable of reproducing measured defect level energies, electronic band gaps, ionization energies, and electron affinities within experimental errors. A general trend of the $F$ center defect level energy with respect to the lattice parameter is found. The results for both the $F$ and the $M$ center of KCl agree with findings from magnetic resonance experiments. The defect orbitals are analyzed and virtual states of the defect electron are identified.

Magnon-polarons are coherently mixed quasiparticles that originate from the strong magnetoelastic coupling of lattice vibrations and spin waves in magnetic-ordered materials. Recently, magnon-polarons have attracted a lot of attention since they provide a powerful tool to manipulate magnons, which is essential for magnon-based spintronic devices. In this work, we report the experimental observation of facet-dependent magnon-polarons in epitaxial ferrimagnetic $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ thin films via spin Seebeck effect measurement. The critical magnetic fields for the magnon-polarons in the (110)- and (100)-oriented $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ films are 1.5 and 1.8 T, respectively, which arises from the different phonon velocities along the [110] and [100] directions. As the temperature decreases, the magnon-polarons-enhanced spin Seebeck voltage decreases in both (110)- and (100)-oriented $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$ films, which could be attributed to the enhanced magnon-polarons scattering at elevated temperatures. This work demonstrates the crystal structure engineering in epitaxial magnetic films as a promising route to manipulate the magnon-polarons for future magnon spintronic applications.

We analyze the effect of different types of fluctuations in internal electron energy on the rates of dark and photon counts in straight current-carrying superconducting nanowires. Dark counts appear due to thermal fluctuations in statistically independent cells with the effective size of the order of the coherence length; each count corresponds to an escape from the equilibrium state through an appropriate saddle point. For photon counts, spectral broadening of the deterministic cutoff in the spectra of the detection efficiency can be phenomenologically explained by local thermal fluctuations in the electron energy within cells with the same effective volume as for dark counts.

The monopole harmonic superconductor (SC), proposed in doped Weyl semimetals as a pairing between the Fermi surfaces enclosing the Weyl points, is rather unusual, as it features the monopole charge inherited from the parent metallic phase. However, this state can compete with more conventional spherical harmonic pairings, such as an $s$ wave. We here demonstrate, within the framework of the weak-coupling mean-field BCS theory, that the monopole and a conventional spherical harmonic SC quite generically coexist, while the repulsion can take place when the absolute value of the monopole charge matches the angular momentum quantum number of the spherical harmonic. As we show, this feature is a direct consequence of the topological nature of the monopole SC, and we dub it topological repulsion. We illustrate the above principle with the example of the conventional $s$- and ($p_x\pm i{p}_y$)-wave pairings competing with the monopole SC $Y_{-1,1,0}(\theta ,\varphi )$, which coexist in a finite region of the parameter space and repel, respectively. Furthermore, the $s$-wave pairing is more stable both when the chemical potentials at the nodes are unequal and in the presence of pointlike charged impurities. Since the phase transition is discontinuous, close to the phase boundary, we predict that the Majorana surface modes at the interfaces between domains featuring the monopole and the trivial phases, such as an $s$-wave, will be the experimental signature of the monopole SC.

We present a technique that enables the evaluation of perturbative expansions based on one-loop-renormalized vertices up to large expansion orders. Specifically, we show how to compute large-order corrections to the random phase approximation in either the particle-hole or particle-particle channels. The algorithm's efficiency is achieved by the summation over contributions of all symmetrized Feynman diagram topologies using determinants, and by integrating out analytically the two-body long-range interactions in order to yield an effective zero-range interaction. Notably, the exponential scaling of the algorithm as a function of perturbation order leads to a polynomial scaling of the approximation error with computational time for a convergent series. To assess the performance of our approach, we apply it to the nonperturbative regime of the square-lattice fermionic Hubbard model away from half-filling and report, as compared to the bare interaction expansion algorithm, significant improvements of the Monte Carlo variance as well as the convergence properties of the resulting perturbative series.

The frequency-dependent magneto-optical response of a topological nodal ring semimetal in a tilted magnetic field is studied systematically. Because of its specific low-energy band structure, we find that the magneto-optical response is affected qualitatively by both the strength and direction of the magnetic field. In the clean limit under the magnetic field along the ring axis, the magneto-optical response is investigated analytically, and several meaningful features are presented. For large ring radii, we find an unusual series of peaks in the transverse optical conductivity. As the ring axis tilts away from the magnetic field, the optical transitions are broken by the vertical projection of the magnetic field. For small ring radii, there is mixing of resonant peaks. The magneto-optical conductivities are explained by optical transitions between the underlying Landau level structure. The influence of chemical potential on the magneto-optical conductivities has been investigated correspondingly.

Nodal-chain fermions, which are novel topological states of matter, have been hotly discussed in the field of nonmagnetic materials. Here, by using first-principles calculations and symmetry analysis, we propose the realization of a fully spin-polarized nodal chain in several spinel half metals, including ${\mathrm{LiV}}_2{\mathrm{O}}_4, {\mathrm{VMg}}_2{\mathrm{O}}_4, {\mathrm{FeAl}}_2{\mathrm{O}}_4$, and ${\mathrm{NiAl}}_2{\mathrm{O}}_4$. In these materials, the ferromagnetic state takes on a half-metal band structure, and only the bands from the single channel are present near the Fermi level. Taking ${\mathrm{LiV}}_2{\mathrm{O}}_4$ as an example, we show how the crossing of bands in the spin-up channel forms two types of nodal loops. These nodal loops arise from band inversion and are under the protection of the glide mirror symmetries. Remarkably, we find that the nodal loops join with each other and form a chainlike nodal structure. Correspondingly, the $\omega$-shaped surface states are also fully spin-polarized. The fully spin-polarized nodal chain identified here has not been proposed in realistic materials before. An effective model is constructed to describe the nature of the nodal chain. The effects of the electron correlation, the lattice strains, and the spin-orbit coupling are discussed. The fully spin-polarized bulk nodal chain and the associated nontrivial surface states for a half metal may pave the way for additional applications in spintronics.

The central role that materials play in human history is exemplified by the three-age division of prehistory into the stone, bronze, and iron ages. References to our present time as the information age or silicon age epitomizes the important role that this semiconducting material came to play in the development of computers and devices that permeate our daily lives. Here we show that the electronic states in silicon have nontrivial topological structures that are captured by a network of Berry flux lines that link at points of high symmetry in the Brillouin zone. This complex network has ice-nodal points where fluxes satisfy ice rules, making silicon a nodal-chain insulator. Fixing the longitudinal momentum parallel to such flux lines yields a two-dimensional Dirac Hamiltonian for the transverse degrees of freedom. Similar to nodal-chain semimetals, we find drumheadlike states in the regions that are delimited by the projections of the bulk Berry flux network on the surface Brillouin zone.

An anomalous optical second-harmonic generation (SHG) signal was previously reported in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ and attributed to a hidden odd-parity bulk magnetic state. Here we investigate the origin of this SHG signal using a combination of bulk magnetic susceptibility, magnetic-field-dependent SHG rotational anisotropy, and overlapping wide-field SHG imaging and atomic force microscopy measurements. We find that the anomalous SHG signal exhibits a twofold rotational symmetry as a function of in-plane magnetic field orientation that is associated with a crystallographic distortion. We also show a change in SHG signal across step edges that tracks the bulk antiferromagnetic stacking pattern. While we do not rule out the existence of hidden order in ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$, our results altogether show that the anomalous SHG signal in parent ${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ originates instead from a surface-magnetization-induced electric-dipole process that is enhanced by strong spin-orbit coupling.