The mobility and disorder in the lattice of Cu atoms as liquidlike behavior is an important characteristic affecting the thermoelectric properties of ${\mathrm{Cu}}_2\mathrm{S}$. In this study, using a theoretical model called an acanthite-like structure for ${\mathrm{Cu}}_2\mathrm{S}$ at a low-temperature range, we systematically investigate the electronic structure, intrinsic defect formation, and transport properties by first-principles calculations. Therefore, previous experimental reports on the indirect band-gap nature of ${\mathrm{Cu}}_2\mathrm{S}$ were confirmed in this work with an energy gap of about 0.9–0.95 eV. As a result, the optical absorption coefficient estimated from this model also gives a potential value of $\alpha >{10}^4{\mathrm{cm}}^{-1}$ in the visible spectrum range. According to the bonding analysis and formation energy aspect, Cu vacancy is the most preferred defect to form in ${\mathrm{Cu}}_2\mathrm{S}$, which primarily affects the conductive behavior as a $p$ type, as experimentally observed. Finally, the transport properties of ${\mathrm{Cu}}_2\mathrm{S}$ system were successfully reproduced using an electron-phonon scattering method, highlighting the important role of relaxation time prediction in conductivity estimation instead of regarding it as a constant.

Hole-spin qubits in quasi-one-dimensional structures are a promising platform for quantum information processing because of the strong spin-orbit interaction (SOI). We present analytical results and discuss device designs that optimize the SOI in Ge semiconductors. We show that at the magnetic field values at which qubits are operated, orbital effects of magnetic fields can strongly affect the response of the spin qubit. We study one-dimensional hole systems in Ge under the influence of electric and magnetic fields applied perpendicular to the device. In our theoretical description, we include these effects exactly. The orbital effects lead to a strong renormalization of the $g$ factor. We find a sweet spot of the nanowire (NW) $g$ factor where charge noise is strongly suppressed and present an effective low-energy model that captures the dependence of the SOI on the electromagnetic fields. Moreover, we compare properties of NWs with square and circular cross sections with ones of gate-defined one-dimensional channels in two-dimensional Ge heterostructures. Interestingly, the effective model predicts a flat band ground state for fine-tuned electric and magnetic fields. By considering a quantum dot (QD) harmonically confined by gates, we demonstrate that the NW $g$-factor sweet spot is retained in the QD. Our calculations reveal that this sweet spot can be designed to coincide with the maximum of the SOI, yielding highly coherent qubits with large Rabi frequencies. We also study the effective $g$ factor of NWs grown along different high-symmetry axes and find that our model derived for isotropic semiconductors is valid for the most relevant growth directions of nonisotropic Ge NWs. Moreover, a NW grown along one of the three main crystallographic axes shows the largest SOI. Our results show that the isotropic approximation is not justified in Ge in all cases.

The effect by which light focuses upon entering a medium with a negative refractive index, known as Veselago lensing, may enable optical imaging below the diffraction limit. Similarly, focusing electrons across a $p-n$ junction could realize a technologically promising electronic Veselago lens. However, its scope remains limited by the lack of three-dimensional platforms and its insensitivity to computational degrees of freedom, such as spin or chirality. Here, we propose a single-material three-dimensional electronic Veselago lens that selectively focuses electrons of a given chirality. Using the chiral anomaly of topological semimetals it is possible to create a sharp $p-n$ junction for a single chirality, a chiral Veselago lens, and tune it with a magnetic field to an ideal lensing condition. We estimate that chiral Veselago lensing is observable in nonlocal transport and spectroscopy experiments. In particular, we show that the chiral Veselago lens leads to giant nonlocal magnetoresistance.

In valley physics, large spin-orbit coupling (SOC) is the key, which is however rather weak for many materials with two-dimensional (2D) hexagonal honeycomb lattice. In contrast to conventional materials such as graphene with active $p_z$ orbitals, we proposed that large SOC splitting could be realized in $p_{x,y}$-orbitals active honeycomb lattice, i.e., the $XYH$ lattice. In particular, our model analysis confirmed that dynamically and magnetically induced valley polarizations are experimentally accessible. In accordance to the first-principles calculation and many-body perturbation theory, quaternary-layer ${\mathrm{BiSbC}}_3$ proves our proposed physics, with extremely large SOC splitting of 567.9 meV, excitonic energy difference of 496 meV for characteristic $A$ and $B$ excitons, and valley polarization of 168.7 meV by V doping.

Alloy oxidation is complex and involves several critical processes that lack understanding on the atomic level. Here, we report an atomistic picture of the initial-stage oxidation of stepped ${\mathrm{Cu}}_3\mathrm{Au}\left(100\right)$ using a combination of surface science tools and modeling to illuminate the microscopic processes underlying oxygen-adsorption-induced structural and compositional changes. Pristine ${\mathrm{Cu}}_3\mathrm{Au}\left(100\right)$ consists of wide CuAu-terminated terraces and narrow Cu-terminated terraces separated by monatomic steps. Counterintuitive to the common expectations of the adsorbate-induced surface segregation of the more reactive alloy component, our observations demonstrate that the oxygen adsorption leads to the exfoliation of the outermost CuAu layer, thereby exposing the inner Cu plane to O attack. This occurs via the oxygen-assisted abstraction of Au and Cu atoms from step edges and CuAu terraces, which generates many Cu adatoms aggregating into Cu clusters and Au adatoms dissolving into the bulk. The oxygen adsorption onto fourfold hollow sites of the exposed Cu plane results in nucleation and growth of the $c(2\times 2)\text{−}\mathrm{O}$ superstructure, which can be fit well by the Johnson-Mehl-Avrami-Kolmogorov theory with a site-saturated nucleation mechanism.

Hydrodynamic fluidity in condensed-matter physics has been experimentally demonstrated only in a limited number of compounds due to the stringent conditions that must be met. Herein, we performed thermal and electrical transport experiments in three-dimensional topological semimetal $\mathrm{Zr}{\mathrm{Te}}_5$. By measuring the thermal properties in a wide temperature range, two representative experimental evidences of the hydrodynamics are observed in the temperature window between the ballistic and diffusive regimes: a faster evolution of the thermal conductivity than in the ballistic regime and the nonmonotonic temperature-dependent effective quasiparticle mean-free-path. In addition, magnetothermal conductivity results indicate that charged quasiparticles, as well as phonons, may also play an important role in this hydrodynamic-like flow in $\mathrm{Zr}{\mathrm{Te}}_5$.

In rare-earth semiconductors, wide ligand $p$ and rare-earth $5d$ bands coexist with localized, partially filled $4f$ shells. A simultaneous description for both extended and localized states represents a significant challenge for first-principles theories. Here, we combine an ab initio dynamical mean-field theory approach to strong local correlations with a perturbative application of the semilocal modified Becke-Johnson exchange potential to correct the semiconducting gap. We apply this method to calculate the electronic structure and optical response of the light rare-earth sesquioxides $R_2{\mathrm{O}}_3$ $(R=\mathrm{La},\mathrm{Ce},\mathrm{Pr},\text{and}\mathrm{Nd})$. Our calculations correctly capture a nontrivial evolution of the optical gap in $R_2{\mathrm{O}}_3$ due to a progressive lowering of the $4f$ states along the series and their multiplet structure. $2p-4f$ hybridization is found to induce a substantial upward shift for the occupied $4f$ states occurring within the $p-d$ gap, thus reducing the magnitude of the optical gap. We show that a characteristic plateau observed in the optical conductivity in the Pr and Nd sesquioxides right above their absorption edge is a fingerprint of $4f$ states located within the $p-d$ gap.

CeBi has an intricate magnetic phase diagram whose fully polarized state has recently been suggested as a Weyl semimetal, though the role of $f$ states in promoting strong interactions has remained elusive. Here we focus on the less-studied but also time-reversal symmetry-breaking ferrimagnetic phase of CeBi, where our density functional theory (DFT) calculations predict additional Weyl nodes near the Fermi level $E_F$. We use spin-polarized scanning tunneling microscopy and spectroscopy to image the surface ferrimagnetic order on the itinerant Bi $p$ states, indicating their orbital hybridization with localized Ce $f$ states. We observe suppression of this spin-polarized signature at $E_F$, coincident with a Fano line shape in the conductance spectra, suggesting the Bi $p$ states partially Kondo screen the $f$ magnetic moments, and this $p-f$ hybridization causes strong Fermi-level band renormalization. The $p$-band flattening is supported by our quasiparticle interference measurements, which also show band splitting in agreement with DFT, painting a consistent picture of a strongly interacting magnetic Weyl semimetal.

The frustrated magnetism on the Kondo lattice system motivates intriguing Kondo breakdown beyond the traditional Doniach scenario. Among them, the fractionalized Fermi liquid (FL*) has drawn particular interest by virtue of its fractionalized nature. Here we study the phase diagram of the $J_1\text{−}{J}_2$ Kondo-Heisenberg model on a honeycomb lattice at a quarter filling. Employing the slave-fermion mean-field theory with $d\pm id$ spin liquid ansatz and exact diagonalization, we discuss the emergence of partial Kondo screening in the frustrated regime with comparable $J_1$ and $J_2$, and the fractionalized superconductor (SC*), which is a superconductor analogy of the FL*. Due to the larger number of local spin moments than itinerant electrons, the magnetic fluctuation is still significant even in the strong-coupling limit, which influences the thermodynamic and transport properties qualitatively. In particular, we estimate the thermal conductance to probe the low-energy excitation and show the anomalous behavior in the SC* phase in contrast to the conventional superconductors.

We present a nonperturbative study of the quantum many-body effects caused by the long-range Coulomb interaction in a two-dimensional semi-Dirac semimetal. This kind of semimetal may be realized in deformed graphene and a class of other realistic materials. In the noninteracting limit, the dispersion of semi-Dirac fermion is linear in one direction and quadratic in the other direction. When the impact of Coulomb interaction is taken into account, such a dispersion can be significantly modified. To reveal the correlation effects, we first obtain the exact self-consistent Dyson-Schwinger equation of the full fermion propagator and then extract the momentum dependence of the renormalized fermion dispersion from the numerical solutions. Our results show that the fermion dispersion becomes linear in two directions. These results are compared to previous theoretical works on semi-Dirac semimetals.

Non-Fermi liquid behavior in some fermionic systems has attracted significant interest in last few decades. Certain pyrochlore iridates with stronger spin-orbit coupling strength have recently been added to the list. Here, we provide evidence of such a non-Fermi liquid ground state in another mixed-valent metallic pyrochlore iridate ${\mathrm{Pb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_{7-\delta }$, through the combined investigation of electronic, magnetic, and thermodynamic properties as a function of temperature ($T$) and applied magnetic field ($H$). Resistivity measurement showed a linear temperature dependence down to 15 K below which it shows $\rho \sim T^{3/2}$ dependence while magnetic susceptibility diverges as $\chi (T$) $\sim T^{-\alpha }$ ($\alpha <1$) below 10 K. While a strong negative ${\mathrm{\Theta }}_{\text{CW}}$ has been observed from Curie-Weiss fitting, the absence of any long-range order down to 80 mK only indicates the presence of strong inherent geometric frustration in the system. Heat capacity data showed $C_p\sim Tln(T_0/T)+\beta T^3$ dependence below 15 K down to 1.8 K. More importantly spin-orbit coupling strength by x-ray absorption spectroscopy was found to be weaker in ${\mathrm{Pb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_{7-\delta }$ compared to other pyrochlore iridates. In the absence of any large moment rare earth magnetic ion, ${\mathrm{Pb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_{7-\delta }$ presents a rare example of an iridate system showing non-Fermi liquid behavior due to disordered distribution of ${\mathrm{Ir}}^{4+}$ and ${\mathrm{Ir}}^{5+}$ having markedly different strengths of spin-orbit coupling which might offer a prescription for achieving new non-Fermi liquid systems.

A kagome lattice, formed by triangles of two different directions, is known to have many emergent quantum phenomena. Under the breathing anisotropy of bond strengths, this lattice can become a higher-order topological insulator (HOTI), which hosts topologically protected corner states. Experimental realizations of HOTI on breathing kagome lattices have been reported for various artificial systems, but not for simple natural materials with an electronic breathing kagome lattice. Here we prove that a breathing kagome lattice and HOTI are hidden inside the electronic structure of hexagonal transition metal dichalcogenides (h-TMD). Due to the trigonal prismatic symmetry, $s{p}^2$-like hybrid $d$ orbitals create an electronic kagome lattice with anisotropic intersite and on-site hopping interactions. We demonstrate that HOTI h-TMD triangular nanoflakes host topologically protected corner states, which could be quantum-mechanically entangled with triple degeneracy. Because $\mathrm{h}\text{−}\mathrm{TM}{\mathrm{D}}_{\mathrm{S}}$ are easily synthesizable and stable with a large bandgap at ambient conditions, our findings open an avenue for quantum physics based on simple condensed matter systems.

We devise a methodology for charge, heat, and entropy transport driven by carriers with finite lifetimes. Combining numerical simulations with analytical expressions for low temperatures, we establish a comprehensive and thermodynamically consistent phenomenology for transport properties in semiconductors. We demonstrate that the scattering rate (inverse lifetime) is a relevant energy scale: It causes the emergence of several characteristic features in each transport observable. The theory is capable to reproduce, with only a minimal input electronic structure, the full temperature profiles measured in correlated narrow-gap semiconductors. In particular, we account for the previously elusive low-$T$ saturation of the resistivity and the Hall coefficient, as well as the (linear) vanishing of the Seebeck and Nernst coefficients in systems such as ${\mathrm{FeSb}}_2, {\mathrm{FeAs}}_2, {\mathrm{RuSb}}_2$, and ${\mathrm{FeGa}}_3$.

Carrier mobility in amorphous semiconductors remained unpredictable due to random electronic states in the absence of the long-range order in a lattice structure, although amorphous semiconductors have been investigated over several decades and widely used in diverse electronic devices. In this work, we develop a method to predict mobility of disordered systems by virtue of the first-principles calculation without using any empirical parameters. Quantum transport modeling based on the nonequilibrium Green's function formalism enables us to establish a formula to connect first-principles results with amorphous-phase mobility. Finally, the developed approach is quantitatively validated by comparing the theoretical predictions with previously measured mobilities of amorphous metal oxides $({\mathrm{SnO}}_2,{\mathrm{In}}_2{\mathrm{O}}_3$, and ZnO) and amorphous silicon. Localization analysis provides further physical insight into a distinct feature between the amorphous metal oxides and amorphous silicon.

Single crystal quality films of dilute $\mathrm{Ga}{\mathrm{Sb}}_x{\mathrm{N}}_{1-x}$ and $\mathrm{Ga}{\mathrm{Bi}}_y{\mathrm{N}}_{1-y}$ alloys were synthesized through a plasma-assisted vapor liquid phase epitaxy technique in which nitrogen radicals, antimony, and bismuth species diffuse through molten gallium and form alloys at the substrate. Incorporation fractions $x$ up to 0.009 (three samples), and $y$ up to 0.011 (two samples) were achieved. Morphology and photoluminescence characteristics as well as x-ray spectra were evaluated confirming the alloys possess gallium nitride wurtzite structure but with a slightly higher lattice parameter as determined with differences of hundredths of degrees in the diffraction peak position of plane (002). UV-Vis spectrometry measurements of the alloys also resemble those of pure gallium nitride, indicating dilute anion substitution at the levels reported in this study are not sufficient to promote significant changes in the band gap versus host material, gallium nitride. First-principles density functional theory incorporating the local-density approximation/generalize gradient approximation+U formalism also reveals that relatively small concentration of Sb impurities is not sufficient to achieve a significant narrowing of gallium nitride band gap.

Coupled Maxwell and quantum-mechanical equations are used to simulate the electron dynamics in nanogaps in systems containing thousands of atoms. It will be shown that besides the carrier-envelop phase, bow-tie or gap shape, and gap size, the atomistic structure also significantly alters the electron dynamics. Atomic-scale interference fringes appear not only in the electron density but in the electron current and field enhancement as well. Electron bursts emerge from individual atoms and scatter on atoms driven by the direction of the laser. The time-dependent orbital-free density functional theory coupled to the Maxwell equations allows us to simulate physical systems approaching the realistic size and to explore the physical mechanism controlling the electron dynamics.

We study the manipulation of Majorana zero modes in a thin disk made from a $p$-wave superconductor, in order to understand their use as a building block for topological quantum computers. We analyze the second-order topological corner modes that arise when an in-plane magnetic field is applied, and we calculate their dynamical evolution when rotating the magnetic field, with special emphasis on nonadiabatic effects. We characterize the phase transition between high-frequency and near-adiabatic evolution using Floquet analysis. We show that oscillations persist even in the adiabatic phase because of a frequency-independent coupling between zero modes and excited states, which we have quantified numerically and analytically. These results show that controlling the rotation frequency can be a simple method to avoid the nonadiabatic errors originated from this coupling and thus increase the robustness of topological quantum computation.

The ensemble geometric phase (EGP) has been proposed as a topological indicator for finite-temperature quantum systems. The ensemble Wilson loop, or the transfer matrix, contains the crucial information in the EGP construction. We propose a proxy index and a proxy EGP directly from the transfer matrix and apply them to time-reversal invariant topological insulators exemplified by the Bernevig-Hughes-Zhang (BHZ) and Kane-Mele (KM) models. The quantized proxy index and proxy EGP smoothly generalize the ground-state topological index to finite temperatures. For the BHZ model, a comparison with another topological indicator, the Uhlmann phase, shows different transition behavior with temperature. For the KM model, the EGP have been generalized to the time-reversal EGP previously, but the proxy EGP does not require any splitting of the contributions. The proxy index and proxy EGP thus offer an efficient means for characterizing finite-temperature topological properties.

Probing phonons, quasiparticle excitations, and their coupling has enriched our understanding of two-dimensional (2D) materials and proved to be crucial for developing their potential applications. Here, we report comprehensive temperature 4–330 K and polarization-dependent Raman measurements on monolayer (1L) and bilayer (2L) ${\mathrm{MoSe}}_2$. Phonon modes up to the fourth order are observed, including forbidden Raman and infrared modes, and understood considering the Fröhlich mechanism of exciton-phonon coupling. Most notably, anomalous variations in the phonon linewidths with temperature point at the significant role of electron-phonon coupling in these systems, especially for the out-of-plane ($A_{1g}$) and shear modes ($E_{2g}^2$), which is found to be more prominent in the narrow-gapped 2L than the large gapped 1L. We deciphered the ambiguity in symmetry assignments, especially to the peaks ∼170 and $\sim 350\mathrm{c}{\mathrm{m}}^{–1}$ via polarization-dependent measurements. Temperature-dependent thermal expansion coefficient, an important parameter for device performance, is carefully extracted for both 1L and 2L by monitoring the temperature dependence of the real part of the phonon self-energy parameter. Our temperature-dependent in-depth Raman studies pave a way for uncovering the deeper role of phonons in these 2D layered materials from a fundamental as well as application point of view.

We analyze the intrinsic stability of spin spiral states in the two-dimensional Heisenberg model isolated from its environment. Our analysis reveals that the SU(2) symmetric point hosts a dynamic instability that is enabled by the existence of energetically favorable transverse deformations—both in real and spin space—of the spiral order. The instability is universal in the sense that it applies to systems with any spin number, spiral wave vector, and spiral amplitude. Unlike the Landau or modulational instabilities which require impurities or periodic potential modulation of an optical lattice, quantum fluctuations alone are sufficient to trigger the transverse instability. We analytically find the most unstable mode and its growth rate, and compare our analysis with phase-space methods. By adding an easy-plane exchange coupling that reduces the Hamiltonian symmetry from SU(2) to U(1), the stability boundary is shown to continuously interpolate between the modulational instability and the transverse instability. This suggests that the transverse instability is an intrinsic mechanism that hinders long-range phase coherence even in the presence of exchange anisotropy.