Strong spin-orbit coupling in transition metal dichalcogenide (TMDC) monolayers results in spin-resolvable band structures about the $K$ and $K^{\prime }$ valleys such that the eigenbasis of a two-dimensional quantum dot (QD) in a TMDC monolayer in zero field is described by the Kramers pairs ${|0\rangle }_{-}=|K^{\prime }\uparrow \rangle ,{|1\rangle }_{-}=|K\downarrow \rangle$ and ${|0\rangle }_{+}=|K\uparrow \rangle ,{|1\rangle }_{+}=|K^{\prime }\downarrow \rangle$. The strong spin-orbit coupling limits the usefulness of single TMDC QDs as spin qubits. Possible regimes of spin-degenerate states, overcoming the spin-orbit coupling in monolayer TMDC QDs, are investigated in both zero field, where the spin and valley degrees of freedom become fourfold degenerate, and twofold degeneracy in some magnetic field, localized to a given valley. Such regimes are shown to be achievable in ${\mathrm{MoS}}_2$, where the spin-orbit coupling is sufficiently low and of the right sign such that the spin-resolved conduction bands intersect at points about the $K$ and $K^{\prime }$ valleys and as such may be exploited by selecting suitable critical dot radii.

Well-ordered stepped semiconductor surfaces attract intense attention owing to the regular arrangements of their atomic steps that makes them perfect templates for the growth of one-dimensional systems, e.g., nanowires. Here, we report on the atomic structure of the vicinal $\mathrm{Si}(111)$ surface with ${10}^{\circ }$ miscut investigated by a joint frequency-modulation scanning force microscopy (FM-SFM) and ab initio approach. This popular stepped surface contains $7\times 7$-reconstructed terraces oriented along the $\mathrm{Si}(111)$ direction, separated by a stepped region. Recently, the atomic structure of this triple step based on scanning tunneling microscopy (STM) images has been subject of debate. Unlike STM, SFM atomic resolution capability arises from chemical bonding of the tip apex with the surface atoms. Thus, for surfaces with a corrugated density of states such as semiconductors, SFM provides complementary information to STM and partially removes the dependency of the topography on the electronic structure. Our FM-SFM images with unprecedented spatial resolution on steps coincide with the model based on a $\left(7710\right)$ orientation of the surface and reveal structural details of this surface. Two different FM-SFM contrasts together with density functional theory calculations explain the presence of defects, buckling, and filling asymmetries on the surface. Our results evidence the important role of charge transfers between adatoms, restatoms, and dimers in the stabilisation of the structure of the vicinal surface.

The optical response of small charged metallic nanodisks of one atomic monolayer thickness is analyzed under the excitation by an incident plane wave and by a localized pointlike dipole. Using the time-dependent density functional theory (TDDFT) and classical electrodynamical calculations we identify the bright and dark plasmon modes and study their evolution under external charging of the nanostructure. For neutral nanodisks, despite their monolayer thickness, the in-plane optical response, as obtained from TDDFT, is in agreement with classical electromagnetic results. The optical response for an incident wave polarized perpendicular to the nanostructure cannot be retrieved classically as it reflects a discrete energy structure of electronic levels. This latter situation appears most sensitive to external charging while the energy of the in-plane plasmon with dipolar character is nearly charge independent.

We use a Kubo formalism to calculate both AC conductivity and DC transport properties of a dirty nodal loop semimetal. The optical conductivity as a function of photon energy $\mathrm{\Omega }$ exhibits an extended flat background ${\sigma }^{\mathrm{BG}}$ as in graphene provided the scattering rate $\mathrm{\Gamma }$ is small as compared to the radius of the nodal ring $b$ (in energy units). Modifications to the constant background arise for $\mathrm{\Omega }\le \mathrm{\Gamma }$ and the minimum DC conductivity ${\sigma }^{\mathrm{DC}}$, which is approached as ${\mathrm{\Omega }}^2/{\mathrm{\Gamma }}^2$ as $\mathrm{\Omega }\to 0$, is found to be proportional to $\frac{\sqrt{{\mathrm{\Gamma }}^2+b^2}}{v_F}$ with $v_F$ the Fermi velocity. For $b=0$ we recover the known three-dimensional point node Dirac result ${\sigma }^{\mathrm{DC}}\sim \frac{\mathrm{\Gamma }}{v_F}$ while for $b>\mathrm{\Gamma }, {\sigma }^{\mathrm{DC}}$ becomes independent of $\mathrm{\Gamma }$ (universal) and the ratio $\frac{{\sigma }^{\mathrm{DC}}}{{\sigma }^{\mathrm{BG}}}=\frac{8}{{\pi }^2}$ where all reference to material parameters has dropped out. As $b$ is reduced and becomes of the order $\mathrm{\Gamma }$, the flat background is lost as the optical response evolves towards that of a three-dimensional point node Dirac semimetal which is linear in $\mathrm{\Omega }$ for the clean limit. For finite $\mathrm{\Gamma }$ there are modifications from linearity in the photon region $\mathrm{\Omega }\le \mathrm{\Gamma }$. When the chemical potential $\mu$ (temperature $T$) is nonzero the DC conductivity increases as ${\mu }^2/{\mathrm{\Gamma }}^2$($T^2/{\mathrm{\Gamma }}^2$) for $\frac{\mu }{\mathrm{\Gamma }} \left(\frac{T}{\mathrm{\Gamma }}\right)\le 1$. Such laws apply as well for thermal conductivity and thermopower with coefficients of the quadratic law only slightly modified from their value in the three-dimensional point node Dirac case. However in the $\mu =T=0$ limit both have the same proportionality factor of $\sqrt{{\mathrm{\Gamma }}^2+b^2}$ as does ${\sigma }^{\mathrm{DC}}$. Consequently the Lorentz number is largely unmodified. For larger values of $\mu >\mathrm{\Gamma }$ away from the nodal region the conductivity shows a Drude-like contribution about $\mathrm{\Omega }\approxeq 0$ which is followed by a dip in the Pauli blocked region $\mathrm{\Omega }\le 2\mu$ after which it increases to merge with the flat background (two-dimensional graphene like) for $\mu <b$ and to the quasilinear (three-dimensional point node Dirac) law for $\mu >b$.

We report experiments demonstrating that a spin current can be converted into a charge current in a film of the paramagnetic semiconductor (Ga,Mn)As at room temperature by means of the inverse spin Hall effect. The spin currents are generated in a thin layer of permalloy (Py) by two different processes: spin-pumping effect (SPE) and spin Seebeck effect (SSE). In the first, we use microwave-driven ferromagnetic resonance of the Py film to generate a SPE spin current that is injected into the (Ga,Mn)As film either in direct contact with Py or through a thin layer of insulating antiferromagnetic NiO. In the second, we use the SSE in the longitudinal configuration in Py with no contamination by the Nernst effect made possible with the use of a thin layer of NiO between the Py and (Ga,Mn)As layers. The results of the two measurements are consistent with each other, and from them, we obtain a spin Hall angle for (Ga,Mn)As at room temperature of ${\theta }_{\mathrm{SH}}=(1.5\pm 0.5)\times {10}^{-3}$, which is one order of magnitude larger than the values reported for $p$-Si and $n$-Ge at room temperature.

By applying Berry-phase theory for the effective half-filled Hubbard model, we derive an analytical expression for the electronic polarization driven by the relativistic spin-orbit (SO) coupling. The model itself is constructed in the Wannier basis, using the input from first-principles electronic structure calculations in the local-density approximation, and then treated in the spirit of the superexchange theory. The obtained polarization has the following form: ${\mathbf{P}}_{ij}={\mathbf{\epsilon }}_{ji}{\mathcal{P}}_{ij}·[{\mathit{e}}_i\times {\mathit{e}}_j]$, where ${\mathbf{\epsilon }}_{ji}$ is the direction of the bond $\langle ij\rangle , {\mathit{e}}_i$ and ${\mathit{e}}_j$ are the directions of spins in this bond, and ${\mathcal{P}}_{ij}$ is the pseudovector containing all the information about the crystallographic symmetry of the considered system. The expression describes the ferroelectric activity in various magnets with noncollinear but otherwise nonpolar magnetic structures, which would yield no polarization without SO interaction, including the magnetoelectric (ME) effect, caused by the ferromagnetic canting of spins in magnetic field, and spin-spiral multiferroics. The abilities of this theory are demonstrated for the analysis of linear ME effect in ${\mathrm{Cr}}_2{\mathrm{O}}_3$ and ${\mathrm{BiFeO}}_3$, and the properties of multiferroics ${\mathrm{MnWO}}_4, \beta -{\mathrm{MnO}}_2, {\mathrm{CuFeO}}_2$, and ${\mathrm{MnI}}_2$. In all considered examples, the theory perfectly describes the symmetry properties of the induced polarization. However, in some cases, the values of this polarization are underestimated, suggesting that other effects, besides the spin and electronic ones, can also play an important role.

We report pressure-driven superconductivity (SC) in the vicinity of a commensurate charge-density wave (CCDW) in transition-metal dichalcogenides (TMDs) $1T\text{−}\mathrm{TaS}{\mathrm{e}}_2$ by simultaneous resistivity and ac susceptibility. The superconducting phase enters at 4.5 GPa and bulk SC emerges along with the collapse of the CCDW phase at a critical pressure $P_c\sim 6.5\mathrm{GPa}$. Higher than $P_c$, the superconducting transition temperature ($T_c$) keeps increasing linearly, without a dome-shaped superconducting diagram in our pressure range. $T_c$ reaches $\sim 5.3\mathrm{K}$ at 15 GPa, which is the highest among all $1T$-TMDs. A comprehensive analysis shows that electronic correlations of the CCDW phase open energy gaps, which prohibit Cooper pairing, while the superconducting channels and CCDW domain wall coexist in three dimensions above $P_c$. The evolutions of the Fermi surface and the softening of phonon modes under pressure are proposed to explain the monotonic increase of $T_c$. The findings reveal the interplay of CCDW and SC in $1T\text{−}\mathrm{TaS}{\mathrm{e}}_2$ by a clean method, viz., high pressure, and shed light on the underlying superconducting mechanism in the relevant systems.

One of central issues in iron-based superconductors is the role of structural change to the superconducting transition temperature ($T_c$). It was found in FeSe that the lattice strain leads to a drastic increase in $T_c$, accompanied by suppression of nematic order. By angle-resolved photoemission spectroscopy on tensile- or compressive-strained and strain-free FeSe, we experimentally show that the in-plane strain causes a marked change in the energy overlap ($\mathrm{\Delta }{E}_{h-e}$) between the hole and electron pockets in the normal state. The change in $\mathrm{\Delta }{E}_{h-e}$ modifies the Fermi-surface volume, leading to a change in $T_c$. Furthermore, the strength of nematicity is also found to be characterized by $\mathrm{\Delta }{E}_{h-e}$. These results suggest that the key to understanding the phase diagram is the fermiology and interactions linked to the semimetallic band overlap.

Transport and magnetic studies of ${\mathrm{PbTaSe}}_2$ under pressure suggest the existence of two superconducting phases with the low temperature phase boundary at $\sim 0.25$ GPa that is defined by a very sharp, first order, phase transition. The first order phase transition line can be followed via pressure dependent resistivity measurements, and is found to be near 0.12 GPa near room temperature. Transmission electron microscopy and x-ray diffraction at elevated temperatures confirm that this first order phase transition is structural and occurs at ambient pressure near $\sim 425$ K. The new, high temperature/high pressure phase has a similar crystal structure and slightly lower unit cell volume relative to the ambient pressure, room temperature structure. Based on first-principles calculations this structure is suggested to be obtained by shifting the Pb atoms from the $1a$ to $1e$ Wyckoff position without changing the positions of Ta and Se atoms. ${\mathrm{PbTaSe}}_2$ has an exceptionally pressure sensitive, structural phase transition with $\mathrm{\Delta }{T}_s/\mathrm{\Delta }P\approx -1400$ K/GPa near room temperature, and $\approx -1700$ K/GPa near 4 K. This first order transition causes a $\sim 1$ K ($\sim 25\%$) steplike decrease in $T_c$ as pressure is increased through 0.25 GPa.

The interest in the properties of edge states in Chern insulators and in ${\mathbb{Z}}_2$ topological insulators has increased rapidly in recent years. We present calculations on how to influence the transport properties of chiral and helical edge states by modifying the edges in the Haldane and in the Kane-Mele model. The Fermi velocity of the chiral edge states becomes direction dependent as does the spin-dependent Fermi velocity of the helical edge states. Additionally, we explicitly investigate the robustness of edge states against local disorder. The edge states can be reconstructed in the Brillouin zone in the presence of disorder. The influence of the width and of the length of the system is studied as well as the dependence of the edge states on the strength of the disorder.

We compare local electronic structure at different atom types of a metallic semiconductor in bulk and nanocrystalline form. Multinuclear magic-angle-spinning nuclear magnetic resonance (MAS NMR) establishes that GaN synthesized as an intentionally doped bulk powder or as annealed nanocrystalline particles exhibits metallic behavior and a wide distribution of differing electronic environments in both forms. Bulk polycrystalline wurtzite GaN doped with 0.13% Ge as a shallow donor exhibits a temperature-independent distribution of ${}^{71}\mathrm{Ga}$ Knight shifts over the temperature range 123–473 K. Each Knight shift frequency in the inhomogeneously broadened spectrum is characterized by a ${}^{71}\mathrm{Ga}$ spin-lattice relaxation time $T_1$ that is in good agreement with the value predicted by the Knight-Korringa relation across the broad range of temperatures. The ${}^{14}\mathrm{N}$ spectrum shows a slightly smaller Knight shift distribution with spin-lattice relaxation time $T_1$ values at 295 K across the distribution also in good agreement with the Knight-Korringa relation. Similarly, annealed nanocrystalline wurtzite GaN (50–100 nm, and without Ge) exhibits a ${}^{71}\mathrm{Ga}$ Knight shift distribution and $T_1$ values (at 295 K) that follow the same Knight-Korringa behavior. Thus, both bulk and nanocrystalline forms of GaN are n type and well above the metal-insulator transition (MIT), the nanocrystals most likely as a result of incorporation of shallow donor oxygen atoms during synthesis. Carriers in both forms of samples exhibit the near-ideal characteristics of a degenerate Fermi gas of noninteracting spins. The observation of NMR signals from both atom types, Ga and N, allows for the direct spatial correlation of the local electronic structure at the two sites in the lattice, specifically the $s$-orbital character of the electronic wave function of conduction band electrons at the Fermi edge. The relative values of these carrier wave-function probabilities (nearly twice as great for the N atom as for the Ga) are in line with theoretical predictions. Analyses of ${}^{71}\mathrm{Ga}, {}^{14}\mathrm{N}$, and ${}^{15}\mathrm{N}$ NMR results, including double-resonance 2D ${}^{15}\mathrm{N}\left\{{}^{71}\mathrm{Ga}\right\}$ measurements, reveal electronic disorder in the form of broad distributions of local metallic properties (Knight shifts) that are shown to be spatially correlated on a subnanometer scale.

We study plasmonic excitations in the limit of few electrons, in one-atom-thick sodium chains. We compare the excitations to classical localized plasmon modes, and we find for the longitudinal mode a quantum-classical transition around 10 atoms. The transverse mode appears at much higher energies than predicted classically for all chain lengths. The electric field enhancement is also considered, which is made possible by considering the effects of electron-phonon coupling on the broadening of the electronic spectra. Large field enhancements are possible on the molecular level allowing us to consider the validity of using molecules as the ultimate small size limit of plasmonic antennas. Additionally, we consider the case of a dimer system of two sodium chains, where the gap can be considered as a picocavity, and we analyze the charge-transfer states and their dependence on the gap size as well as chain size. Our results and methods are useful for understanding and developing ultrasmall, tunable, and novel plasmonic devices that utilize quantum effects that could have applications in quantum optics, quantum metamaterials, cavity-quantum electrodynamics, and controlling chemical reactions, as well as deepening our understanding of localized plasmons in low-dimensional molecular systems.

We performed effective mass measurements employing microwave-induced resistance oscillation in a tunable-density GaAs/AlGaAs quantum well. Our main result is a clear observation of an effective mass increase with decreasing density, in general agreement with earlier studies which investigated the density dependence of the effective mass employing Shubnikov–de Haas oscillations. This finding provides further evidence that microwave-induced resistance oscillations are sensitive to electron-electron interactions and offer a convenient and accurate way to obtain the effective mass.

The (111) surface of noble metals is usually treated as an isolated two-dimensional (2D) triangular lattice completely decoupled from the bulk. However, unlike in topological insulators, bulk bands also cross the Fermi level. We here introduce an effective tight-binding model that accurately reproduces results from first-principles calculations, accounting for both surface and bulk states. We numerically solve the many-body problem of two quantum impurities sitting on the surface by means of the density matrix renormalization group. By performing simulations in a star geometry, we are able to study the nonperturbative problem in the thermodynamic limit with machine precision accuracy. We find that there is a nontrivial competition between Kondo and RKKY physics and as a consequence, ferromagnetism is never developed, except at short distances. The bulk introduces a variation in the period of the RKKY interactions, and therefore the problem departs considerably from the simpler 2D case. In addition, screening and the magnitude of the effective indirect exchange are enhanced by the contributions from the bulk states.

We measured the magnetoresistance of the 2D electron liquid formed at the (111) ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface. The hexagonal symmetry of the interface is manifested in a six-fold crystalline component appearing in the anisotropic magnetoresistance (AMR) and planar Hall data, which agree well with symmetry analysis we performed. The six-fold component increases with carrier concentration, reaching 15% of the total AMR signal. Our results suggest the coupling between higher itinerant electronic bands and the crystal as the origin of this effect and demonstrate that the (111) oxide interface is a unique hexagonal system with tunable magnetocrystalline effects.

We observe variation in the resistively detected nuclear magnetic resonance (RDNMR) line shapes in quantum Hall breakdown. The breakdown occurs locally in a gate-defined quantum point contact (QPC) region. Of particular interest is the observation of a dispersive line shape occurring when the bulk two-dimensional electron gas (2DEG) set to ${\nu }_{\mathrm{b}}=2$ and the QPC filling factor to the vicinity of ${\nu }_{\mathrm{QPC}}=1$, strikingly resemble the dispersive line shape observed on a 2D quantum Hall state. This previously unobserved line shape in a QPC points to a simultaneous occurrence of two hyperfine-mediated spin flip-flop processes within the QPC. Those events give rise to two different sets of nuclei polarized in the opposite direction and positioned at a separate region with different degrees of electronic spin polarization.

A Weyl semimetal exhibits unique properties with Weyl nodes in the bulk and Fermi arcs on the surface. Recently, ${\mathrm{MoTe}}_2$ was found to be a type-II Weyl semimetal, providing a platform for realizing these Weyl physics. Here, we report visualization of topological surface states on the surface of ${\mathrm{MoTe}}_2$ using a scanning tunneling microscope. Scattering between topological states forms quasiparticle interference (QPI) patterns in the Fourier transform of conductance maps. The complete existence of topological surface states in energy momentum space is revealed by $dI/dV$ mapping. By comparing QPI results with a first-principles calculation, we further unveil the locations of Weyl nodes in the surface Brillouin zone. Our work provides spectroscopic information in the unoccupied states, especially those around the Weyl nodes energy, demonstrating the node-arc correlation in Weyl semimetals.

The link between sub-bandgap states and optoelectronic properties is investigated for amorphous zinc tin oxide (a-ZTO) thin films deposited by RF sputtering. a-ZTO samples were annealed up to 500 °C in oxidizing, neutral, and reducing atmospheres before characterizing their structural and optoelectronic properties by photothermal deflection spectroscopy, near-infrared-visible UV spectrophotometry, Hall effect, Rutherford backscattering, hydrogen forward scattering and transmission electron microscopy. By combining the experimental results with density functional theory calculations, oxygen deficiencies and resulting metal atoms clusters are identified as the source of subgap states, some of which act as electron donors but also as free electron scattering centers. The role of hydrogen on the optoelectronic properties is also discussed. Based on this detailed understanding of the different point defects present in a-ZTO, their impact on optoelectronic properties, and how they can be suppressed by postdeposition annealing treatments, an amorphous indium-free transparent conductive oxide, with a high thermal stability and an electron mobility up to $35\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, is demonstrated by defect passivation.

In this paper we predict the existence of a novel species of Wannier excitons when exposed to crossed electric and magnetic fields. In particular, we present a theory of giant-dipole excitons in ${\mathrm{Cu}}_2\mathrm{O}$ in crossed fields. Within our theoretical approach we perform a pseudoseparation of the center-of-mass motion for the field-dressed excitonic species, thereby obtaining an effective single-particle Hamiltonian for the relative motion. For arbitrary gauge fields we exactly separate the gauge-dependent kinetic-energy terms from the effective single-particle interaction potential. Depending on the applied field strengths and the specific field orientation, the potential for the relative motion of electron and hole exhibits an outer well at spatial separations up to several micrometers and depths up to $380\mu \mathrm{eV}$, leading to possible permanent excitonic electric dipole moments of around $3\times {10}^6$ D.