We derive from a microscopic model the effective theory of nematic order in a system with a spontaneous quantum anomalous Hall effect in two dimensions. Starting with a model of two-component fermions (a spinor field) with a quadratic band crossing and short-range four-fermion marginally relevant interactions we use $1/N$ expansion and bosonization methods to derive the effective field theory for the hydrodynamic modes associated with the conserved currents and with the local fluctuations of the nematic order parameter. We focus on the vicinity of the quantum phase transition from the isotropic Mott Chern insulating phase to a phase in which time-reversal symmetry breaking coexists with nematic order, the nematic Chern insulator. The topological sector of the effective field theory is a background field (BF)/Chern-Simons gauge theory. We show that the nematic order parameter field couples with the Maxwell-type terms of the gauge fields as the space components of a locally fluctuating metric tensor. The nematic field has $z=2$ dynamic scaling exponent. The low-energy dynamics of the nematic order parameter is found to be governed by a Berry phase term. By means of a detailed analysis of the coupling of the spinor field of the fermions to the changes of their local frames originating from long-wavelength lattice deformations, we calculate the Hall viscosity of this system and show that in this system it is not the same as the Berry phase term in the effective action of the nematic field, but both are related to the concept of torque Hall viscosity, which we introduce here.

A combined experimental and theoretical study of the local chemistry of cerium oxide films and islands on Ru(0001) is presented. Based on intensity-voltage low-energy electron microscopy [$I\left(V\right)$-LEEM] and resonant x-ray photoemission spectroscopy, we establish a one-to-one correspondence between the local oxidation state of ${\mathrm{Ce}}^{3+}$ [cubic Ce${}_2$O${}_3$(111)] and ${\mathrm{Ce}}^{4+}$ [cubic CeO${}_2$(111)] and their respective spatially resolved $I\left(V\right)$ curves. Ab initio scattering theory explains the difference between the $I\left(V\right)$ curves in the low-energy range in terms of the $k_{\parallel }=0$ projected band structure arising from the different structure of the Ce 5d states in fully oxidized and reduced ceria. The theoretical analysis unambiguously attributes the LEEM contrast observed for chemically reduced cerium oxide to a variation in oxidation state on the nanometer scale, which is not present for the as-grown islands.

We calculate the finite-temperature conductance of clean, weakly interacting one-dimensional quantum wires subject to Rashba spin-orbit coupling and a magnetic field. For chemical potentials near the center of the Zeeman gap ($\mu =0$), two-particle scattering causes the leading deviation from the quantized conductance at finite temperatures. On the other hand, for $\left|\mu \right|>0$, three-particle scattering processes become more relevant. These deviations are a consequence of the strongly nonlinear single-particle spectrum, and are thus not accessible using Luttinger liquid theory. We discuss the observability of these predictions in current experiments on InSb nanowires and in “spiral liquids,” where a spontaneous ordering of the nuclear spins at low temperatures produces an effective Rashba coupling.

We have investigated the electronic structure of graphene supported on Re(0001) before and after the intercalation of one monolayer of Ag by means of angle-resolved photoemission spectroscopy measurements and density functional theory calculations. The intercalation of Ag reduces the graphene-Re interaction and modifies the electronic band structure of graphene. Although the linear dispersion of the $\pi$ state of graphene in proximity of the Fermi level highlights a rather weak graphene–noble-metal layer interaction, we still observe a significant hybridization between the Ag bands and the $\pi$ state in lower energy regions. These results demonstrate that covering a surface with a noble-metal layer does decouple the electronic states, but still leads to a noticeable change in the electronic structure of graphene.

We investigate the influence of SiO${}_2$, Au, Ag, Cu, and Pt substrates on the Raman spectrum of graphene. Experiments reveal particularly strong modifications to the Raman signal of graphene on platinum, compared to that of suspended graphene. The modifications strongly depend on the relative orientation of the graphene and platinum lattices. These observations are theoretically investigated and shown to originate basically from hybridization of electronic states in graphene and $d$ orbitals in platinum. It is expected that, quite generally, hybridization between graphene and any material with $d$ orbitals near the Fermi level will result in an imprint on the graphene Dirac cone, which depends sensitively on the relative orientation of the respective lattices.

The magnetic structure and metal-insulator transition in antiferromagnetic (AFM) BaMn${}_2$As${}_2$ and Ba${}_{1-x}$K${}_x$Mn${}_2$As${}_2$ single crystals have been investigated by ${}^{55}$Mn and ${}^{75}$As nuclear magnetic resonance (NMR) measurements. In the parent AFM insulator BaMn${}_2$As${}_2$ with a Néel temperature $T_{\mathrm{N}}=625$ K, we observed a ${}^{55}$Mn zero-field NMR (ZFNMR) spectrum and confirmed the G-type AFM structure from the field dependence of the ${}^{55}$Mn spectra and ${}^{75}$As NMR spectra below $T_{\mathrm{N}}$. In hole-doped crystals with $x>0.01$, similar ${}^{55}$Mn ZFNMR spectra were observed and the AFM state was revealed to be robust up to $x=0.4$ with the ordered moment nearly independent of $x$. The nuclear spin-lattice relaxation rates ($1/T_1$) for both nuclei in the doped samples follow the Korringa relation $T_{1}T=\text{const}$, indicating a metallic state. This confirms the coexistence of AFM ordered localized Mn spins and conduction carriers from a microscopic point of view. From the $x$ dependence of ${\left(T_{1}T\right)}^{-1/2}$ for both nuclei, we conclude that this transition is caused by vanishing of the hole concentration as the transition is approached from the metallic side.

We investigate persistent currents in metallic rings interrupted by a Coulomb-blockaded topological superconducting segment. We show that the presence of Majorana bound states in the superconductor is reflected in the emergence of an $h/e$ harmonic in the persistent current. The Majorana bound states further render the current finite at zero flux, with a sign that is determined by the fermion parity of the superconductor. The resulting peculiar symmetry of the persistent current is compatible with a free energy that is even in time-reversal symmetry-breaking fields. These unique features of the persistent currents are robust against disorder and provide unambiguous signatures of the presence of Majorana fermions.

We study the temperature dependence of the electrical resistivity of interacting two-dimensional Fermi liquids. We perform a numerical simulation of the nonequilibrium state based on semiclassical Boltzmann transport theory. A two-dimensional system of quasiparticles on a square lattice serves as a model system. We use a single-orbital tight-binding model of the dispersion. For conceptual purposes, we choose a simple repulsive onsite interaction that leads to quasiparticle scattering and delta-potential scatterers as a model of nonmagnetic impurity scattering. Through our simulation, we demonstrate that deviations from the predictions of standard Fermi-liquid theory can arise due to the nontrivial scattering geometry of umklapp processes, in special cases even in the ultralow-temperature limit. We show through qualitative arguments how these unconventional scaling properties of the electrical resistivity, which are often interpreted as indication of a non-Fermi-liquid state, can arise due to special geometric conditions of the Fermi surface. The appearance of robust deviations from the predictions of Fermi-liquid theory within our simple model presents a viewpoint in order to interpret unconventional transport properties in electron-electron scattering dominated metallic systems.

We show that the colossal thermoelectric power $S\left(T\right)$ observed in the correlated semiconductor FeSb${}_2$ below 30 K is accompanied by a huge Nernst coefficient $\nu \left(T\right)$ and magnetoresistance MR$\left(T\right)$. Markedly, the latter two quantities are enhanced in a strikingly similar manner. While in the same temperature range, $S\left(T\right)$ of the reference compound FeAs${}_2$, which has a seven-times-larger energy gap, amounts to nearly half of that of FeSb${}_2$, its $\nu \left(T\right)$ and MR$\left(T\right)$ are intrinsically different to FeSb${}_2$: They are smaller by two orders of magnitude and have no common features. Underlying the essentially different thermoelectric properties between FeSb${}_2$ and FeAs${}_2$, a large mismatch between the electrical and thermal Hall mobilities is found only in the former compound. With the charge transport of FeAs${}_2$ successfully captured by the density functional theory, we emphasize a significantly dispersive electron-relaxation time $\tau \left({\epsilon }_k\right)$ related to electron-electron correlations to be at the heart of the peculiar thermoelectricity and magnetoresistance of FeSb${}_2$.

The surface termination of the promising topological insulator TlBiSe${}_2$ has been studied by surface- and bulk-sensitive probes. Our scanning tunneling microscopy has unmasked for the first time the unusual surface morphology of TlBiSe${}_2$ obtained by cleaving, where islands are formed by residual atoms on the cleaved plane. The chemical condition of these islands was identified using core-level spectroscopy. We observed thallium core-level spectra that are strongly deformed by a surface component in sharp contrast to the other elements. We propose a simple explanation for this behavior by assuming that the sample cleaving breaks the bonding between thallium and selenium atoms, leaving the thallium layer partially covering the selenium layer. These findings will assist the interpretation of future experimental and theoretical studies of this surface.

Graphene electrodes provide a suitable alternative to metal contacts in molecular conduction nanojunctions. Here, we propose to use graphene electrodes as a platform for effective photon assisted tunneling through molecular conduction nanojunctions. We predict dramatic increasing currents evaluated at side-band energies $\sim n\hslash \omega$ ($n$ is a whole number) related to the modification of graphene gapless spectrum under the action of external electromagnetic field of frequency $\omega$. A side benefit of using doped graphene electrodes is the polarization control of photocurrent related to the processes occurring either in the graphene electrodes or in the molecular bridge. The latter processes are accompanied by surface plasmon excitation in the graphene sheet that makes them more efficient. Our results illustrate the potential of graphene contacts in coherent control of photocurrent in molecular electronics, supporting the possibility of single-molecule devices.

We study the conduction band spin splitting that arises in transition metal dichalcogenide (TMD) semiconductor monolayers such as MoS${}_2$, MoSe${}_2$, WS${}_2$, and WSe${}_2$ due to the combination of spin-orbit coupling and lack of inversion symmetry. Two types of calculation are done. First, density functional theory (DFT) calculations based on plane waves that yield large splittings, between 3 and 30 meV. Second, we derive a tight-binding model that permits to address the atomic origin of the splitting. The basis set of the model is provided by the maximally localized Wannier orbitals, obtained from the DFT calculation, and formed by 11 atomiclike orbitals corresponding to $d$ and $p$ orbitals of the transition metal (W, Mo) and chalcogenide (S, Se) atoms respectively. In the resulting Hamiltonian, we can independently change the atomic spin-orbit coupling constant of the two atomic species at the unit cell, which permits to analyze their contribution to the spin splitting at the high symmetry points. We find that—in contrast to the valence band—both atoms give comparable contributions to the conduction band splittings. Given that these materials are most often $n$-doped, our findings are important for developments in TMD spintronics.

Image charge effects in nanoscale junctions with strong electron-phonon coupling open the way to unexplored physical scenarios. We propose a simple and still accurate many-body approach to deal with the simultaneous occurrence of the Franck-Condon blockade and the screening-induced enhancement of the polaron mobility. A transparent analytic expression for the polaron decay rate is derived and the dependence on the strength and range of the screening is highlighted. This allows us to interpret and explain several transient and steady-state features of the electrical current. Remarkably, we find that the competition between the charge blocking due to the electron-phonon interaction and the charge deblocking due to the image charges gives rise to a novel mechanism of negative differential conductance. An experimental setup to observe this phenomenon is discussed.

A comprehensive study on thermoelectric effects is performed for the transverse Pt strips attached to a longitudinal permalloy (Py) layer or sandwiched between two longitudinal Py layers. Thermal voltages jointly determined by planar Nernst effect (PNE), anomalous Nernst effect (ANE), and spin Seebeck effect (SSE) are detected across the Pt strip when a thermal gradient exists. By comparing the data of two differently structured samples, the respective contributions of the PNE, ANE, and SSE to thermal voltage are determined, and they have the ratio of 64:26:10, without considering the shunting effect of conductive Py, or 35:15:50, after the shunting-effect correction.

Magnetoresistance (MR), polarized neutron reflectometry (PNR), and magnetometry measurements in MnSi thin films and rigorous analytical solutions of the micromagnetic equations show that the field-induced unwinding of confined helicoids occurs via discrete steps. A comparison between the theoretical results and the PNR and magnetometry data shows that finite-size effects confine the wavelength and lead to a quantization of the number of turns in the helicoid. We demonstrate that the magnetic state of these finite helicoids can be read by electrical means.

Quantum Diesel cycles are numerically realized using the electronic states of a Ni${}_2$ dimer. The quantum nature and the complexity of the electronic structure of the Ni${}_2$ dimer result in new features in the evolution of the pressure as well as in the heat-work transformation. The multitude of internal degrees of freedom in the isobaric process in molecules can result in crossing of the two adiabatic processes in the P-V diagram. The interplay of heat and work, originating from thermal nonequilibrium effects, can lead to a thermal efficiency of up to $100\%$. The spin moment of the Ni${}_2$ can be decreased by the isobaric process. To link the molecular heat capacity to easily accessible experimental quantities, we also calculate the Kerr effect and the magnetic susceptibility at different temperatures and magnetic fields.

LiBH${}_4$ is a fast ionic conductor in its high-temperature phase, which is stabilized at room temperature by various chemical substitutions, making it a potential solid electrolyte material for Li-ion batteries. Using first-principles variable-cell-shape molecular dynamics simulations, we reproduce the experimentally observed low- and high-temperature structures. Using the height of a structure-factor-like peak as a collective coordinate, we calculate the free energy differences between the two structures as a function of temperature and substitutional ion concentration. We get good agreement with experiment for I, Cl, and Na, and predict that K is even more effective for lowering the critical temperature. Decomposition of the free energy into enthalpy and entropy reveals that the mechanism driving this lowering varies among substitutional elements. Calculating the full free energy, rather than simply the enthalpy, is therefore crucial to understanding how chemical substitution stabilizes the high conductivity phase.

The lattice dynamics of samarium is determined by in situ low-temperature nuclear inelastic scattering on a single crystalline (0001)Sm film, a polycrystalline Sm foil, and by first-principles theory. The ab initio calculated phonon dispersion relations and phonon density of states for the Sm-type structure and the double hexagonal-close-packed (dhcp) lattice, characteristic for light lanthanides, are compared. The dhcp unit cell, which is a factor of 2.24 smaller in height, exhibits more pronounced vibrational anisotropy in comparison to the Sm-type structure. The analysis reveals a minor influence of the spin-orbit coupling in the Sm atom on the lattice dynamics. A broadening of the longitudinal peak, not found in the calculations, suggests the influence of electron correlations on lattice dynamics in metallic samarium.

Diamond irradiated with an ultrashort intense laser pulse in the regime of photon energies from soft up to hard x rays can undergo a phase transition to graphite. This transition is induced by an excitation of electrons from the valence band or from atomic deep shells of the material into its conduction band, which is followed by a transient rapid change of the interatomic potential. Such a nonthermal phase transition occurs on a femtosecond time scale, shortly after or even during the laser pulse. In this work we show that the duration of the graphitization depends on the incoming photon energy: the higher the photon energy is, the longer the secondary electron cascading which promotes the electrons into the conduction band will take. The transient kinetics of the electronic and atomic processes during the graphitization is analyzed in detail. The damage threshold fluence is calculated in the broad photon energy range and is found to be always $\sim$0.7 eV/atom in terms of the average dose absorbed per atom. It is confirmed that the temporal characteristics of a femtosecond laser pulse (at a fixed pulse duration and fluence) do not significantly influence the transient damage kinetics. Finally, the influence of an additional surface layer of high-$Z$ material on the damage within diamond is discussed.

We report a high-precision finite-size scaling study of the critical behavior of the three-dimensional Ising Edwards-Anderson model (the Ising spin glass). We have thermalized lattices up to $L=40$ using the Janus dedicated computer. Our analysis takes into account leading-order corrections to scaling. We obtain $T_{\mathrm{c}}=1.1019\left(29\right)$ for the critical temperature, $\nu =2.562\left(42\right)$ for the thermal exponent, $\eta =-0.3900\left(36\right)$ for the anomalous dimension, and $\omega =1.12\left(10\right)$ for the exponent of the leading corrections to scaling. Standard (hyper)scaling relations yield $\alpha =-5.69\left(13\right)$, $\beta =0.782\left(10\right)$, and $\gamma =6.13\left(11\right)$. We also compute several universal quantities at $T_{\mathrm{c}}$.