The weakness of electron-electron correlations in the itinerant antiferromagnet Cr doped with V has long been considered the reason that neither new collective electronic states nor even non-Fermi-liquid behavior are observed when antiferromagnetism in ${\mathrm{Cr}}_{1-x}{\mathrm{V}}_x$ is suppressed to zero temperature. We present the results of neutron and electron diffraction measurements of several lightly doped single crystals of ${\mathrm{Cr}}_{1-x}{\mathrm{V}}_x$ in which the archetypal spin density wave instability is progressively suppressed as the V content increases, freeing the nesting-prone Fermi surface for a new striped charge instability that occurs at $x_c=0.037$. This novel nesting driven instability relieves the entropy accumulation associated with the suppression of the spin density wave and avoids the formation of a quantum critical point by stabilizing a new type of charge order at temperatures in excess of 400 K. Restructuring of the Fermi surface near quantum critical points is a feature found in materials as diverse as heavy fermions, high-temperature copper oxide superconductors and now even elemental metals such as Cr.

We present a functional renormalization group (fRG) formalism for interacting fermions on lattices that captures the flow into states with commensurate spin-density-wave order. During the flow, the growth of the order parameter is fed back into the flow of the interactions and all modes can be integrated out. This extends previous fRG flows in the symmetric phase that run into a divergence at a nonzero RG scale, i.e., that have to be stopped at the ordering scale. We use the corresponding Ward identity to check the accuracy of the results. We apply our method to a model with two Fermi pockets that have perfect particle-hole nesting. The results obtained from the fRG are compared with those in random-phase approximation.

We report magnetic susceptibility $\left(\chi \right)$ and heat capacity $\left(C_p\right)$ measurements along with ab initio electronic structure calculations on ${\mathrm{PbCuTe}}_2{\mathrm{O}}_6$, a compound made up of a three-dimensional (3D) network of corner-shared triangular units. The presence of antiferromagnetic interactions is inferred from a Curie-Weiss temperature $({\theta }_{\mathrm{CW}})$ of about $-22$ K from the $\chi \left(T\right)$ data. The magnetic heat capacity $C_m$ data show a broad maximum at $T^{\max }\simeq 1.15$ K (i.e., $T^{\max }/{\theta }_{\mathrm{CW}}\simeq 0.05)$, which is analogous to the the observed broad maximum in the $C_m/T$ data of a hyper-kagome system, ${\mathrm{Na}}_4{\mathrm{Ir}}_3{\mathrm{O}}_8$. In addition, $C_m$ data exhibit a weak kink at $T^{*}\simeq 0.87$ K. While the $T^{\max }$ is nearly unchanged, the $T^{*}$ is systematically suppressed in an increasing magnetic field $\left(H\right)$ up to 80 kOe. For $H\ge 80$ kOe, the $C_m$ data at low temperatures exhibit a characteristic power-law $\left(T{}^{\alpha }\right)$ behavior with an exponent $\alpha$ slightly less than 2. Hopping integrals obtained from the electronic structure calculations show the presence of strongly frustrated 3D spin interactions along with non-negligible unfrustrated couplings. Our results suggest that ${\mathrm{PbCuTe}}_2{\mathrm{O}}_6$ is a candidate material for realizing a 3D quantum spin liquid state at high magnetic fields.

Many ab initio calculations, in particular in solid state physics, rely on the antiresonant approximation. In this paper we discuss the derivation, the validity, and the accuracy of this approximation, analyzing how the optical properties of several bulk semiconducting materials and surfaces can be affected. We present accurate results for different spectroscopic quantities in the linear and nonlinear responses and we analyze the discrepancies between approximated and exact formulas. An investigation is made of the effect of the approximation on absorption spectra for different materials, showing the reliability of the approximation even in the presence of local field and excitonic effects. The energy loss is shown to be drastically affected. The effect of the band gap of materials on the quality of the approximation is also discussed. Finally, we report on the influence of the antiresonant approximation on second harmonic generation spectra.

Electromagnetic fields bound tightly to charge carriers in a two-dimensional sheet, namely surface plasmons, are shielded by metallic plates that are a part of a device. It is shown that for epitaxial graphenes, the propagation velocity of surface plasmons is suppressed significantly through a partial screening of the electron charge by the interface states. On the basis of analytical calculations of the electron lifetime determined by the screened Coulomb interaction, we show that the screening effect gives results in agreement with those of a recent experiment.

The quantum confinement effect may play an important role in the gap modulation of armchair graphene nanoribbons (AGNRs) under strain. Using the phase accumulation model, we have investigated the energy-dependent phase shift $\varphi \left(\varepsilon \right)$ at the $\Gamma$ point of AGNRs under various strains using first-principles calculation. The calculation results show that although the energy dispersion of the phase shift is modified by strain, the phase shift near the Fermi level is close to $0.75\pi$, indicating that strain has little effect near that energy level. We can approximate the energy-dependent phase shift by a constant, $\varphi \left(\varepsilon \right)=0.75\pi$, for AGNRs under various $x$ strains. Due to the structural similarity between AGNRs and zigzag carbon nanotubes (ZCNTs), the electronic properties of AGNRs should be similar to those of ZCNTs. The quantization condition of the wave vector of ZCNTs governed by the periodic boundary condition along the circumference direction is similar to that of AGNRs except that the phase shift is equal to zero, $\varphi \left(\varepsilon \right)=0$. Using the zone-folding (ZF) method, we can calculate the band gap of any strained AGNR (ZCNT) from the phase shift $\varphi =0.75\pi$ $(\varphi =0)$ and the electronic structure of the strained graphene. The AGNR shows a zigzag behavior of the dependence of the band gap on strain which is very similar to the ZCNT. The zigzag patterns are significantly shifted by different phase shifts. The peak value of the band gap and the period of the pattern decreases as the width of the ribbon increases. For a given AGNR, the peak value and the period of the pattern increase as the strain increases. A flattening of the peaks appears at the strain where the maximum band gap occurs due to large compressive strain. All these observations can be understood easily from our ZF calculations. The agreement between our model and real local-density approximation calculations indicates that our model can provide an efficient and accurate method to estimate the band gap of AGNRs and ZCNTs under strain, and therefore can provide a better understanding of the effect of quantum confinement on the electronic properties of AGNRs.

We discuss the emergence of nonadiabatic behavior in the dynamics of the order parameter in a low-dimensional quantum many-body system subject to a linear ramp of one of its parameters. While performing a ramp within a gapped phase seems to be the most favorable situation for adiabaticity, we show that such a change leads eventually to the disruption of the order, no matter how slowly the ramp is performed. We show this in detail by studying the dynamics of the one-dimensional quantum Ising model subject to linear variation of the transverse magnetic field within the ferromagnetic phase, and then propose a general argument applicable to other systems.

We examine the role of spin-orbit coupling in the electronic structure of $\alpha -{\mathrm{RuCl}}_3$, in which Ru ions in $4d^5$ configuration form a honeycomb lattice. Our x-ray absorption spectroscopy measurements at the Ru $L$ edges exhibit distinct spectral features associated with the presence of substantial spin-orbit coupling, as well as an anomalously large branching ratio. Furthermore the measured optical spectra can be described very well with first-principles electronic structure calculations obtained by taking into account both spin-orbit coupling and electron correlations. We propose that $\alpha -{\mathrm{RuCl}}_3$ is a spin-orbit assisted Mott insulator, and that the bond-dependent Kitaev interaction may be important for understanding magnetism of this compound.

We study a model (i.e., the $t_2\text{−}V$ model involving next-nearest-neighbor hopping and nearest-neighbor repulsion) in one dimension that generically depicts the dominant transport mechanism in cooperative strong electron-phonon interaction systems. Using analytic and numerical approaches, hard-core bosons are shown to typically undergo a striking discontinuous transition from a superfluid to a supersolid. Topological inequivalence of rings with even and odd number of sites is manifested through observable differences (in structure factor peaks) at the transition. Connections are also identified between the $t_2\text{−}V$ model and other topologically interesting models.

A nonperturbative quantum impurity solver is proposed based on a formally exact hierarchical equations of motion (HEOM) formalism for open quantum systems. It leads to quantitatively accurate evaluation of physical properties of strongly correlated electronic systems, in the framework of dynamical mean-field theory (DMFT). The HEOM method is also numerically convenient to achieve the same level of accuracy as that using the state-of-the-art numerical renormalization group impurity solver at finite temperatures. The practicality of the HEOM+DMFT method is demonstrated by its applications to the Hubbard models with Bethe and hypercubic lattice structures. We investigate the metal-insulator transition phenomena, and address the effects of temperature on the properties of strongly correlated lattice systems.

The low-energy band-structure of electrons propagating on a lateral surface of a heterostructure consisting of three dimensional topological insulator (TI) and magnetic insulator layers has been calculated. The energy spectrum is highly tunable depending on the relation between the interlayer tunneling amplitude, Zeeman energy and surface potential. A ratio between the first two parameters controls a topological transition between chiral and helical surface state band-structures. In the former case localized states, whose wave-functions are confined near single TI layers, emerge in a narrow parameter range where they coexist with itinerant states. In regular superlattices these localized states form a flat band in the vertical direction. Such a localization has a topological origin. It is associated with a specific spatial spin texture of these states. The localized states vanish upon the topological phase transition from the anomalous quantum Hall regime to a trivial phase characterized by an anisotropic Dirac cone on the lateral surface of the heterostructure.

The exothermic nature of oxidation causes nearly all semiconductor applications in various fields like electronics, medicine, photonics, and sensor technology to acquire an oxidized semiconductor surface part during the application manufacturing. The significance of understanding and controlling the atomic scale properties of oxidized semiconductor surfaces is expected to increase even further with the development of nanoscale semiconductor crystals. The nature of oxidized semiconductor layers is, however, hard to predict and characterize as they are usually buried and amorphous. To shed light on these issues, we pursue a different approach based on oxidized III-V semiconductor layers that are crystalline. We present a comprehensive characterization of oxidized crystalline InSb(100)$\left(1\times 2\right)$-O layers by ab initio calculations, photoelectron spectroscopy, scanning tunneling microscopy, and spectroscopy, and demonstrate the electronic band structures of different oxidized phases of the semiconductor, which elucidate the previous contradictory semiconductor-oxidation effects. At 0.5 monolayer (ML) oxidation, oxygen atoms tend to occupy subsurface Sb sites, leading to metallic states in the semiconductor band gap, which arise from top dimers. When the oxidation is increased to the 1.0–2.0 ML concentration, oxygen occupies also interstitial sites, and the insulating band structure without gap states is stabilized with unusual occupied In dangling bonds. In contrast, the 2.5–3.0 ML oxide phases undergo significant changes toward a less ordered structure. The findings suggest a methodology for manipulating the electronic structure of oxidized semiconductor layers.

Metal-oxide ${\mathrm{[Fe}}_K/\left(\text{MgO}\right){}_L{\mathrm{]}}_N$ multilayers were grown on MgO(001) substrates for various integer numbers $(K,L)$ of the Fe(001) and MgO(001) monolayers, respectively, and the number of repetitions $N$ varied from 6 to 30. Room-temperature conversion electron Mössbauer spectroscopy (CEMS) measurements proved that the magnetic properties of these monoatomic multilayers were extremely sensitive to both the Fe and MgO sublayer thicknesses. A stable ferromagnetic state and a strong perpendicular magnetization component at room temperature were obtained by changing the sublayer thickness and the number of multilayer repetitions. The analysis of the CEMS spectra in correlation with the magneto-optic Kerr effect measurements indicated a complicated domain structure in this special type of metal-insulator material. The vortexlike domain structure was confirmed by micromagnetic simulations.

The thermoelectric properties of 1.6-nm-thick Si square nanowires with $\left[100\right]$ crystalline orientation are calculated over a wide temperature range from 0 K to 1000 K, taking into account atomistic electron-phonon interaction. In our model, the $\left[010\right]$ and $\left[001\right]$ facets are passivated by hydrogen and there are Si-Si dimers on the nanowire surface. The electronic structure was calculated by using the $s{p}^3$ spin-orbit-coupled atomistic second-nearest-neighbor tight-binding model. The phonon dispersion was calculated from a valence force field model of the Brenner type. A scheme for calculating electron-phonon matrix elements from a second-nearest-neighbor tight-binding model is presented. Based on Fermi's golden rule, the electron-phonon transition rate was obtained by combining the electron and phonon eigenstates. Both elastic and inelastic scattering processes are taken into consideration. The temperature dependence of transport characteristics was calculated by using a solution of the linearized Boltzmann transport equation obtained by means of the iterative orthomin method. At room temperature, the electron mobility is 195 ${\mathrm{cm}}^2$ ${\mathrm{V}}^{-1}$ ${\mathrm{s}}^{-1}$ and increases with temperature, while a figure of merit $ZT=0.38$ is reached for n-type doping with a concentration of $n={10}^{19}$ ${\mathrm{cm}}^{-3}$.

A first-principles investigation of $E^{\prime }$  centers in vitreous silica $(v-{\mathrm{SiO}}_2)$ based on calculations of the electron paramagnetic resonance (EPR) parameters is presented. The EPR parameters are obtained by exploiting the gauge including projector augmented wave method as implemented in the quantum-espresso package. First, we analyze the EPR parameters of a large number of ${\mathrm{Si}}_2$ dimers. The $g$ tensor of the ${\mathrm{Si}}_2$ dimers is shown to possess an average rhombic symmetry and larger $g$ principal values with respect to those observed, e.g., for the $E^{\prime }$${}_{\gamma }$ center in silica. Furthermore, the $g$ principal values clearly show a linear trend with the Si-Si dimer length. Our results suggest that the ${\mathrm{Si}}_2$ dimers could correspond to an unidentified paramagnetic center, though occasionally the calculated $g$ principal values of the ${\mathrm{Si}}_2$ dimer might be compatible with those found experimentally for the $E^{\prime }$${}_{\delta }$ center. Next, we generate nondimer configurations by a procedure involving structural relaxations in the subsequent positively charged states. In particular, puckered, unpuckered, doubly puckered, and forward-oriented configurations are generated. The distributions of the calculated EPR parameters of the puckered and unpuckered configurations further support the assignment of the $E^{\prime }$${}_{\gamma }$ center to an unpaired spin localized at a threefold coordinated silicon dangling bond. Moreover, by analyzing Fermi contacts and $g$ tensors of the puckered and forward-oriented configurations, we suggest the assignment of the $E^{\prime }$${}_{\alpha }$ center to the latter type of configurations. This work also suggests that the differences in the EPR parameters of $E^{\prime }$${}_{\alpha }$ and $E^{\prime }$${}_{\gamma }$ centers mainly arise from the strained geometry of the silicon dangling bond. In the forward-oriented configurations, one Si-O bond is about 0.2 Å longer than the remaining two, whereas in the silicon dangling bond of the puckered and unpuckered configurations, all three bonds have a length of $\sim$1.6 Å  each.

We study the dynamics of excitations in a system of $O\left(N\right)$ quantum rotors in the presence of random fields and random anisotropies. Below the lower critical dimension $d_{\mathrm{lc}}=4$ the system exhibits a quasi-long-range order with a power-law decay of correlations. At zero temperature the spin waves are localized at the length scale $L_{\mathrm{loc}}$ beyond which the quantum tunneling is exponentially suppressed, $c\sim e^{-{(L/L_{\mathrm{loc}})}^{2(\theta +1)}}$. At finite temperature $T$ the spin waves propagate by thermal activation over energy barriers that scale as $L^{\theta }$. Above $d_{\mathrm{lc}}$ the system undergoes an order-disorder phase transition with activated dynamics such that the relaxation time grows with the correlation length $\xi$ as $\tau \sim e^{C{\xi }^{\theta }/T}$ at finite temperature and as $\tau \sim e^{C^{\prime }{\xi }^{2(\theta +1)}/{\hslash }^2}$ in the vicinity of the quantum critical point.

For fcc and tetragonal distorted fct iron a large number of magnetic configurations as a function of crystal structural parameters were studied by means of density functional theory concepts. The stability of magnetic structures was defined by the magnetic reorientation energy $\Delta E_{\mathrm{reor}}^i$ as the difference of the total energy of configuration $i$ and that of the fcc ferromagnetic state. The cluster expansion technique was applied to six volumes deriving $\Delta E_{\mathrm{reor}}$ for more than 90 000 collinear spin structures at each volume. Structures with low $\Delta E_{\mathrm{reor}}$ were tetragonally distorted according to a two-dimensional mesh defined by volume per atom $V$ and $c/a$ ratio. At each mesh point $\Delta E_{\mathrm{reor}}$ for all collinear structures were compared to results for spin spirals (SSs) which were calculated on a grid of propagation directions, and then the lowest $\Delta E_{\mathrm{reor}}$ defined the magnetic structure map. Three local minima were identified and for each of the minima SSs were calculated on a fine grid of propagation vectors. At the minimum with $V=10.6 \AA$${}^3$ and $0.94\le c/a\le 1.01$ a hitherto unknown simple collinear spin structure with four atoms per fct unit cell was the most stable one. It consists of two atoms with antiferromagnetically ordered local moments of $\pm 1.8{\mu }_{\mathrm{B}}$ and of two atoms with zero local moment.

We investigate the current-induced motion of ferromagnetic domain walls in the presence of a Rashba spin-orbit interaction of the itinerant electrons. We show how a Rashba interaction can stabilize the domain wall motion, such that the Walker breakdown is shifted to larger current densities. The Rashba spin-orbit interaction creates a fieldlike contribution to the spin torque, which breaks the symmetry of the system and modifies the internal structure of the domain wall. Moreover, it can induce an additional switching of the chirality of the domain wall for sufficiently strong Rashba interactions. This allows one to choose the desired chirality by choosing the direction of the applied spin-polarized current. Both the suppression of the Walker breakdown and the chirality switching affect the domain wall velocity significantly. This is even more pronounced for short current pulses, where an additional domain wall drift in either positive or negative direction appears after the pulse ends. By this, we can steer the final position of the domain wall. This mechanism may help to overcome the current limitations of the domain wall motion due to the Walker breakdown which occurs for rather low current densities in systems without a Rashba spin-orbit interaction.

It has recently been shown that domain walls (DWs) in ferromagnets can be moved in the presence of thermal gradients. In this work we study the motion of narrow domain walls in low-dimensional systems when subjected to thermal gradients. The system chosen is a monolayer of Fe on W(110) which is known to exhibit a large anisotropy while having a soft exchange, resulting in a very narrow domain wall. The study is performed by means of atomistic spin dynamics simulations coupled to first-principles calculations. By subjecting this system to thermal gradients we observe a temperature-dependent movement of the domain wall. The thermal gradient always makes the domain wall move towards the hotter region of the sample with a velocity proportional to the gradient. Our material specific study is complemented by model simulations to discern the interplay between the thermal gradient, magnetic anisotropy, and the exchange interaction and shows that the larger DW velocities are found for materials with broader domain-wall width. The relatively slow DW motion of the Fe/W(110) system is hence primarily caused by its narrow domain-wall width, which results from its large magnetic anisotropy and soft exchange.

We investigate the charge and spin transport of a voltage-biased superconducting point contact coupled to a nanomagnet. The magnetization of the nanomagnet is assumed to precess with the Larmor frequency ${\omega }_L$ when exposed to ferromagnetic resonance conditions. The Larmor precession locally breaks the spin-rotation symmetry of the quasiparticle scattering and generates spin-polarized Shapiro steps for commensurate Josephson and Larmor frequencies that lead to magnetization reversal. This interplay between the ac Josephson current and the magnetization dynamics occurs at voltages $\left|V\right|=\hslash {\omega }_L/2en$ for $n=1,2,...$, and the subharmonic steps with $n>1$ are a consequence of multiple Andreev reflection (MAR). Moreover, the spin-precession-assisted MAR generates quasiparticle scattering amplitudes that, due to interference, lead to current-voltage characteristics of the dc charge and spin currents with subharmonic gap structures displaying an even-odd effect.