To generate a moderate band gap in a graphene monolayer is a very important but rather difficult task. A rare working solution of this problem is to cut it into one-dimensional (1D) nanometer-wide ribbons. Here we show that, instead of cutting the graphene monolayer, a wide band gap can be created in a unique 1D strained structure, i.e., a wrinkled graphene-nanoribbon-like (GNR-like) structure, of a continuous graphene sheet via strong hybridization between the graphene and the metal substrate. The wrinkled GNR-like structures with widths of only a few nanometers are observed in a continuous graphene sheet grown on a Rh foil by using thermal strain engineering. Spatially resolved scanning tunneling spectroscopy revealed a band-gap opening of a few hundred meV in the GNR-like structure in an otherwise continuous metallic graphene sheet, directly demonstrating the realization of a metallic-semiconducting-metallic junction entirely in a graphene monolayer.

Double-layered $\mathrm{S}{\mathrm{r}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ has received phenomenal consideration because it exhibits a plethora of exotic phases when perturbed. New phases emerge with the application of pressure, magnetic field, or doping. Here, we show that creating a surface is an alternative and effective way to reveal hidden phases that are different from those seen in the bulk by investigating the surface properties of $\mathrm{S}{\mathrm{r}}_3{\left(\mathrm{R}{\mathrm{u}}_{1-x}\mathrm{M}{\mathrm{n}}_x\right)}_2{\mathrm{O}}_7$. Driven by the tilt distortion of $\mathrm{Ru}{\mathrm{O}}_6$ octahedra, the surface of $\mathrm{S}{\mathrm{r}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ is less metallic than the bulk. In contrast, because of the vanishing of tilt and enhanced rotation with Mn doping, the surface of $\mathrm{S}{\mathrm{r}}_3{\left(\mathrm{R}{\mathrm{u}}_{0.84}\mathrm{M}{\mathrm{n}}_{0.16}\right)}_2{\mathrm{O}}_7$ is metallic, while the bulk is insulating. Our result demonstrates that the electronic and structural properties at the surface are intimately coupled and consistent with quasitwo-dimensional character.

Optical conductivity of graphene is studied using quantum Monte Carlo calculations. We start from a Euclidean current-current correlator and extract $\sigma \left(\omega \right)$ from Green-Kubo relations using the Backus-Gilbert method. Calculations were performed both for long-range interactions and taking into account only the contact term. In both cases we vary interaction strength and study its influence on optical conductivity. We compare our results with previous theoretical calculations choosing $\omega \approx \kappa$, thus working in the region of the plateau in $\sigma \left(\omega \right)$ which corresponds to optical conductivity of Dirac quasiparticles. No dependence of optical conductivity on interaction strength is observed unless we approach the antiferromagnetic phase transition in the case of an artificially enhanced contact term. Our results strongly support previous theoretical studies that claimed very weak regularization of graphene conductivity.

The evolution of local ferroelectric lattice distortions in multiferroic ${\mathrm{Ge}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ is studied by x-ray diffraction, x-ray absorption spectroscopy, and density functional theory. We show that the anion/cation displacements smoothly decrease with increasing Mn content, thereby reducing the ferroelectric transition from 700 to 100 K at $x=0.5$, where the ferromagnetic Curie temperature reaches its maximum. First principles calculations explain this quenching by different local bond contributions of the Mn $3d$ shell compared to the Ge $4s$ shell in excellent quantitative agreement with the experiments.

The paper deals with the spontaneous coherence building up between exciton-polaritons trapped in an array of deep potential wells in the presence of an incoherent pump. A theoretical approach based on a standard tight-binding mean-field approximation is used to reduce the continuous periodic problem to a discrete model. The typical dynamics of the nonlinear exciton-polariton system for the cases of spatially uniform and for localized pumps are discussed. Special attention is paid to the “staggered” coherent steady states with $\pi$ jumps in the phases between neighboring sites and to “uniform” states with a smooth phase distribution. It is shown that, apart from the states with a single frequency, mixed states with spectra with several harmonics can form in the system. The selection mechanism that controls the type of steady state growing from a weak noise is studied. It is found that in the case of localized pumps the decaying tails of the solutions play a crucial role in the dynamics of the polaritons. The applicability of the obtained theoretical results for a qualitative explanation of the complex phenomena observed in recent experiments is discussed.

This paper presents the results of specific-heat and magnetization measurements, in particular their field-orientation dependence, on the first discovered heavy-fermion superconductor ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ ($T_{\mathrm{c}}\sim 0.6$ K). We discuss the superconducting gap structure and the origin of the anomalous pair-breaking phenomena, leading, e.g., to the suppression of the upper critical field $H_{\mathrm{c}2}$, found in the high-field region. The data show that the anomalous pair breaking becomes prominent below about 0.15 K in any field direction, but occurs closer to $H_{\mathrm{c}2}$ for $H\parallel c$. The presence of this anomaly is confirmed by the fact that the specific-heat and magnetization data satisfy standard thermodynamic relations. Concerning the gap structure, field-angle dependencies of the low-temperature specific heat within the $ab$ and $ac$ planes do not show any evidence for gap nodes. From microscopic calculations in the framework of a two-band full-gap model, the power-law-like temperature dependencies of $C$ and $1/T_1$, reminiscent of nodal superconductivity, have been reproduced reasonably. These facts further support multiband full-gap superconductivity in ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$.

We consider the phase diagram of a ferromagnetic system driven to a quantum phase transition with a tuning parameter $p$. Before being suppressed, the transition becomes of the first order at a tricritical point, from which wings emerge under application of the magnetic field $H$ in the $T-p-H$ phase diagram. We show that the edge of the wings merge with tangent slopes at the tricritical point.

The intention of this paper is twofold. First, the mode-width probability density function (PDF) is introduced as a powerful statistical tool to study and compare the transverse Anderson localization properties of a disordered quasi-one-dimensional optical waveguide. Second, by analyzing the scaling properties of the mode-width PDF with the transverse size of the waveguide, it is shown that the mode-width PDF gradually converges to a terminal configuration. Therefore, it may not be necessary to study a real-sized disordered structure in order to obtain its statistical localization properties and the same PDF can be obtained for a substantially smaller structure. This observation is important because it can reduce the often demanding computational effort that is required to study the statistical properties of Anderson localization in disordered waveguides. Using the mode-width PDF, substantial information about the impact of the waveguide parameters on its localization properties is extracted. This information is generally obscured when disordered waveguides are analyzed using other techniques such as the beam propagation method. As an example of the utility of the mode-width PDF, it is shown that the cladding refractive index can be used to quench the number of extended modes, hence improving the contrast in image transport properties of disordered waveguides.

We predict the electrical generation and injection of domain walls into a ferromagnetic nanowire without the need of an assisting magnetic field. Our analytical and numerical results show that above a critical current $j_c$ domain walls are injected into the nanowire with a period $T\sim {(j-j_c)}^{-1/2}$. Importantly, domain walls can be produced periodically even in a simple exchange ferromagnet with uniaxial anisotropy, without requiring any standard “twisting” interaction such as Dzyaloshinskii-Moriya or dipole-dipole interactions. We show analytically that this process and the period exponents are universal and do not depend on the peculiarities of the microscopic Hamiltonian. Finally we give a specific proposal for an experimental realization.

Nonuniform metasurfaces (electrically thin composite layers) can be used for shaping refracted and reflected electromagnetic waves. However, known design approaches based on the generalized refraction and reflection laws do not allow realization of perfectly performing devices: there are always some parasitic reflections into undesired directions. In this paper we introduce and discuss a general approach to the synthesis of metasurfaces for full control of transmitted and reflected plane waves and show that perfect performance can be realized. The method is based on the use of an equivalent impedance matrix model which connects the tangential field components at the two sides on the metasurface. With this approach we are able to understand what physical properties of the metasurface are needed in order to perfectly realize the desired response. Furthermore, we determine the required polarizabilities of the metasurface unit cells and discuss suitable cell structures. It appears that only spatially dispersive metasurfaces allow realization of perfect refraction and reflection of incident plane waves into arbitrary directions. In particular, ideal refraction is possible only if the metasurface is bianisotropic (weak spatial dispersion), and ideal reflection without polarization transformation requires spatial dispersion with a specific, strongly nonlocal response to the fields.

The ground state phase of a spin-$\frac{1}{2} J_1\text{−}{J}_2$ antiferromagnetic Heisenberg model on a square lattice around the maximally frustrated regime $\left(J_2\sim 0.5J_1\right)$ has been debated for decades. Here we study this model using the cluster update algorithm for tensor-product states (TPSs). The ground state energies at finite sizes and in the thermodynamic limit (with finite size scaling) are in good agreement with exact diagonalization study. Through finite size scaling of the spin correlation function, we find the critical point $J_2^{c_1}=0.572\left(5\right)J_1$ and critical exponents $\nu =0.50\left(8\right),{\eta }_s=0.28\left(6\right)$. In the range of $0.572<J_2/J_1\le 0.6$ we find a paramagnetic ground state with an exponentially decaying spin-spin correlation. Up to a $24\times 24$ system size, we observe power law decaying dimer-dimer and plaquette-plaquette correlations with an anomalous plaquette scaling exponent ${\eta }_p=0.24\left(1\right)$ and an anomalous columnar scaling exponent ${\eta }_c=0.28\left(1\right)$ at $J_2/J_1=0.6$. These results are consistent with a potential gapless $U\left(1\right)$ spin-liquid phase. However, since the $U\left(1\right)$ spin liquid is unstable due to the instanton effect, a valence bond solid order with very small amplitude might develop in the thermodynamic limit. Thus, our numerical results strongly indicate a deconfined quantum critical point at $J_2^{c_1}$. Remarkably, all the observed critical exponents are consistent with the $J\text{−}Q$ model.

Two-dimensional (2D) transition metal dichalcogenides have shown numerous interesting physical and chemical properties, making them promising materials for electronic, optoelectronic, and energy applications. Tuning thermal conductivity of 2D materials could expand their applicability in many of these fields. In this paper, we propose a strategy of using alloying and nanodomains to suppress the thermal conductivity of 2D materials. To predict the thermal conductivity of a 2D alloy embedded with nanodomains, we employ the Green's function approach to assess the phonon scattering strength due to alloying and nanodomain embedding. Our first-principles-driven phonon Boltzmann transport equation calculations show that the thermal conductivity of single-layer $\mathrm{Mo}{\mathrm{S}}_2$ can be reduced to less than one-tenth of its intrinsic thermal conductivity after alloying with W and introducing nanodomains due to the strong scattering for both high- and low-frequency phonons. Strategies to further reduce thermal conductivity are also discussed.

Predicting time-dependent quantum transport in the transient regime is important for understanding the intrinsic dynamic response of a nanodevice and for predicting the limit of how such a device can switch on or off a current. Theoretically, this problem becomes quite difficult to solve when the nanodevice contains disorder because the calculated transient current must be averaged over many disorder configurations. In this work, we present a theoretical formalism to calculate the configuration averaged time-dependent current flowing through a phase coherent device containing disorder sites where the transient current is driven by sharply turning on and off the external bias voltage. Our theory is based on the Keldysh nonequilibrium Green's function (NEGF) formalism and is applicable in the far from equilibrium nonlinear response quantum transport regime. The effects of disorder scattering are dealt with by the coherent potential approximation (CPA) extended in the time domain. We show that after approximations such as CPA and vertex corrections for calculating the multiple impurity scattering in the transient regime, the derived NEGFs perfectly satisfy a Ward identity. The theory is quantitatively verified by comparing its predictions to the exact solution for a tight-binding model of a disordered two-probe transport junction.

We study the transmission phase shift across a Kondo correlated quantum dot in a GaAs heterostructure at temperatures below the Kondo temperature $\left(T<T_{\mathrm{K}}\right)$, where the phase shift is expected to show a plateau at $\pi /2$ for an ideal Kondo singlet ground state. Our device is tuned such that the ratio $\mathrm{\Gamma }/U$ of level width $\mathrm{\Gamma }$ to charging energy $U$ is quite large ($\lesssim 0.5$ rather than $\ll 1$). This situation is commonly used in GaAs quantum dots to ensure Kondo temperatures large enough ($\simeq 100$ mK here) to be experimentally accessible; however, it also implies that charge fluctuations are more pronounced than typically assumed in theoretical studies focusing on the regime $\mathrm{\Gamma }/U\ll 1$ needed to ensure a well-defined local moment. Our measured phase evolves monotonically by $\pi$ across the two Coulomb peaks, but without being locked at $\pi /2$ in the Kondo valley for $T\ll T_{\mathrm{K}}$, due to a significant influence of large $\mathrm{\Gamma }/U$. Only when $\mathrm{\Gamma }/U$ is reduced sufficiently does the phase start to be locked around $\pi /2$ and develops into a plateau at $\pi /2$. Our observations are consistent with numerical renormalization group calculations, and can be understood as a direct consequence of the Friedel sum rule that relates the transmission phase shift to the local occupancy of the dot, and thermal average of a transmission coefficient through a resonance level near the Fermi energy.

We describe the finite-size spectrum in the vicinity of the quantum critical point between a ${\mathbb{Z}}_2$ spin liquid and a coplanar antiferromagnet on the torus. We obtain the universal evolution of all low-lying states in an antiferromagnet with global SU(2) spin rotation symmetry, as it moves from the fourfold topological degeneracy in a gapped ${\mathbb{Z}}_2$ spin liquid to the Anderson “tower-of-states” in the ordered antiferromagnet. Due to the existence of nontrivial order on either side of this transition, this critical point cannot be described in a conventional Landau-Ginzburg-Wilson framework. Instead, it is described by a theory involving fractionalized degrees of freedom known as the $\mathrm{O}{\left(4\right)}^{*}$ model, whose spectrum is altered in a significant way by its proximity to a topologically ordered phase. We compute the spectrum by relating it to the spectrum of the $\mathrm{O}\left(4\right)$ Wilson-Fisher fixed point on the torus, modified with a selection rule on the states, and with nontrivial boundary conditions corresponding to topological sectors in the spin liquid. The spectrum of the critical $\mathrm{O}(2N$) model is calculated directly at $N=\infty$, which then allows a reconstruction of the full spectrum of the $\mathrm{O}{\left(2N\right)}^{*}$ model at leading order in $1/N$. This spectrum is a unique characteristic of the vicinity of a fractionalized quantum critical point, as well as a universal signature of the existence of proximate ${\mathbb{Z}}_2$ topological and antiferromagnetically ordered phases, and can be compared with numerical computations on quantum antiferromagnets on two-dimensional lattices.

We investigate effects of the filled Dirac sea on the $SU\left(4\right)$ symmetry breaking for the integer quantum Hall states of graphene with long-ranged electrostatic Coulomb interactions. Our model also includes Hubbard and nearest-neighbor repulsive interactions with strengths $U$ and $V$, respectively. We find the symmetry breaking of $n=0$ Landau level is accompanied by an $SU\left(4\right)$ polarization of the filled Dirac sea. We compute the phase diagram for filling factor $\nu =0,\pm 1$ in the $U\text{−}V$ space at magnetic fields relevant to experiments. The Dirac sea polarization significantly influences the phase diagram. It suppresses the Kekulé ordering at $\nu =0$ and induces it at $\nu =\pm 1$. We also calculate the excitation gaps at small tilted magnetic fields. This enables us to conclude that the ground state is a charge density wave at $\nu =0$ and valley-spin polarized at $\nu =\pm 1$. We delineate a region in the $U\text{−}V$ space consistent with the experiments.

In single-molecule magnets, the exchange between a localized spin moment and the electronic background provides a suitable laboratory for studies of dynamical aspects of both local spin and transport properties. Here we address the time evolution of a localized spin moment coupled to an electronic level in a molecular quantum dot embedded in a tunnel junction between metallic leads. The interactions between the localized spin moment and the electronic level generate an effective interaction between the spin moment at different instances in time. Therefore, we show that, despite being a single-spin system, there are effective contributions of isotropic Heisenberg and anisotropic Ising and Dzyaloshinski-Moriya character acting on the spin moment. The interactions can be controlled by gate voltage, voltage bias, the spin polarization in the leads, in addition to external magnetic fields. Signatures of the spin dynamics are found in the transport properties of the tunneling system, and we demonstrate that measurements of the spin current may be used for readout of the local spin moment orientation.

We present a resonant x-ray diffraction study of the magnetic order in ${\mathrm{NdFe}}_{\text{3}}{\left({\mathrm{BO}}_{\text{3}}\right)}_{\text{4}}$ and its coupling to applied electric fields. Our high-resolution measurements reveal two different coexisting magnetic phases, which can directly be controlled and manipulated by external electric fields. More specifically, the volume fraction of the collinear magnetic phase is found to strongly increase at the expense of helically ordered regions when an electric field is applied. These results confirm that the collinear magnetic phase is responsible for the ferroelectric polarization of ${\mathrm{NdFe}}_{\text{3}}{\left({\mathrm{BO}}_{\text{3}}\right)}_{\text{4}}$ and, more importantly, demonstrate that coexisting magnetic phases provide a route towards materials with a strong but yet hysteresis-free magnetoelectric response.

We discuss the minimum value of the zero-bias differential conductance $G_{\min }$ in a junction consisting of a normal metal and a nodal superconductor preserving time-reversal symmetry. Using the quasiclassical Green function method, we show that $G_{\min }$ is quantized at $(4e^2/h)N_{\mathrm{ZES}}$ in the limit of strong impurity scatterings in the normal metal at the zero temperature. The integer $N_{\mathrm{ZES}}$ represents the number of perfect transmission channels through the junction. An analysis of the chiral symmetry of the Hamiltonian indicates that $N_{\mathrm{ZES}}$ corresponds to the Atiyah-Singer index in mathematics.

We have varied the oxygen defect concentration in ${\mathrm{Pr}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_{4+\delta }$ and characterized the mean-free path length $\ell$ and the superconducting coherence length $\xi$ of ${\mathrm{Pr}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_{4+\delta }$ by means of low-temperature transport measurements. Concomitant with the clean-to-dirty crossover around $\ell \approx \xi$ we find the emergence of linear resistivity behavior being persistent far into the clean limit $\left(\ell \gg \xi \right)$. We find a violation of Matthiessen's rule that can be explained by a model of $\sigma$ involving a defect-assisted interlayer channel which acts in parallel with the bandlike conductivity. We observe an unusual dependence of $T_c$ on residual resistivity, inconsistent with the generalized Abrikosov-Gor'kov theory for an order parameter with a single component, providing an important constraint on models of the superconductivity in this material system.