We study the effect of disordered ripples on the conductivity of monolayer graphene flakes. We calculate the relaxation times and the Boltzmann conductivities associated with two mechanisms. First, we study the conductivity correction due to an external in-plane magnetic field $B_{\parallel }$. Due to the irregular local curvature found in graphene sheets deposited over a substrate, $B_{\parallel }$ can be mapped into an effective random magnetic field perpendicular to the graphene surface. Second, we study the electron momentum relaxation due to intrinsic pseudomagnetic fields originated from deformations and strain. We find that the competition between these mechanisms gives rise to a strong anisotropy in the conductivity tensor. This result provides a new strategy to quantitatively infer the strength of pseudomagnetic fields in rippled graphene flakes.

We demonstrate that the physical reason for the nontrivial topological properties of Dirac semimetals $A_3\mathrm{Bi}$ $\left(A=\mathrm{Na},\mathrm{K},\mathrm{Rb}\right)$ is connected with a discrete symmetry of the low-energy effective Hamiltonian. By making use of this discrete symmetry, we argue that all electron states can be split into two separate sectors of the theory. Each sector describes a Weyl semimetal with a pair of Weyl nodes and broken time-reversal symmetry. The latter symmetry is not broken in the complete theory because the time-reversal transformation interchanges states from different sectors. Our findings are supported by explicit calculations of the Berry curvature. In each sector, the field lines of the curvature reveal a pair of monopoles of the Berry flux at the positions of Weyl nodes. The ${\mathbb{Z}}_2$ Weyl semimetal nature is also confirmed by the existence of pairs of surface Fermi arcs, which originate from different sectors of the theory.

RuP single crystals of MnP-type orthorhombic structure were synthesized by the Sn flux method. Temperature-dependent x-ray diffraction measurements reveal that the compound experiences two structural phase transitions, which are further confirmed by enormous anomalies shown in temperature-dependent resistivity and magnetic susceptibility. Particularly, the resistivity drops monotonically upon temperature cooling below the second transition, indicating that the material shows metallic behavior, in sharp contrast with the insulating ground state of polycrystalline samples. Optical conductivity measurements were also performed in order to unravel the mechanism of these two transitions. The measurement revealed a sudden reconstruction of band structure over a broad energy scale and a significant removal of conducting carriers below the first phase transition, while a charge-density-wave-like energy gap opens below the second phase transition.

The method of continuous unitary transformations (CUTs) is applied to the Anderson impurity and the Kondo model aiming at the systematic derivation of convergent effective models. If CUTs are applied in a conventional way, diverging differential equations occur. Similar to poor man's scaling, the energy scale, below which the couplings diverge, corresponds to the Kondo temperature $T_K$. We present a way to apply CUTs to the Kondo and to the Anderson impurity model so that no divergences occur but a converged effective low-energy model is derived with small finite parameters at arbitrarily small energies. The ground state corresponds to a bound singlet with a binding energy given by the Kondo temperature $T_K$.

Spin-momentum locked states, notably those found on the surfaces of topological insulators, are promising for low-power electronic devices based on spin transport. Here we report a much more versatile case involving a Dirac semimetal film, theoretically modeled by low-$Z$ substitution of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, on a $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ topological insulator substrate. Such a film can carry highly spin-polarized conduction channels by electronic coupling to the substrate. The spin channel width, defined by the film thickness, is at the designer's disposal, thus permitting optimization of the system parameters. The results demonstrate Dirac semimetals as an important class of material for spintronic applications.

We present low-temperature specific heat and magnetization measurements of the $S$ = 1/2 organic spin ladder 3-I-V [3-(3-iodophenyl)-1,5-diphenylverdazyl]. $Abinitio$ molecular orbital calculations indicate the existence of an intraladder diagonal interaction $J_{\mathrm{diag}}$ and three types of interladder interactions. The ferromagnetic $J_{\mathrm{diag}}$ induces frustration in the spin ladder, which has an essential contribution to the ground state. The obtained phase diagram exhibits a low-field three-dimensional order and a nontrivial phase near the saturation field. The absence of successive phase transition with decreasing temperature and the anomalous behavior of the magnetization curve near the saturation field suggest that these two phases are longitudinal spin-density-wave and spin-multipolar orders, respectively.

We study the photoinduced manipulation of charge carriers in monolayer silicene subject to intense electromagnetic terahertz radiation. Considering the Dirac cone approximation and going beyond the off-resonant condition for large frequencies of the radiation field, where only virtual photon processes are allowed, we present the exact zero-momentum pseudospin polarization and numerical results for the quasienergy band structure and time-averaged density of states. We find that resonant processes, due to real photon emission and absorbtion processes, induce a band inversion that qualitatively modifies the quasienergy spectrum. These band-structure changes manifest themselves as an inversion of the averaged pseudospin polarization. Through the analysis of the time-averaged density of states we find that effective photoinduced gap manipulation can only be achieved in the intermediate and strong matter-radiation coupling regime where the off-resonant approximation breaks down.

From a systematic study of the electrical resistivity $\rho (T,H)$, magnetic susceptibility $\chi (T,H)$, isothermal magnetization $M\left(H\right)$, and the specific heat $C(T,H)$, a temperature-magnetic field ($T-H$) phase diagram has been established for ${\mathrm{GdCo}}_{1-x}{\mathrm{Fe}}_x\mathrm{AsO}$ ($x=0$ and $0.05$) polycrystalline compounds. GdCoAsO undergoes two long-range magnetic transitions: ferromagnetic (FM) transition of Co $3d$ electrons ($T_{\mathrm{C}}^{\mathrm{Co}}$) and antiferromagnetic (AFM) transition of Gd $4f$ electrons ($T_{\mathrm{N}}^{\mathrm{Gd}}$). For the Fe-doped sample ($x=0.05$), an extra magnetic reorientation transition takes place below $T_{\mathrm{N}}^{\mathrm{Gd}}$, which is likely associated with Co moments. The two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic (FIM) behavior in the magnetic susceptibility. Upon decreasing the temperature ($T<T_{\mathrm{C}}^{\mathrm{Co}}$), the magnetocrystalline anisotropy breaks up the FM order of Co by aligning the moments with the local easy axes of the various grains, leading to a spin reorientation transition at $T_{\mathrm{R}}^{\mathrm{Co}}$. By applying a magnetic field, $T_{\mathrm{R}}^{\mathrm{Co}}$ monotonically decreases to lower temperatures, while the $T_{\mathrm{N}}^{\mathrm{Gd}}$ is relatively robust against the external field. On the other hand, the applied magnetic field pulls the magnetization of grains from the local easy direction to the field direction via a first-order reorientation transition, with the transition field ($H_{\mathrm{M}}$) increasing upon cooling the temperature.

We report on ${}^7\mathrm{Li}$ and ${}^{77}\mathrm{Se}$ nuclear magnetic resonance (NMR) measurements in the newly discovered iron-based superconductor (${\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}$)OHFeSe. Both remarkable change in spectrum and divergence of spin-spin relaxation (${\mathrm{T}}_2$) are observed with decreasing temperature, suggesting a static magnetic ordering in this material. Moreover, the broadened ${}^7\mathrm{Li}$ spectrum also shows a characteristic three-peak structure. Comparison of linewidth between ${}^7\mathrm{Li}$ and ${}^{77}\mathrm{Se}$ spectra indicates that there is a ferromagnetic component along the external field direction and it happens on Fe sites in the (${\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}$)OH layer rather than the FeSe layer, which is also responsible for the three-peak structure of the ${}^7\mathrm{Li}$ spectrum with (${\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}$) random occupation. The field-dependent spectrum and ${\mathrm{T}}_2$ suggest that the above ferromagnetic component is induced by external magnetic field. Our present results indicate that the superconductivity in (${\mathrm{Li}}_{0.8}{\mathrm{Fe}}_{0.2}$)OHFeSe is very robust against the observed field-induced ferromagnetism and both of them could coexist under external magnetic field.

We present a joint experimental theoretical study of the coupling of coherent phonons in bulk GaAs with a nonequilibrium electron-hole plasma following photoexcitation at the $E_1$ gap by ultrafast laser pulses. In contrast to prior coherent phonon experiments where photoexcitation across the $E_0$ gap generated electrons in the $\Gamma$ valley, for the $E_1$ gap excitation, the majority of the electrons are generated in the satellite $L$ valleys. This leads to a drastically different situation from the previous studies because the damping of electrons is now faster due to the higher scattering rates in the $L$ valley, and, in addition, the diffusion of carriers has a significant effect on the plasma response due to the shorter optical absorption depth of the pump-probe light. Reflectivity measurements show coherent phonon-plasmon oscillations, whose frequencies lie between the transverse and longitudinal optical phonon frequencies due to the heavy damping and change with time due to the diffusion of the plasma. We analyze the experimental data with a theoretical model that describes the time and density-dependent coupling of the coherent phonon and the electron-hole plasma as the photoexcited carriers diffuse into the sample on a subpicosecond time scale. The calculated phonon-plasmon dynamics qualitatively reproduce the experimentally observed time-dependent frequency.

Epitaxial $p$-type transparent conducting oxide (TCO) $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_3:\mathrm{Mg}$ was grown by electron-beam evaporation in a molecular beam epitaxy system on $c$-plane sapphire. The influence of Mg dopants and the oxygen partial pressure were investigated by thermoelectric and electrical measurements. The conduction mechanism is analyzed using the small-polaron hopping model, and hopping activation energies have been determined, which vary with doping concentration in the range of 210–300 ± 5 meV. Films with better conductivity were obtained by postannealing. The effect of postannealing is discussed in terms of a crystallographic reordering of the Mg dopant. The highest Seebeck mobilities obtained from thermoelectric measurements are of the order of ${10}^{-4}{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. We investigate the fundamental properties of a Mg dopant in a high crystalline quality epitaxial film of a binary oxide, helping us understand the role of short range crystallographic order in a $p$-type TCO in detail.

We report on the optical characterization of single (Zn,Cd)Se quantum dots (QDs) embedded in vapor-liquid-solid-grown ZnSe nanowires (NWs). The temperature dependent quenching of the QD luminescence demonstrates that their electronic structure is comparable to that of self-assembled (Zn,Cd)Se QDs in ZnSe matrices. The photoluminescence excitation (PLE) spectrum of single nanowire QDs reveals the presence of both zinc blende (ZB) and wurtzite (WZ) crystal modifications of ZnSe in the NW shafts. PLE provides, therefore, a complementary technique to transmission electron microscopy imaging to reveal polytypism in ZnSe NWs. A transient quenching of the PL emission suggests a type II staggered band alignment at the ZB/WZ interface in our ZnSe NWs.

We consider a single-level quantum dot tunnel-coupled to one normal and one superconducting lead. We employ a diagrammatic real-time approach to calculate the finite-frequency current noise for subgap transport. The noise spectrum gives direct access to the internal dynamics of the dot. In particular, the noise spectrum shows sharp dips at the frequency of the coherent oscillations of Cooper pairs between the dot and the superconductor. This feature is most pronounced when the superconducting correlation is maximal. Furthermore, in the quantum-noise regime, $\omega >k_{\mathrm{B}}T,{\mu }_{\mathrm{N}}$, the noise spectrum exhibits steps at frequencies equal to the Andreev addition energies. The height of these steps is related to the effective coupling strength of the excitations. The finite-frequency noise spectrum hence provides a full spectroscopy of the system.

A new magnetic phase transition, which we assign to the separation of two different spiral phases, has been observed by ultrasound studies below the Néel temperature ($T_N=7.8$ K) in the frustrated antiferromagnet ${\mathrm{CdCr}}_2{\mathrm{O}}_4$. This transition renormalizes the velocity and amplitude of the transverse acoustic mode $c_T$ whereas the longitudinal mode $c_L$ is not affected. The specific heat does not show any significant change in the entropy at this transition. Furthermore, in an applied magnetic field, the mode $c_T$ exhibits extended metastable magnetostructural states neighboring the one-half magnetization plateau in ${\mathrm{CdCr}}_2{\mathrm{O}}_4$. By applying an exchange-striction model we can quantitatively describe the field dependence of the sound velocity below and above the one-half magnetization plateau.

We determine the phase diagram of copper nitrate $\text{Cu}{\left({\text{NO}}_3\right)}_2·2.5{\mathrm{D}}_2\text{O}$ in the context of quantum phase transitions and novel states of matter. We establish this compound as an ideal candidate to study quasi-1D Luttinger liquids, 3D Bose-Einstein-Condensation of triplons, and the crossover between 1D and 3D physics. Magnetocaloric effect, magnetization, and neutron scattering data provide clear evidence for transitions into a Luttinger liquid regime and a 3D long-range ordered phase as a function of field and temperature. Theoretical simulations of this model material allow us to fully establish the phase diagram and to discuss it in the context of dimerized spin systems.

Scanning tunneling microscopy and spectroscopy have been employed to investigate the superconductivity in single unit-cell FeSe nanoflakes on ${\mathrm{SrTiO}}_3$ substrates. We find that the differential conductance $dI/dV$ spectra are spatially nonuniform and fluctuate within the flakes as their area is reduced to below $\sim 150{\mathrm{nm}}^2$. An enhancement in the superconductivity-related gap size as large as $25\%$ is observed. The superconductivity behavior disappears when the FeSe nanoflakes reduce to $\sim 40{\mathrm{nm}}^2$. Compared to a previous report [Wang et al., Chin. Phys. Lett. 29, 037402 (2012)], the gap is asymmetric relative to the Fermi energy $E_{\mathrm{F}}$. All the features, particularly the fluctuating gap and quenched superconductivity, could be accounted for by quantum size effects. Our study helps to understand nanoscale superconductivity in low-dimensional systems.

The relationship between charge-density wave (CDW) orders and superconductivity in arsenide superconductor ${\text{SrPt}}_2{\text{As}}_2$ with $T_c=5.2\mathrm{K}$ which crystallizes in the ${\text{CaBe}}_2{\text{Ge}}_2$-type structure was studied by ${}^{75}\text{As}$ nuclear magnetic resonance (NMR) measurements up to 520 K, and ${}^{75}\text{As}$ nuclear quadrupole resonance (NQR) and ${}^{195}\mathrm{Pt}-\text{NMR}$ measurements down to 1.5 K. At high temperature, ${}^{75}\mathrm{As}-\text{NMR}$ spectrum and nuclear-spin-relaxation rate $(1/T_1)$ have revealed two distinct CDW orders, one realized in the As-Pt-As layer below $T_{\mathrm{CDW}}^{\mathrm{As}\left(1\right)}=410\mathrm{K}$ and the other in the Pt-As-Pt layer below $T_{\mathrm{CDW}}^{\mathrm{As}\left(2\right)}=255\mathrm{K}$. The $1/T_1$ measured by ${}^{75}\mathrm{As}-\text{NQR}$ shows a clear Hebel-Slichter peak just below $T_c$ and decreases exponentially well below $T_c$. Concomitantly, ${}^{195}\mathrm{Pt}$ Knight shift decreases below $T_c$. Our results indicate that superconductivity in ${\text{SrPt}}_2{\text{As}}_2$ is in the spin-singlet state with an $s$-wave gap and is robust under the two distinct CDW orders in different layers.

The recent discovery of superconductivity at 190 K in highly compressed ${\mathrm{H}}_2\mathrm{S}$ is spectacular not only because it sets a record high critical temperature, but because it does so in a material that appears to be, and we argue here that it is, a conventional strong-coupling BCS superconductor. Intriguingly, superconductivity in the observed pressure and temperature range was predicted theoretically in a similar compound, ${\mathrm{H}}_3\mathrm{S}.$ Several important questions about this remarkable result, however, are left unanswered: (1) Does the stoichiometry of the superconducting compound differ from the nominal composition, and could it be the predicted ${\mathrm{H}}_3\mathrm{S}$ compound? (2) Is the physical origin of the anomalously high critical temperature related only to the high H phonon frequencies, or does strong electron-ion coupling play a role? We show that at experimentally relevant pressures ${\mathrm{H}}_2\mathrm{S}$ is unstable, decomposing into ${\mathrm{H}}_3\mathrm{S}$ and S, and that ${\mathrm{H}}_3\mathrm{S}$ has a record high $T_c$ due to its covalent bonds driven metallic, which make this compound rather similar to ${\mathrm{MgB}}_2$, but unlike most other good conventional superconductors.