We present an experimental study of the local and collective magnetism of ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ that is isostructural with the high-temperature superconductor parent compound ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. In contrast to ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, where only Fe spins order, ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ has an additional magnetic transition below 20 K due to the ordering of the ${\mathrm{Eu}}^{2+}$ spins $(J=7/2$, with $L=0$ and $S=7/2)$ in an $A$-type antiferromagnetic texture (ferromagnetic layers stacked antiferromagnetically). This may potentially affect the FeAs layer and its local and correlated magnetism. $\mathrm{Fe}K\beta$ x-ray emission experiments on ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ single crystals reveal a local magnetic moment of $1.3\pm 0.15{\mu }_B$ at 15 K that increases slightly to $1.45\pm 0.15{\mu }_B$ at 300 K. Resonant inelastic x-ray scattering experiments performed on the same crystals show dispersive broad (in energy) magnetic excitations along $(0,0)\to (1,0)$ and $(0,0)\to (1,1)$ with a bandwidth on the order of 170–180 meV. These results on local and collective magnetism are in line with other parent compounds of the ${\mathrm{AFe}}_2{\mathrm{As}}_2$ series $(A=\mathrm{Ba}$, Ca, and Sr), especially the well-characterized ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. Thus, our experiments lead us to the conclusion that the effect of the high magnetic moment of Eu on the magnitude of both the Fe local magnetic moment and spin excitations is small and confined to low-energy excitations.

Recent studies of core-level x-ray photoelectron spectroscopy (XPS) spectra of silicene on ${\mathrm{ZrB}}_2\left(0001\right)$ were found to be inconsistent with the density of states (DOS) of a planar-like structure that has been proposed as the ground state by density functional theory (DFT). To resolve the discrepancy, a reexamination of the XPS spectra and direct theoretical access of accurate single-particle excitation energies are desired. By analyzing the XPS data using symmetric Voigt functions, different binding energies and their sequence of Si $2p$ orbitals can be assigned from previously reported ones where asymmetric pseudo-Voigt functions are adopted. Theoretically, we have adopted an approach developed very recently, which follows the sophisticated $\mathrm{\Delta }$ self-consistent field $\left(\mathrm{\Delta }\mathrm{SCF}\right)$ methods, to study the single-particle excitation of core states. In the calculations, each single-particle energy and the renormalized core-hole charge density are calculated straightforwardly via two SCF calculations. By comparing the results, the theoretical core-level absolute binding energies including the splitting due to spin-orbit coupling are in good agreement with the observed high-resolution XPS spectra. The good agreement not only resolves the puzzling discrepancy between experiment and theory (DOS) but also advocates the success of DFT in describing many-body interactions of electrons at the surface.

We present experimental evidence of emergent density-wave instability in single-crystalline low-dimensional wires of yttrium-based pyrochlore iridates. We demonstrate electric-field-induced nonlinear hysteretic switching of the density wave at low temperature, followed by smooth nonlinear conduction at higher temperature $(T>40$ K) in ${\mathrm{Y}}_{2-x}{\mathrm{Bi}}_x{\mathrm{Ir}}_2{\mathrm{O}}_7$, with $x=0$ and 0.3. The ac transport measurements reveal the presence of four different collective relaxation processes which dominate at different temperature scales. There is a strong coupling of the normal charge carriers with the density-wave condensate, which is reflected in the linear scaling of the dc conductivity with the collective relaxation rate across a wide range of frequencies and temperatures. The evidence of the density wave in low-dimensional single crystals of pyrochlore iridate could be a precursor to the possible experimental confirmation of the Weyl semimetallic ground state with broken chiral symmetry.

Electrical currents at the surface or edge of a topological insulator are intrinsically spin polarized. We show that such surface or edge currents can be used to switch the orientation of a molecular magnet weakly coupled to the surface or edge of a topological insulator. For the edge of a two-dimensional topological insulator as well as for the surface of a three-dimensional topological insulator the application of a well-chosen surface or edge current can lead to a complete polarization of the molecule if the molecule's magnetic anisotropy axis is appropriately aligned with the current direction. For a generic orientation of the molecule a nonzero but incomplete polarization is obtained. We calculate the probability distribution of the magnetic states and the switching rates as a function of the applied current.

We present a theoretical study of the electronic transport through Pt nanocontacts. We show that the analysis of the tunneling regime requires a very careful treatment of the technical details. For instance, an insufficient size of the system can cause unphysical charge oscillations to arise along the transport direction; moreover, the use of an inappropriate basis set can deviate the distance dependence of the conductance from the expected exponential trend. While the conductance decay can be either corrected by employing ghost atoms or a large-cutoff-radius basis set, the same does not apply to the corrugation, for which only the second option is recommended. Interestingly, these details were not found to have a remarkable impact in the contact regime. These findings are important for theoretical studies of distance-dependent phenomena in scanning-probe and break-junction experiments.

Small but significant static and dynamic strain coupling effects have been detected in $\mathrm{C}{\mathrm{u}}_2\mathrm{OSe}{\mathrm{O}}_3$ through elastic and anelastic anomalies associated with magnetic phase transitions observed as a function of temperature (1.5–150 K) and magnetic field (0–300 mT). The magnetic transition near 60 K is accompanied by a small increase in single-crystal elastic constants which can be understood in terms of biquadratic coupling between shear strain and the magnetic order parameter, even though the shear strain itself is almost negligibly small. The conical-collinear transition is associated with distinct minima in the elastic properties, while weaker anomalies at lower fields may be related to changes in the configuration of magnetic domains. A distinctive acoustic loss peak at $\sim 42\text{K}$, independent of magnetic field, is attributed to freezing of a defect which is coupled with shear strain, has an associated activation energy of $\sim 5\text{kJ}\text{mo}{\mathrm{l}}^{-1}$, and may play a role in pinning the magnetic microstructures. Anomalies below $\sim 10\text{K}$ indicate the presence of some additional relaxation process which could signify a change in magnetic structure.

Novel magnetic phases are expected to occur in highly frustrated spin systems. Here, we study the structurally perfect kagome antiferromagnet $\mathrm{CdC}{\mathrm{u}}_3{\left(\mathrm{OH}\right)}_6{\left({\mathrm{NO}}_3\right)}_2·{\mathrm{H}}_2\mathrm{O}$ by magnetization, magnetic torque, and heat capacity measurements using single crystals. An antiferromagnetic order accompanied by a small spontaneous magnetization that surprisingly is confined in the kagome plane sets in at $T_{\mathrm{N}}\sim 4\mathrm{K}$, well below the nearest-neighbor exchange interaction $J/k_{\mathrm{B}}=45\mathrm{K}$. This suggests that a unique “$\mathbf{q}=0$” type ${120}^{\circ }$ spin structure with “negative” (downward) vector chirality, which breaks the underlying threefold rotational symmetry of the kagome lattice and thus allows a spin canting within the plane, is exceptionally realized in this compound rather than a common one with “positive” (upward) vector chirality. The origin is discussed in terms of the Dzyaloshinskii-Moriya interaction.

We uncover the previously inaccessible overdoped regime to attain the complete superconducting dome in a pristine high temperature cuprate superconductor, by applying pressures up to 280 kbar to single crystals near stoichiometric ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$. The obtained superconducting phase boundary as a function of hole doping closely follows the form of the superconducting dome in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. Measurements are now enabled to trace the evolution of various entangled phases and the Fermi surface from the underdoped to overdoped regime in a single high purity cuprate superconducting family of materials.

The development of plasmonic and metamaterial devices requires the research of high-performance materials alternative to standard noble metals. Renewed as a refractory stable compound for durable coatings, titanium nitride has recently been proposed as an efficient plasmonic material. Here, by using a first-principles approach, we investigate the plasmon dispersion relations of TiN bulk and we predict the effect of pressure on its optoelectronic properties. Our results explain the main features of TiN in the visible range and prove a universal scaling law which relates its mechanical and plasmonic properties as a function of pressure. Finally, we address the formation and stability of surface-plasmon polaritons at different TiN-dielectric interfaces proposed by recent experiments. The unusual combination of plasmonics and refractory features paves the way for the realization of plasmonic devices able to work at conditions not sustainable by the usual noble metals.

Magnetic, transport, and thermal properties of ${\mathrm{CeCu}}_2\mathrm{Mg}$ are investigated to elucidate the lack of magnetic order in this heavy-fermion compound with a specific heat value, $C_{\mathrm{mag}}{/T|}_{T\to 0}\approx 1.2$ J/mol ${\mathrm{K}}^2$ and robust effective magnetic moments (${\mu }_{\mathrm{eff}}\approx 2.46{\mu }_{\mathrm{B}}$). The lack of magnetic order is attributed to magnetic frustration favored by the hexagonal configuration of the Ce sublattice. In fact, the effect of magnetic field on $C_{\mathrm{mag}}/T$ and residual resistivity ${\rho }_0$ does not correspond to that of a Fermi liquid (FL) because a broad anomaly appears at $T_{\max }\approx 1.2$ K in $C_{\mathrm{mag}}\left(T\right)/T$, without changing its position up to ${\mu }_{0}H=7.5$ T. However, the flattening of $C_{\mathrm{mag}}{/T|}_{T\to 0}$ and its magnetic susceptibility ${\chi }_{T\to 0}$, together with the $T^2$ dependence of $\rho \left(T\right)$, reveal a FL behavior for $T\le 2$ K which is also supported by Wilson and Kadowaki-Woods ratios. The unusual coexistence of FL and frustration phenomena can be understood by placing paramagnetic ${\mathrm{CeCu}}_2\mathrm{Mg}$ in an intermediate section of a frustration-Kondo model. The entropy, $S_{\mathrm{mag}}$, reaches $0.87\mathrm{R}ln6$ at $T\simeq 100$ K, with a tendency to approach the expected value $S_{\mathrm{mag}}=\mathrm{R}ln6$ of the $J=5/2$ ground state of ${\mathrm{Ce}}^{3+}$.

We report an observation of phase coexistence near the metal-insulator transition (MIT) in a film of $\mathrm{NdNi}{\mathrm{O}}_3$ grown on crystalline substrate $\mathrm{LaAl}{\mathrm{O}}_3$. This was established through a combination of three techniques, namely, scanning tunneling spectroscopy, $1/f$ noise spectroscopy, and impedance spectroscopy experiments. The spatially resolved scanning tunneling spectroscopy showed that the two coexisting phases have different types of density of states (DOS) at the Fermi level. One phase showed a depleted DOS close to $E_{\mathrm{F}}$ with a small yet finite correlation gap, while the other coexisting phase showed a metal-like DOS that had no depletion. The existence of the phase separation leads to a jump in the resistance fluctuation (as seen through $1/f$ noise spectroscopy) at the transition, and, notably, the fluctuation becomes non-Gaussian when there is a phase separation even in the metallic phase. This was corroborated by the impedance spectroscopy, which showed a broad hump in capacitance at the transition region as a signature of the existence of two phases that have widely different electrical conductivities. The phase separation starts well within the metallic phase much above the transition temperature and makes the sample electronically “inhomogeneous” in nanoscopic scale close to the transition. We discuss certain scenarios that lead to such a phase separation in the general context of strongly correlated oxides.

One of the most straightforward ways to study thermal properties beyond linear response is to monitor the relaxation of an arbitrarily large left-right temperature gradient $T_L-T_R$. In one-dimensional systems which support ballistic thermal transport, the local energy currents $\langle j\left(t\right)\rangle$ acquire a nonzero value at long times, and it was recently investigated whether or not this steady state fulfills a simple additive relation $\langle j(t\to \infty )\rangle =f\left(T_L\right)-f\left(T_R\right)$ in integrable models. In this paper, we probe the nonequilibrium dynamics of the Hubbard chain using density matrix renormalization group (DMRG) numerics. We show that the above form provides an effective description of thermal transport in this model; violations are below the finite-time accuracy of the DMRG. As a second setup, we study how an initially equilibrated system radiates into different nonthermal states (such as the vacuum).

We study the half-filled extended Hubbard model on a two-dimensional square lattice using cluster dynamical mean-field theory on clusters of size 8–20. We show that the model exhibits metallic, Mott-insulating, and charge-ordered phases, and determine the location of the charge-ordering phase-transition line and the properties of the phases as a function of temperature, local interaction, and nearest-neighbor interaction. We find strong nonlocal correlations outside the charge-ordered phase and a pronounced screening effect in the vicinity of the phase transition, where nonlocal interactions push the system towards metallic behavior. In contrast, correlations in the charge-ordered phase are mostly local to the unit cell. Finally, we demonstrate how strong nonlocal electron-electron interactions can increase electron mobility by turning a charge-ordered insulator into a metal. We analyze finite-size effects and the convergence of our data to the thermodynamic limit. Control of all sources of errors allows us to assess the regime of applicability of simpler approximation schemes for systems with nonlocal interactions.

The indirect exchange interaction between magnetic impurities located in the bulk of a two-dimensional topological insulator decays exponentially with the distance. The indirect exchange interaction for magnetic impurities mediated by the helical states at the edge of the topological insulator demonstrates behavior which is typical for the Ruderman-Kittel-Kasuya-Yosida interaction in a one-dimensional metal. We have shown that interference between the bulk and the edge states in the two-dimensional topological insulator results in existence of an unusual contribution to the indirect exchange interaction which, on the one hand, decays exponentially with a distance at the length scale controlled by the Fermi energy of the edge states and, on the other hand, oscillates with distance along the helical edge with the period determined by the Fermi wavelength. We found that this interference contribution to the indirect exchange interaction becomes dominant for such configurations of two magnetic impurities that one of them is situated close to the helical edge whereas the other one is located far away in the bulk.

The possibility to inject spin currents in topological insulators (TIs) by ultrashort optical pulses has stimulated intense studies of their out-of-equilibrium electronic properties. However, a comprehensive description of the electronic relaxation dynamics has been elusive, so far. In order to reveal the role of the bulk and surface states in the microscopic scattering mechanisms, we have investigated, by means of time- and angle-resolved photoemission spectroscopy, a wide set of TIs. These have been selected in order to display different positions of the Fermi energy ($E_{\text{F}}$) within the bulk band structure. When three-dimensional bulk states lie at $E_{\text{F}}$, we observe a fast relaxation dynamics with a characteristic time scale of a few picoseconds (ps). On the contrary, a long lasting excited state is recorded when only the two-dimensional surface state crosses $E_{\text{F}}$. These findings suggest the important role played by spatial diffusion in the direction orthogonal to the surface in governing the relaxation mechanisms. We propose that this electron diffusive mechanism is driven by the optically induced temperature gradient that is at play only for electrons residing in bulk states.

Landau levels and shallow donor states in multiple GaAs/AlGaAs quantum wells (MQWs) are investigated by means of the cyclotron resonance at megagauss magnetic fields. Measurements of magneto-optical transitions were performed in pulsed fields up to 140 T and temperatures from 6–300 K. The ${\displaystyle 14\times 14}$ P${\displaystyle ·}$p band model for GaAs is used to interpret free-electron transitions in a magnetic field. Temperature behavior of the observed resonant structure indicates, in addition to the free-electron Landau states, contributions of magnetodonor states in the GaAs wells and possibly in the AlGaAs barriers. The magnetodonor energies are calculated using a variational procedure suitable for high magnetic fields and accounting for conduction band nonparabolicity in GaAs. It is shown that the above states, including their spin splitting, allow one to interpret the observed magneto-optical transitions in MQWs in the middle infrared region. Our experimental and theoretical results at very high magnetic fields are consistent with the picture used previously for GaAs/AlGaAs MQWs at lower magnetic fields.

We present a detailed theoretical analysis of the Wigner crystal states in confined semiconducting carbon nanotubes. We show by detailed semimicroscopic calculations that the effective exchange interaction has an SU(4) symmetry, and can reach values as large as $J\sim 100\mathrm{K}$ in weakly screened, small diameter nanotubes, close to the Wigner crystal—electron liquid crossover. This large value of the exchange coupling in the crossover region also follows from robust scaling arguments. Modeling the nanotube carefully and analyzing the magnetic structure of the inhomogeneous electron crystal, we recover the experimentally observed “phase boundaries” of Deshpande and Bockrath [V. V. Deshpande and M. Bockrath, Nat. Phys. 4, 314 (2008)]. Spin-orbit coupling only slightly modifies these boundaries, but breaks the spin symmetry down to $\mathrm{SU}\left(2\right)\times \mathrm{SU}\left(2\right)$, and in Wigner molecules it gives rise to interesting excitation spectra, reflecting the underlying SU(4) as well as the residual $\mathrm{SU}\left(2\right)\times \mathrm{SU}\left(2\right)$ symmetries.