Perfect fluids are characterized as having the smallest ratio of shear viscosity to entropy density, $\eta /s$, consistent with quantum uncertainty and causality. So far, nearly perfect fluids have only been observed in the quark-gluon plasma and in unitary atomic Fermi gases, exotic systems that are amongst the hottest and coldest objects in the known universe, respectively. We use angle resolved photoemission spectroscopy to measure the temperature dependence of an electronic analog of $\eta /s$ in an optimally doped cuprate high-temperature superconductor, finding it too is a nearly perfect fluid around, and above, its superconducting transition temperature $T_c$.

True inorganic spin-Peierls materials are extremely rare, but ${\mathrm{NaTiSi}}_2{\mathrm{O}}_6$ was at one time considered to be an ideal candidate owing to its well separated chains of edge-sharing ${\mathrm{TiO}}_6$ octahedra. At low temperatures, this material undergoes a phase transition from $C2/c$ to $P\overline{1}$ symmetry, where ${\mathrm{Ti}}^{3+}\text{−}{\mathrm{Ti}}^{3+}$ dimers begin to form within the chains. However, it was quickly realized with magnetic susceptibility that simple spin fluctuations do not progress to the point of enabling such a transition. Since then, considerable experimental and theoretical endeavors have been undertaken to find the true ground state of this system and explain how it manifests. Here, we employ the use of x-ray diffraction, neutron spectroscopy, and magnetic susceptibility to directly and simultaneously measure the symmetry loss, spin singlet-triplet gap, and phonon modes. A gap of 53(3) meV was observed, fit to the magnetic susceptibility, and compared to previous theoretical models to unambiguously assign ${\mathrm{NaTiSi}}_2{\mathrm{O}}_6$ as having an orbital-assisted Peierls ground state.

A recently discovered magnetic semiconductor ${\mathrm{Ba}}_{1-\mathrm{x}}{\mathrm{K}}_{\mathrm{x}}{\left({\mathrm{Zn}}_{1-\mathrm{y}}{\mathrm{Mn}}_{\mathrm{y}}\right)}_2{\mathrm{As}}_2$, with its decoupled spin and charge doping, provides a unique opportunity to elucidate the microscopic origin of the magnetic interaction and ordering in dilute magnetic semiconductors (DMSs). We show that (i) the conventional density functional theory accurately describes this material, and (ii) the magnetic interaction emerges from the competition of the short-range superexchange and a longer-range interaction mediated by the itinerant As holes, coupled to Mn via the Schrieffer-Wolff $p\text{−}d$ interaction representing an effective Hund's rule coupling $J_H^{\text{eff}}$. The key difference between the classical double exchange and the actual interaction in DMSs is that an effective $J_H^{\text{eff}}$, as opposed to the standard Hund's coupling $J_H$, depends on the Mn $d$-band position with respect to the Fermi level, and thus allows tuning of the magnetic interactions. The physically transparent description of this material may also be applicable in more complicated DMS systems.

The structure of inverse opal crystals based on nickel was probed on the mesoscopic and atomic levels by a set of complementary techniques such as scanning electron microscopy and synchrotron microradian and wide-angle diffraction. The microradian diffraction revealed the mesoscopic-scale face-centered-cubic (fcc) ordering of spherical voids in the inverse opal-like structure with unit cell dimension of $750\pm 10\mathrm{nm}$. The diffuse scattering data were used to map defects in the fcc structure as a function of the number of layers in the Ni inverse opal-like structure. The average lateral size of mesoscopic domains is found to be independent of the number of layers. 3D reconstruction of the reciprocal space for the inverse opal crystals with different thickness provided an indirect study of original opal templates in a depth-resolved way. The microstructure and thermal response of the framework of the porous inverse opal crystal was examined using wide-angle powder x-ray diffraction. This artificial porous structure is built from nickel crystallites possessing stacking faults and dislocations peculiar for the nickel thin films.

The dynamic behavior of magnetic domain walls incident upon a $\mathsf{Y}$-shaped junction between three nanowires is explored. Details in the micromagnetic structure of the domain wall and the local spin texture at a vertex result in a complex interaction which ultimately determines the propagation direction of the domain wall following the vertex. This interaction has been explored through both magneto-optical Kerr effect experiments and micromagnetic simulations on single-vertex structures. Differences in the micromagnetic structure of incident domain walls in the dynamic regime have a significant influence on the interaction. We observe an oscillatory path selection with increasing domain wall propagation distance preceding the vertex. Our analysis shows that this behavior originates from cyclic reversals of the transverse domain wall chirality due to Walker breakdown processes.

We report an intriguing twofold spin reorientation transition of type ${\Gamma }_4\left(G_x,A_y,F_z\right)\to {\Gamma }_1\left(A_x,G_y,C_z\right)\to {\Gamma }_2\left(F_x,C_y,G_z\right)$ for the ${\mathrm{Fe}}^{3+}$ magnetic sublattice near spin reorientation temperatures $T_{\mathrm{SR}1}\sim 77 \mathrm{K}$ and $T_{\mathrm{SR}2}\sim 45 \mathrm{K}$ in a rare-earth orthoferrite ${\mathrm{Dy}}_{0.5}{\mathrm{Pr}}_{0.5}{\mathrm{FeO}}_3$ single crystal. Magnetic-field-induced incomplete spin-configuration transitions (${\Gamma }_4\to {\Gamma }_{41}\to {\Gamma }_{42}$ for $H =20\mathrm{kOe}$ and ${\Gamma }_4\to {\Gamma }_{42}$ for $H =40\mathrm{kOe}$) were observed by measurement of magnetization as a function of temperature. The spin reorientation temperature of a ${\mathrm{Dy}}_{0.5}{\mathrm{Pr}}_{0.5}{\mathrm{FeO}}_3$ single crystal can be controlled by changing the magnitude of the applied magnetic field. We also show that spin reorientation of the ${\Gamma }_1\to {\Gamma }_4$ type between $T_{\mathrm{SR}2}$ and $T_{\mathrm{SR}1}$ can be induced by an applied magnetic field along the $c$ axis. The origin of the intriguing magnetic behavior is ascribed to the anisotropic effective field whose strength is determined by the interactions with $R^{3+}$($R =\mathrm{Dy},\mathrm{Pr}$) spins and can be modified by the external applied magnetic field. It provides deeper insight into the ${\mathrm{Fe}}^{3+}-R^{3+}$ magnetic interaction which dominates the sophisticated magnetic phase transitions in the rare-earth orthoferrites.

A structural and magnetic investigation of ${\mathrm{NaFe}}_4{\mathrm{Sb}}_{12}$ with filled skutterudite structure is carried out by means of powder neutron diffraction and magnetization measurements in order to investigate the magnetic ground state of this compound. The existence of an ordered magnetic moment on the Fe site of $\approx 0.6{\mu }_B$/Fe atom is in good agreement with magnetization data. The ratio between the number of spins in the paramagnetic state and in the saturated ferromagnetic state is $\approx 1.7$, indicating some degree of delocalization of the Fe magnetic moment in ${\mathrm{NaFe}}_4{\mathrm{Sb}}_{12}$. Theoretical density-functional calculations of real-space spin-density maps are performed and compared to the experimental findings of weak ferromagnetism in ${\mathrm{NaFe}}_4{\mathrm{Sb}}_{12}$ ($T_C\approx 83$ K). Diffuse magnetic scattering with polarized neutrons revealed that a localized moment exists in the paramagnetic phase which decreases with increasing temperature. The results are discussed in the light of comparison with other intermetallic compounds.

Using nonresonant Fe $K\beta$ x-ray emission spectroscopy, we reveal that Sr substitution into ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ decouples the Fe moment from the volume collapse transition, yielding a collapsed-tetragonal, paramagnetic normal state out of which superconductivity develops. X-ray diffraction measurements implicate the $c$-axis lattice parameter as the controlling criterion for the Fe moment, promoting a generic description for the appearance of pressure-induced superconductivity in the alkaline-earth-based 122 ferropnictides (A${\mathrm{Fe}}_2{\mathrm{As}}_2)$. The evolution of $T_c$ with pressure lends support to theories for superconductivity involving unconventional pairing mediated by magnetic fluctuations.

In this work we revisit the vortex matter phase diagram in layered superconductors solving still open questions by means of ac and dc local magnetic measurements in the paradigmatic ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ compound. We show that measuring with ac magnetic techniques is mandatory in order to probe the bulk response of vortex matter, particularly at high temperatures where surface barriers for vortex entrance dominate. From the $T_{\mathrm{FOT}}$ evolution of the enthalpy and latent heat at the transition we find that, contrary to previous reports, the nature of the dominant interlayer coupling is electromagnetic in the whole temperature range. By studying the dynamic properties of the phase located at $T\gtrsim T_{\mathrm{FOT}}$, we reveal spanning in a considerable fraction of the phase diagram of a nonlinear vortex phase suggesting that bulk pinning might play a role even in the liquid vortex phase.

We show that finite temperature variational cluster approximation (VCA) calculations on an extended Falicov-Kimball model can reproduce angle-resolved photoemission spectroscopy (ARPES) results on ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ across a semiconductor-to-semiconductor structural phase transition at 325 K. We demonstrate that the characteristic temperature dependence of the flat-top valence band observed by ARPES is reproduced by the VCA calculation on the realistic model for an excitonic insulator only when the strong excitonic fluctuation is taken into account. The present calculations indicate that ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ falls in the Bose-Einstein condensation regime of the excitonic insulator state.

The surface structure of epitaxial $\beta \text{−}{\mathrm{FeSi}}_2$(100) thin film grown on Si(001) was analyzed using the quantitative low-energy electron diffraction intensity-voltage (LEED I-V) method, ab initio density functional theory (DFT) calculations, and scanning tunneling microscopy (STM). LEED patterns measured on the $\beta \text{−}{\mathrm{FeSi}}_2$(100) surface reveal two domains of a $p(2\times 2)$ reconstruction with $p2gg$ diffraction symmetry. The iron-silicide film truncation and atomic surface structure were determined by LEED I-V method: The smallest Pendry's reliability factor $R_P=0.22\pm 0.02$ was achieved for the bare $\beta \text{−}{\mathrm{FeSi}}_2$ film truncated by an Si layer, whereas Si and Fe ad-atom structures were excluded. Significant atomic relaxations within the topmost surface layers were revealed by the LEED I-V method and confirmed by DFT. The simulated STM patterns from the best-fit model agree well with the measured STM images on the $\beta \text{−}{\mathrm{FeSi}}_2\left(100\right)/\mathrm{Si}\left(001\right)\text{−}p(2\times 2)$ surface: Four Si atoms on a surface form one bright protrusion on STM patterns. Electronic band structure analysis of the bulk and epitaxial $\beta \text{−}{\mathrm{FeSi}}_2$(100) was carried out. A bare truncated epitaxial film was found to be metallic. Surface electronic states were identified by a partial $k$-resolved atomic-orbital based local density-of-state analysis.

We analyze the linewidth of intersubband absorption features observed in $n$-type $s\text{−}\mathrm{Ge}/{\mathrm{Ge}}_{0.82}{\mathrm{Si}}_{0.18}$ multiquantum wells grown on Si(001) substrates. Supported by a thorough theoretical modeling, we discuss the role on the line broadening of electron scattering due to interface roughness, spatial distribution and density of ionized donors, phonons, alloy disorder, and extended defects. To this aim, the absorption spectra of a set of samples featuring different well widths and doping profiles are measured obtaining a very good agreement with the theoretical results. We demonstrate values of the half width at half maximum of the $E_0\to E_1$ absorption peak as small as 2 meV. Our analysis suggests that the coherence time of electron wave functions in $n$-doped Ge/SiGe quantum wells is not yet limited by intrinsic scattering mechanisms up to room temperature, opening new perspectives for the quantum design of silicon-based light emitting devices.

We propose a scheme to perform single-shot quantum nondemolition (QND) readout of the spin of an electron trapped in a semiconductor quantum dot (QD). Our proposal relies on the interaction of the QD electron spin with optically excited, quantum well (QW) microcavity exciton-polaritons. The spin-dependent Coulomb exchange interaction between the QD electron and cavity polaritons causes the phase and intensity response of left circularly polarized light to be different than that of right circularly polarized light, in such a way that the QD electron's spin can be inferred from the response to a linearly polarized probe reflected or transmitted from the cavity. We show that with careful device design it is possible to essentially eliminate spin-flip Raman transitions. Thus a QND measurement of the QD electron spin can be performed within a few tens of nanoseconds with fidelity $\sim 99.95\%$. This improves upon current optical QD spin readout techniques across multiple metrics, including speed and scalability.

The observation of reaction outcomes at the bulk scale provides an average view of chemical processes, obscuring potentially significant differences in the behavior of matter at the atom scale. Through a series of atom-resolved scanning tunneling microscopy experiments, the inhomogeneous reactivity of silicon surface dangling bond states is revealed. The differences in reactivity provide evidence for the coexistence of neutral and negative surface states on these surfaces. It is shown that reactivity can be modulated through the density of surface states and by the bulk dopant level. These findings demonstrate that site-specific surface reactivity at the atom scale can be modulated by nonlocal charge allocation and provide opportunities for controlling chemistry at this scale.

We study the electronic contribution to the thermal conductivity and the thermopower of Weyl and Dirac semimetals using a semiclassical Boltzmann approach. We investigate the effect of various relaxation processes including disorder and interactions on the thermoelectric properties, and also consider doping away from the Weyl or Dirac point. We find that the thermal conductivity and thermopower have an interesting dependence on the chemical potential that is characteristic of the linear electronic dispersion, and that the electron-electron interactions modify the Lorenz number. For the interacting system, we also use the Kubo formalism to obtain the transport coefficients. We find exact agreement between the Kubo and Boltzmann approaches at high temperatures. We also consider the effect of electric and magnetic fields on the thermal conductivity in various orientations with respect to the temperature gradient. Notably, when the temperature gradient and magnetic field are parallel, we find a large contribution to the longitudinal thermal conductivity that is quadratic in the magnetic field strength, similar to the magnetic field dependence of the longitudinal electrical conductivity due to the presence of the chiral anomaly when no thermal gradient is present.

Au-induced atomic wires on the Ge(001) surface were recently claimed to be an ideal one-dimensional (1D) metal and their tunneling spectra were analyzed as the manifestation of a Tomonaga-Luttinger liquid (TLL) state. We reinvestigate this system for atomically well-ordered areas of the surface with high-resolution scanning tunneling microscopy and spectroscopy (STS). The local density-of-states maps do not provide any evidence of a metallic 1D electron channel along the wires. Moreover, the atomically resolved tunneling spectra near the Fermi energy are dominated by local density-of-states features, deviating qualitatively from the power-law behavior. On the other hand, the defects strongly affect the tunneling spectra near the Fermi level. These results do not support the possibility of a TLL state for this system. A 1D metallic system with well-defined 1D bands and without defects are required for the STS study of a TLL state.

In the context of fluctuation relations, we study the distribution of energy dissipated by a driven two-level system. Incorporating an energy counting field into the well-known spin-boson model enables us to calculate the distribution function of the amount of energy exchanged between the system and the bath. We also derive the conditional distribution functions of the energy exchanged with the bath for particular initial and/or final states of the two-level system. We confirm the symmetry of the conditional distribution function expected from the theory of fluctuation relations. We also find that the conditional distribution functions acquire considerable quantum corrections at times shorter or of the order of the dephasing time. Our findings can be tested using solid-state qubits.

Our first-principles simulations demonstrate the precise engineering of edge states by chemical decoration in topological Bi(111) bilayer nanoribbons. The chemical passivation removes the trivial edge states, recovers the Dirac linear dispersion of the topological edge state, and significantly influences both transport and optical signatures. Our transport simulations show that the Bi(111) bilayer nanoribbon offers an ideal system for assessing conductance fluctuation of edge states of a quantum spin Hall system by comparing the edge state transport with and without chemical decoration. The spatial extent and spin texture of edge states also become engineerable with the chemical decoration.