Using the quasiclassical formalism, we determine the low-temperature phase diagram of a quasi-one-dimensional superconductor, taking into account the interchain Josephson coupling and the paramagnetic spin splitting. We show that the anisotropy of the onset of superconductivity changes in the FFLO state as compared with the conventional superconducting phase. It can result in anomalous peaks in the field-direction dependence of the upper critical field when the magnetic field length equals to the FFLO period. This regime is characterized by the lock-in effect of the FFLO modulation wave vector, which is governed by the magnetic length. Furthermore, in the FFLO phase, the anisotropy of the upper critical field is inverted at $T_1^{**}=0.5T_{c0}$, where the orbital anisotropy disappears. We suggest that an experimental study of the anisotropy of the upper critical field can provide very reach information about the parameters of the FFLO phase in quasi-1D samples.

We study the electronic current through a quantum dot coupled to two superconducting leads which is driven by either a voltage $V$ or temperature $\Delta T$ bias. Finite biases beyond the linear response regime are considered. The local two-particle interaction $U$ on the dot is treated using an approximation scheme within the functional renormalization group approach set up in Keldysh-Nambu space with $U$ being the small parameter. For $V>0$, we compare our renormalization group enhanced results for the dc component of the current to earlier weak coupling approaches such as the Hartree-Fock approximation and second-order perturbation theory in $U$. We show that in parameter regimes in which finite-bias-driven multiple Andreev reflections prevail, small-$\left|U\right|$ approaches become unreliable for interactions of appreciable strength. In the complementary regime, the convergence of the current with respect to numerical parameters becomes an issue, but can eventually be achieved, and interaction effects turn out to be smaller than expected based on earlier results. For $\Delta T>0$, we find a surprising increase of the current as a function of the superconducting phase difference in the regime which at $T=0$ becomes the $\pi$ (doublet) phase.

Positive-muon (${\mu }^{+}$) Knight shifts have been measured in the paramagnetic states of ${\mathrm{Pr}}_{1-x}{\mathrm{Nd}}_x{\mathrm{Os}}_4{\mathrm{Sb}}_{12}$ alloys, where $x=0$, 0.25, 0.45, 0.50, 0.55, 0.75, and 1.00. In Pr-substituted ${\mathrm{NdOs}}_4{\mathrm{Sb}}_{12}$ ($x\le 0.75$), but not in ${\mathrm{NdOs}}_4{\mathrm{Sb}}_{12}$, Clogston-Jaccarino plots of ${\mu }^{+}$ Knight shift $K$ versus magnetic susceptibility $\chi$ exhibit an anomalous saturation of $K\left(\chi \right)$ at $\sim -0.5$% for large susceptibilities (low temperatures), indicating a reduction of the coupling strength between ${\mu }^{+}$ spins and $4f$ paramagnetism for temperatures $\lesssim 15$ K. We speculate that itinerant ${\mathrm{Pr}}^{3+}$ quadrupolar excitations, invoked to mediate the superconducting Cooper-pair interaction, might modify the ${\mu }^{+}$-$4f$ ion indirect spin-spin interaction.

In the field of topological insulators, the topological properties of quantum states in samples with simple geometries, such as a cylinder or a ribbon, have been classified and understood during the past decade. Here we extend these studies to a Möbius band and argue that its lack of orientability prevents a smooth global definition of parity-odd quantities such as pseudovectors. In particular, the Chern number, the topological invariant for the quantum Hall effect, lies in this class. The definition of spin on the Möbius band translates into the idea of the orientable double cover, an analogy used to explain the possibility of having the quantum spin Hall effect on the Möbius band. We also provide symmetry arguments to show the possible lattice structures and Hamiltonian terms for which topological states may exist in a Möbius band, and we locate our systems in the classification of topological states. Then, we propose a method to calculate Möbius dispersions from those of the cylinder, and we show the results for a honeycomb and a kagome Möbius band with different types of edge termination. Although the quantum spin Hall effect may occur in these systems when intrinsic spin-orbit coupling is present, the quantum Hall effect is more intricate and requires the presence of a domain wall in the sample. We propose an experimental setup which could allow for the realization of the elusive quantum Hall effect in a Möbius band.

We report on the observation of the random walk of solid-state silicon islands on ${\mathrm{SiO}}_2$ substrates during annealing at high temperatures. The mean square displacement (MSD) of the islands exhibits three regimes. At short times, the islands undergo equilibrium diffusion and begin to etch the surface thereby creating ringlike trenches. Then, an unusual size independent diffusionlike behavior is observed with a linear increase of the MSD. This behavior is attributed to a pinning instability of the triple line. Finally, as etching proceeds pits are formed in the substrate, and the MSD saturates as the islands are self-trapped in their own pits. Kinetic Monte Carlo simulations reproduce the main features of the three regimes, and provide a consistent picture of the microscopic reaction mechanisms at play in the experiments.

Effects of disorder on the electronic transport properties of graphene are strongly affected by the Dirac nature of the charge carriers in graphene. This is particularly pronounced near the Dirac point, where relativistic charge carriers cannot efficiently screen the impurity potential. We have studied time-dependent quantum conductance fluctuations in graphene in the close vicinity of the Dirac point at low temperatures. We show that the low-frequency noise arises from the quantum interference effects due to scattering on slowly fluctuating impurities. An unusually large reduction of the noise power in magnetic field suggests that an additional symmetry plays an important role in this regime.

We utilized the field ion microscope and density functional theory to investigate surface diffusion and surface adsorption of W on W(100). We demonstrated experimental proof for the occurrence of the exchange diffusion mechanism on W(100) and its coexistence with adatom jump. From our study it is evident that the primary mechanism of motion is atom exchange which is activated on the time scale of seconds at a temperature of around 650 K and is associated with an activation energy of 1.6 eV. Additionally, at a temperature around 700 K we observe a second surface diffusion mechanism with the activation energy estimated as ∼2.1 eV, which we associate with adatom jump. Our findings are in excellent agreement with DFT investigations. We have performed the adsorption-desorption experiments as a method helping with determining the morphology of a W(100) surface.

We study the formation of a coordination network consisting of the organic pyridine-based 2,4,6-tris(4-pyridine)-1,3,5-triazine (T4PT) species and Cu atoms on Cu(111) and Ag(111) metal surfaces. Using scanning tunneling microscopy, we find that the organic molecule T4PT forms stable two-dimensional porous networks on the surface of Cu(111) and, by codeposition of Cu atoms, also on the Ag(111) crystal, in which Cu atoms are twofold coordinated by T4PT molecules. X-ray absorption spectroscopy measurements of the metal-organic network Cu–T4PT on Ag(111) accompanied by density-functional theory calculations show that the nitrogen atoms of the pyridine end groups of the T4PT molecules are the active sites in coordinating the Cu adatoms. X-ray magnetic circular dichroism experiments reveal that the Cu atom in such a metal-organic motif is in a low-valent $d^{10}$ state and has no magnetic moment.

In this Rapid Communication we present the results from two high-quality, low-density GaAs quantum wells. In sample A of electron density $n$ = 5.0 × 10${}^{10}$ cm${}^{-2}$, anisotropic electronic transport behavior was observed at ν = 7/2 in the second Landau level. We believe that the anisotropy is due to the large Landau level mixing effect in this sample. In sample B of density 4.1 × 10${}^{10}$ cm${}^{-2}$, strong 8/3, 5/2, and 7/3 fractional quantum Hall states were observed. Furthermore, our energy gap data suggest that, similar to the 8/3 state, the 5/2 state may also be spin unpolarized in the low-density limit. The results from both samples show that the strong electron-electron interactions and a large Landau level mixing effect play an import role in the competing ground states in the second Landau level.

We derive an exact equation of motion for the reduced density matrices of individual subsystems of quantum many-body systems of any lattice dimension and arbitrary system size. Our projection operator based theory yields a highly efficient analytical and numerical approach. Besides its practical use it provides an interpretation and systematic extension of mean-field approaches and an adaption of open quantum systems theory to settings where a dynamically evolving environment has to be taken into account. We show its high accuracy for two significant classes of complex quantum many-body dynamics, unitary evolutions of nonequilibrium states in closed and stationary states in driven-dissipative systems.

Results of high-pressure infrared (IR) and Raman spectroscopy measurements are presented for the mixed valence compound ${\mathrm{CsAuI}}_3$, where Au adopts ${\mathrm{Au}}^{\mathrm{I}}$ and ${\mathrm{Au}}^{\mathrm{III}}$ valency. Raman spectroscopy shows softening with pressure of the vibration modes in the ${\mathrm{Au}}^{\mathrm{III}}$-${\mathrm{I}}_4$ square planar units in the tetragonal phase, indicating a similar pressure-induced lattice distortion as found for the closely related compounds ${\mathrm{CsAuCl}}_3$ and ${\mathrm{CsAuBr}}_3$. Multiple features in the higher pressure spectra confirm that the high-pressure phase has a lower symmetry than the ambient pressure tetragonal structure, consistent with an orthorhombic structure discovered recently by x-ray diffraction measurements. From IR spectroscopy, we observed rapid bandgap closure at a rate of 0.2 eV/GPa in the tetragonal phase of ${\mathrm{CsAuI}}_3$, close to the tetragonal-orthorhombic phase transition. The IR reflectivity shows a Drude-like behavior implying metallic conductivity. However, as the compound fully transforms to the orthorhombic phase, the bandgap reopens and the Drude behavior in the reflectivity disappears.

We report the electronic structure and optical absorption spectra of monolayer black phosphorus (phosphorene) nanoribbons (PNRs) via first-principles simulations. The band gap of PNRs is strongly enhanced by quantum confinement. However, differently orientated PNRs exhibit distinct scaling laws for the band gap vs the ribbon width $w$. The band gaps of armchair PNRs scale as $1/w^2$, while zigzag PNRs exhibit a $1/w$ behavior. These distinct scaling laws reflect a significant implication of the band dispersion of phosphorene: electrons and holes behave as nonrelativistic particles along the zigzag direction but resemble relativistic particles along the armchair direction. This unexpected merging of nonrelativistic and relativistic properties in a single material may produce novel electrical and magnetotransport properties of few-layer black phosphorus and its ribbon structures. Finally, the respective PNRs host electrons and holes with markedly different effective masses and optical absorption spectra, which are suitable for a wide range of applications.

The Dirac dispersion of graphene limits the phase space available for energy transport between electrons and acoustic phonons at temperatures above the Bloch-Grüneisen temperature. Consequently, energy relaxation can be dominated by supercollision events [Phys. Rev. Lett. 109, 106602 (2012)] also involving other scattering processes. Scattering from out-of-plane (flexural) phonons can compensate for the large momentum transfer involved in scattering from thermal acoustic phonons and enables similar supercollision events, such as disorder. Such multiphonon processes are also allowed by selection rules. I show that for the energy relaxation, the acoustic-flexular process can be of the same order of magnitude as direct flexural and acoustic phonon processes, depending on electronic screening and mechanical strain.

The optical properties of Si nanoribbons grown on Ag(110) under ultrahigh vacuum have been experimentally determined by use of in situ surface differential reflectance spectroscopy. Real-time measurements showed a clear transition of the optical response of the Si deposit at full coverage of the Ag(110) surface, corresponding to 0.8 monolayer of silicon. The spectra measured for the complete self-assembled nanoribbon layer are different from the reflectance spectrum calculated for a layer of silicene on silver, which rules out the possible silicenelike character of this layer. Furthermore, the dielectric function of the nanoribbons is calculated from the experimental data and is similar to the one of amorphous silicon, with a red shift of about 0.6 to 0.8 eV of the main absorption feature. This result indicates that the Si nanoribbons display a preferential $s{p}^3$ hybridization as in amorphous Si and not a partial $s{p}^2$ hybridization as expected for silicene.

We investigate how the electron transport properties of graphene quantum dot (GQD) arrays transition from two dimensions (2D) to one dimension (1D) in lithographically defined reduced graphene oxide nanoribbons (RGONRs). From the low-temperature electron transport measurements of 200-, 100-, and 50-nm-wide RGONRs, we find that the energy barrier for charge transport increases with decreasing RGONR width in both the Coulomb blockade and the variable-range hopping regime. Different charge transport parameters for 200-nm RGONR are in agreement with 2D transport while these parameters show a gradual transition to 1D transport in 50-nm RGONR.

In the range of incidence angles between 58${}^{\circ }$ and 79${}^{\circ }$, Si develops erosion patterns through room-temperature exposure to 2 keV ${\text{Kr}}^{+}$, while for other incidence angles it remains flat. We investigate the formation of these patterns through all in situ sample preparation and investigation under ultrahigh vacuum conditions. The ion fluence is varied by a factor of 1000 for the incidence angles of 63${}^{\circ }$ and 75${}^{\circ }$. The resulting morphologies are imaged by scanning tunneling microscopy and quantitatively analyzed in view of roughness, wavelength, disorder, and surface slopes. We find it necessary to distinguish between low-fluence and high-fluence regimes of pattern formation. While in both low-fluence regimes a similar parallel mode ripple pattern evolves, the high-fluence regimes are distinctly different and evidence either the evolution of disordered perpendicular ripples or a roof-tile structure for 63${}^{\circ }$ or 75${}^{\circ }$, respectively. The data are compared to other experimental data for ion beam erosion of Si and Ge enabling us to assess the universality of our observations. Comparison to existing models for the surface evolution under ion exposure allows us to draw conclusions on the applicability of these models for pattern formation on Si(001).

We have used x-ray diffraction to study the structure of strained, epitaxial ${\text{BiFeO}}_3$ (BFO) films, which exhibit ordered arrays of stripelike ferroelectric domains, in which the polarization vector P alternates by either 109${}^{\circ }$ or 71${}^{\circ }$. Diffraction maps exhibit an intricate satellite structure that arises from coherent, gratinglike diffraction from the domain structure. In the case of the 109${}^{\circ }$ arrays, the domain structure was found to exert a strain modulation on the ${\text{DyScO}}_3$ substrate, with the same periodicity, indicating that domains in BFO can have an influence on the substrate structure. In the case of the 71${}^{\circ }$ arrays, in which there is no contrast between neighboring domains and coherent scattering is not expected, weak scattering is nonetheless observed, which we interpret as evidence for previously unobserved, internal strains in these domain walls. To understand the x-ray data, we introduce a simple, single-scattering model that incorporates Gaussian disorder and fits the diffraction maps, providing domain periods and surface “puckering” angles that are in good agreement with atomic force microscopy and piezoresponse force microscopy measurements. Our study demonstrates a simple, computationally inexpensive technique for semiquantitatively interpreting coherent domain scattering in ferroelectric films, and suggests that tuning domain structures is a potential route to engineering the near-surface properties of perovskite oxides.

We report on calorimetry under applied hydrostatic pressure and magnetic field at the antiferromagnetic-ferromagnetic (AFM/FM) transition of ${\mathrm{Fe}}_{49}{\mathrm{Rh}}_{51}$. Results demonstrate the existence of a giant barocaloric effect in this alloy, a functional property that adds to the magnetocaloric and elastocaloric effects previously reported for this alloy. All caloric effects originate from the AFM/FM transition which encompasses changes in volume, magnetization, and entropy. The strong sensitivity of the transition temperatures to both hydrostatic pressure and magnetic field confers to this alloy outstanding values for the barocaloric and magnetocaloric strengths ($\left|\Delta S\right|$/$\Delta p$ $\sim$ 12 J kg${}^{-1}$K${}^{-1}$kbar${}^{-1}$ and $\left|\Delta S\right|$/${\mu }_0\Delta H$ $\sim$ 12 J kg${}^{-1}$K${}^{-1}$T${}^{-1}$). Both barocaloric and magnetocaloric effects have been found to be reproducible upon pressure and magnetic field cycling. Such a good reproducibility and the large caloric strengths make Fe-Rh alloys particularly appealing for solid-state cooling technologies at weak external stimuli.