We address the problem of persistent charge and spin currents on a Corbino disk built from a graphene sheet. We consistently derive the Hamiltonian including kinetic, intrinsic (ISO), and Rashba spin-orbit interactions in cylindrical coordinates. The Hamiltonian is carefully considered to reflect hermiticity and covariance. We compute the energy spectrum and the corresponding eigenfunctions separately for the intrinsic and Rashba spin-orbit interactions. In order to determine the charge persistent currents, we use the spectrum equilibrium linear response definition. We also determine the spin and pseudospin polarizations associated with such equilibrium currents. For the intrinsic case, one can also compute the correct currents by applying the bare velocity operator to the ISO wave functions or alternatively the ISO group velocity operator to the free wave functions. Charge currents for both SO couplings are maximal in the vicinity of half-integer flux quanta. Such maximal currents are protected from thermal effects because the contributing levels plunge ($\sim$1 K) into the Fermi sea at half-integer flux values. Such a mechanism, makes them observable at readily accessible temperatures. Spin currents only arise for the Rashba coupling, due to the spin symmetry of the ISO spectrum. For the Rashba coupling, spin currents are canceled at half-integer fluxes but they remain finite in the vicinity, and the same scenario as above protects spin currents.

Lithium cobalt dioxide (LiCoO${}_2$) belongs to a family of layered CoO${}_2$-based materials and has considerable interests in both fundamental physics and technological applications in lithium-ion batteries. We report the results of structural, electrochemical, magnetic susceptibility (χ), electron paramagnetic resonance (EPR), and muon-spin rotation and relaxation (μSR) measurements on powder Li${}_x$${}_0$CoO${}_2$ samples, where the nominal Li/Co ratios ($x_0$) were 0.95, 1.00, 1.02, 1.05, and 1.10, respectively. Structural, electrochemical, and χ measurements suggested that the sample with $x_0$ = 1.02 is very close to single stoichiometric LiCoO${}_2$ (ST-LCO) phase and that the Co ions in the $x_0$ = 1.02 sample are in a nonmagnetic low-spin state with $S$ = 0 ($t_{2g}^6$). However, both EPR and μSR revealed that the $x_0$ = 1.02 (ST-LCO) sample includes a large amount of nonordered magnetic phase in the temperature ($T$) range between 100 and 500 K. The volume fraction of such magnetic phase was found to be ∼45 vol% at 300 K by μSR, indicating an intrinsic bulk feature for ST-LCO. In fact, structural and photoelectron spectroscopic analyses clearly excluded the possibility that the nonordered magnetism is caused by impurities, defects, or surfaces. Because EPR and μSR sense static and dynamic nature of local magnetic environments, we concluded that Co spins in ST-LCO are fluctuating in the EPR and μSR time-windows. We also proposed possible origins of such nonordered magnetism, that is, a spin-state transition and charge disproportionation.

The interface between $\mathrm{FePt}-\mathrm{L}{1}_0$ and MgO(001) alloys has been studied using density-functional calculations. Because the stacking of the face-centered tetragonal L1${}_0$ phase is formed by alternating Fe and Pt planes, both the Fe- and Pt-terminated contact layers were studied. Furthermore, due to the large mismatch between the in-plane lattice constants of both systems, we have chosen some common $a$ values for both alloys in order to explore in detail the adsorption energy, the electronic structure, and the interface magnetism. The adsorption energy has been calculated by subtracting the energy of clean FePt and MgO alloys from the total energy. The preferred adsorbed geometric sites for Fe/Pt atoms are when they lie on top of the O species, having a smaller adsorption energy for the remaining positions. We found that expanding the MgO lattice enhances the magnetic moment of the Fe species but the Pt moments remain almost constant.

We report on the synthesis and the physical properties of Ba${}_{1-x}$Rb${}_x$Ti${}_2$Sb${}_2$O ($x\le 0.4$) by x-ray diffraction, superconducting quantum interference device magnetometry, resistivity, and specific-heat measurements. Upon hole doping by substituting Ba with Rb, we find superconductivity with a maximum $T_{\mathrm{c}}=5.4$ K. Simultaneously, the charge-density-wave transition temperature is strongly reduced from $T_{\text{CDW}}\approx 55$ K in the parent compound ${\mathrm{BaTi}}_2{\mathrm{Sb}}_2\mathrm{O}$ and seems to be suppressed for $x\ge 0.2$. The bulk character of the superconducting state for the optimally doped sample ($x=0.2$) is confirmed by the occurrence of a well developed discontinuity in the specific heat at $T_{\mathrm{c}}$, with $\Delta C/T_{\mathrm{c}}\approx 22$ mJ/mol K${}^2$, as well as a large Meissner-shielding fraction of $\approx$40%. The isotropically averaged lower and upper critical fields of the optimally doped sample ($x=0.2$) are estimated to ${\mu }_0{H}_{\mathrm{c},1}\left(0\right)\approx 3.8$ mT and ${\mu }_0{H}_{\mathrm{c},2}\left(0\right)\approx 2.3$ T, respectively, indicating that these compounds are strongly type-II superconductors.

We theoretically study the transition radiation of bulk and surface electromagnetic waves by an electron crossing an interface between a layered superconductor and an isotropic dielectric. We assume that the direction of the electron motion and the orientation of the superconducting layers are perpendicular to the interface. We derive the analytical expressions for the strongly anisotropic radiation fields and for the time-integrated energy fluxes of bulk and oblique surface electromagnetic waves (OSWs). We show that the OSWs with frequencies close to the Josephson plasma frequency ${\omega }_J$ provide the main contribution to the OSWs energy flux. Moreover, for frequencies close to the Josephson plasma frequency, the spectral density of the OSWs radiation diverges at some critical value of the azimuth angle $\phi$. At the angles $\phi =0$ and $\phi ={90}^{\mathrm{o}}$, the radiation field has a transverse magnetic polarization. We have also studied the Cherenkov radiation by the electron escaping from the layered superconductor and show that this radiation is almost monochromatic. A remarkable feature of the Cherenkov radiation in a layered superconductor is that, contrary to the isotropic case, the Cherenkov radiation distinctly manifests itself in the angular dependence of the radiation energy flux.

The random-field Ising model (RFIM) is one of the simplest statistical-mechanical models that captures the anomalous irreversible collective response seen in a wide range of physical, biological, or socioeconomic situations in the presence of interactions and intrinsic heterogeneity or disorder. When slowly driven at zero temperature, it can display an out-of-equilibrium phase transition associated with critical scaling (“crackling noise”), while it undergoes at equilibrium, under either temperature or disorder-strength changes, a thermodynamic phase transition. We show that the out-of-equilibrium and equilibrium critical behaviors are in the same universality class: they are controlled, in the renormalization-group (RG) sense, by the same zero-temperature fixed point. We do so by combining a field-theoretical formalism that accounts for the multiple metastable states and the exact (functional) RG. As a spin-off, we also demonstrate that critical fluids in disordered porous media are in the same universality class as the RFIM, thereby unifying a broad spectrum of equilibrium and out-of-equilibrium phenomena.

Amorphous solids, and many disordered lattices, exhibit a remarkable qualitative and quantitative universality in their acoustic properties at temperature $\lesssim 3$ K. This phenomenon is attributed to the existence of tunneling two-level systems (TTLSs), characterized by a homogeneous density of states (DOS) at energies much lower than the disorder energy ($\approx 0.1$ eV). Here we calculate numerically, from first principles, the DOS of KBr:CN glass, the archetypal disordered lattice showing universality. In contrast to the standard tunneling model, we find that the DOS diminishes abruptly at $\approx 3$ K, and that tunneling states differ essentially by their symmetry under inversion. This structure of the TTLSs dictates the low temperature below which universality is observed, and the quantitative universality of the acoustic properties in glasses. Consequences to the properties of glasses at intermediate temperatures, as well as to the microscopic structure of amorphous solids, are discussed.

We investigate experimentally and analytically the impact of thermal noise on the sustained gyrotropic mode of vortex magnetization in spin transfer nano-oscillators and its consequence on the linewidth broadening due to the different nonlinear contributions. Performing time domain measurements, we are able to extract separately the phase noise and the amplitude noise at room temperature for several values of dc current and perpendicular field. For a theoretical description, we extend the general model of nonlinear auto-oscillators to the case of vortex core dynamics and provide analytical expressions of the parameters describing the response to noise of the system. From the analysis of our experimental results, we demonstrate the major role of the amplitude-to-phase noise conversion on the linewidth broadening, and propose several solutions to increase even more the spectral coherence of vortex-based spin transfer nano-oscillators.

Rather generically, multicomponent superconductors and superfluids have intercomponent current-current interaction. We show that in superconductors with substantially strong intercomponent drag interaction, the topological defects which form in an external field are characterized by a skyrmionic topological charge. We then demonstrate that they can be distinguished from ordinary vortex matter by a very characteristic magnetization process due to the dipolar nature of inter-skyrmion forces. The results provide an experimental signature to confirm or rule out the formation $p$-wave state with reduced spin stiffness in $p$-wave superconductors.

The first four flexural vibrational modes of single-walled carbon nanotubes (SWCNTs) of various lengths under different axial strains were studied using atomistic molecular dynamics within the framework of the Brenner interatomic potential and Fourier analysis. The simulated results are in excellent agreement with the Timoshenko beam model, which includes the effect of both rotary inertia and of shearing deformation. From the crossing points of the simulation data with the expected resonance frequencies of the unstrained tubes an upper limit for the effective SWCNT thickness is found ($\le 0.1$ nm), with no adjustable parameters. This partially resolves Yakobson's paradox concerning scattered estimates for nanotube width.

Bulk Rashba systems $\mathrm{BiTe}X$ ($X=\text{I}$, Br, Cl) are emerging as important candidates for developing spintronics devices because of the coexistence of spin-split bulk and surface states, along with the ambipolar character of the surface charge carriers. The need to study the spin texture of strongly spin-orbit-coupled materials has recently promoted circular dichroic angular resolved photoelectron spectroscopy (CD-ARPES) as an indirect tool to measure the spin and the angular degrees of freedom. Here we report a detailed photon-energy-dependent study of the CD-ARPES spectra in $\mathrm{BiTe}X$ ($X=\text{I}$, Br, Cl). Our work reveals a large variation in the magnitude and sign of the dichroism. Interestingly, we find that the dichroic signal modulates differently for the three compounds and for the different spin-split states. These findings show a momentum and photon-energy dependence for the CD-ARPES signals in the bulk Rashba semiconductor $\mathrm{BiTe}X$ ($X=\text{I}$, Br, Cl). Finally, the outcome of our experiment indicates the important relation between the modulation of the dichroism and the phase differences between the wave functions involved in the photoemission process. This phase difference can be due to initial- or final-state effects. In the former case the phase difference results in possible interference effects among the photoelectrons emitted from different atomic layers and characterized by entangled spin-orbital polarized bands. In the latter case the phase difference results from the relative phases of the expansion of the final state in different outgoing partial waves.

We present a theory of electron-phonon interaction and phonon decay in Si nanocrystals based on $s{p}^3{d}^5{s}^{*}$ empirical tight-binding model and anharmonic Keating model. We demonstrate that the time of optical phonon emission by hot carriers in Si nanocrystal lies in the subpicosecond time range. However, due to the fast phonon recycling, the energy relaxation rate is determined not by the phonon emission but by the phonon decay rate. The decay rate of the optical phonon into two phonons of smaller energy is found to be in the 1–10 ps range. It is this anharmonic phonon decay process that may control the energy relaxation rate of excited carriers in Si nanocrystals.

We report a study of the atomic and electronic structures of the ordered ${\mathrm{Ag}}_2\mathrm{Ge}$ surface alloy containing ⅓ monolayer of Ge. Low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and angle-resolved photoelectron spectroscopy (ARPES) data reveal a symmetry breaking of the expected √3 × √3 periodicity, which is established for other ${\mathrm{Ag}}_2$$M$ alloys ($M$ = Bi, Sb, Pb, and Sn). The deviation from a simple √3 × √3 structure manifests itself as a splitting of diffraction spots in LEED, as a striped structure with a 6× periodicity including a distortion of the local hexagonal structure in STM, and as a complex surface band structure in ARPES that is quite different from those of the other ${\mathrm{Ag}}_2$$M$ alloys. These results are interesting in view of the differences in the atomic and electronic structures exhibited by different group IV elements interacting with Ag(111). Pb and Sn form √3 × √3 surface alloys on Ag(111), of which ${\mathrm{Ag}}_2\mathrm{Pb}$ shows a surface band structure with a clear spin-orbit split. Si and C form silicene and graphene structures, respectively, with linear band dispersions and the formation of Dirac cones as reported for graphene. The finding that ${\mathrm{Ag}}_2\mathrm{Ge}$ deviates from the ideal (√3 × √3) ${\mathrm{Ag}}_2\mathrm{Sn}$ and ${\mathrm{Ag}}_2\mathrm{Pb}$ surface alloys makes Ge an interesting “link” between the heavy group IV elements (Sn, Pb) and the light group IV elements (Si, C).

The magnetic and transport properties of the novel $R$${}_3$$T$${}_4$As${}_4$O${}_{2-\delta }$ ($R=$ La, Ce, Pr, Nd and Sm, $T=$ Ni and Cu) layered materials were studied using structural and physical properties measurements. Varying the rare-earth ion led to the observation of diverse physical properties including superconductivity for $R=$ La and $T=$ Ni, ferromagnetic or antiferromagnetic order for $R=$ Ce, Pr, and Sm, or spin-glass behavior in Nd${}_3$Ni${}_4$As${}_4$O${}_{2-\delta }$. These complex magnetic and electronic properties are discussed in light of the crystalline anisotropy in these layered compounds.

We show how the two-band nature of superconductivity in noncentrosymmetric compounds leads to a variety of different nonuniform superconducting states induced by a magnetic field. At low fields, a two-band helical state is realized, with a distinctly non-BCS quasiparticle spectrum. At high fields, the superconducting state becomes unstable towards the formation of a lattice of topological phase solitons.

Muon-spin rotation has been used to probe the vortex state in ${\mathrm{Sr}}_2\mathrm{Ru}{\mathrm{O}}_4$. At moderate fields and temperatures a lattice of triangular symmetry is observed, crossing over to a lattice of square symmetry with increasing field and temperature. At lower fields it is found that there are large regions of the sample that are completely free from vortices which grow in volume as the temperature falls. Importantly this is accompanied by increasing vortex density and increasing disorder within the vortex-cluster-containing regions. Both effects are expected to result from the strongly temperature-dependent long-range vortex attractive forces arising from the multiband chiral-order superconductivity.

$S$ = 1/2 quantum-spin systems of the kagome lattice have received intense recent attention because they are expected to be promising candidates for novel quantum states such as spin liquid, spin nematic, and other unknown exotic states. Here we report an unusual transition in a highly two-dimensional and slightly distorted $S$ = 1/2 quantum kagome system ${\mathrm{Cd}}_2{\mathrm{Cu}}_3{\mathrm{(OH)}}_6({\mathrm{SO}}_4){}_2{\mathrm{4H}}_2$O, which is a purified compound of the recently identified mineral edwardsite, Cd${}_{1.89}$Zn${}_{0.11}$${\mathrm{Cu}}_3{\mathrm{(OH)}}_6({\mathrm{SO}}_4){}_2{\mathrm{4H}}_2$O. The $S$ = 1/2 spins of Cu${}^{2+}$ ions form a highly two-dimensional kagome lattice with an extraordinarily large interlayer spacing >10 Å, thus providing a unique kagome lattice with minimum interlayer perturbation. The magnetization shows a Curie-Weiss temperature of 40.5 K and moment freezing below around $T_F$ = 5 K and a magnetic transition feature near 3 K. The muon spin relaxation shows static magnetism below 5 K and saturation below 3 K. However, distinct features have been observed distinguishing it from a spin glass or a conventional order: High-field magnetization until 50 T showed a saturated value of only 1/3 of the full expected moment. Analysis of specific heat suggests that only 1/3 of the spins are related to the magnetic order; on the other hand, the order is destroyed in external field and 1/3 of the spins respond to the field with a Schottky-like behavior while the other 2/3 of the spins remain inactive. These experimental results suggest an unconventional coexistence of the partial order and most probably spin singlets.

Motivated by interest in the elastic properties of high-strength amorphous metals, we examine the elastic properties of select crystalline phases. Using first-principles methods, we calculate elastic moduli in various chemical systems containing transition metals, specifically early (Ta,W) and late (Co,Ni). Theoretically predicted alloy elastic properties are verified for Ni-Ta by comparison with experimental measurements using resonant ultrasound spectroscopy. Comparison of our computed elastic moduli with effective medium theories shows that alloying leads to enhancement of bulk moduli relative to averages of the pure elements and considerable deviation of predicted and computed shear moduli. Specifically, we find an enhancement of bulk modulus relative to effective medium theory and propose a candidate system for high-strength, ductile amorphous alloys. Trends in the elastic properties of chemical systems are analyzed using force constants, electronic densities of state, and crystal overlap Hamilton populations. We interpret our findings in terms of the electronic structure of the alloys.