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We investigate the spin-Nernst effect in time-reversal invariant topological superconductors, and show that it provides a smoking-gun evidence for helical Cooper pairs. The spin-Nernst effect stems from asymmetric, in spin space, scattering of quasiparticles at nonmagnetic impurities, and generates a transverse spin current by the temperature gradient. Both the sign and the magnitude of the effect sensitively depend on the scattering phase shift at impurity sites. Therefore the spin-Nernst effect is uniquely suitable for identifying time-reversal invariant topological superconducting orders., 5 pages, 3 figures (Supplemental Material: 8 pages)

Optical properties of four model systems (Na-, K-, Rb- or Cs-doped quasi-two-dimensional X 1.95Al1.95Si2.05O8.00; X = Na, K, Rb, or Cs) used to study the metal-insulator transition (MIT) in a deformable lattice are investigated. The doping evolution of the optical absorption band(s) originating from small bipolarons show strong variations depending on the electron-lattice coupling strength [Formula: see text]. Despite the increasing number density of small (bi)polarons, the Na-system remains a stubborn (bi)polaronic insulator due to strong [Formula: see text], while the other three systems show closing of the respective mobility gaps giving way to conducting phases with differing properties. These interesting evolutions and dynamical properties are compared and discussed. We conjecture that the manifestation of anomalous electronic transport properties and Mooij correlations near the MIT or superconductor-insulator transition in systems with non-negligible electron-lattice coupling effects may be linked to the coexistence of competing polaronic phases and the dynamical intertwining of the deformable lattice and the random electronic potential.

We performed electron spin-resonance (ESR), magnetic, and optical absorption studies of a model system used to investigate the metal-insulator transition in a deformable lattice. The ESR and magnetic studies revealed that the small polarons have no correlation to the conductivity below or above the mobility edge. The conducting transition is marked by the appearance of an asymmetric ESR signal simultaneously with a significant suppression of the small bipolaron optical absorption band indicating that the extended states result from an abrupt dissociation of the small bipolarons. Immediately above the mobility edge competing phases of extended states, small polarons, and small bipolarons coexist. We propose a phase diagram for the charge and spin dynamics of an interacting gas of small (bi)polarons across the discontinuous conducting transition that has implications for understanding the long-standing conundrum of Mooij correlations.

Source differential phase contrast data showing the chiral soliton lattice dislocation magnetic configuration at low and high applied fields.

Understanding and controlling atomic motions in solid are essential in ionic materials, such as ion conductors, electrolyte and electrodes of batteries, and gas separators. Although photoinduced proton transport in solid materials has been demonstrated, photoinduced transport of heavier ions, such as the oxide ion, in solid materials remains entirely elusive. Herein, we apply ultrafast time-resolved electron diffraction to observe the large displacement of oxide ions in a strongly correlated material, EuBaCo2O5.39, under near-ultraviolet photoexcitation at room temperature. The mechanism of the photoinduced transport of oxide ions is revealed via the combination of ultrafast optical spectroscopy and density functional theory calculations. Our findings show that photoexcitation induces ionic motions in a ceramic crystal at room temperature through the charge transfer that strongly couples with large ionic displacement under the nonequilibrium condition.

We investigate charge densities around the vortex cores of an $s$-wave and a chiral $p$-wave superconductor (SC) in two dimensions within quasiclassical theory. We consider contributions of particle-hole asymmetry through gradient expansions in first order of a quasiclassical parameter using augmented quasiclassical theory. The chiral $p$-wave SC has two inequivalent vortices: One is charged and the other is uncharged on the basis of Bogoliubov-de Gennes (BdG) equation. We explain this qualitative difference in vortex charges also by the augmented quasiclassical theory. In addition, we find that much larger charge is induced for the charged vortex of the chiral $p$-wave SC compared with the vortex of the $s$-wave SC in the clean system. We also confirm this enhancement through quantitative comparison with results based on the BdG theory. We also study effects of Born-type impurities using self energy formalism and find that the larger charge density is rather suppressed in contrast to the $s$-wave's one. Furthermore we study the local density of states as another equilibrium property of particle-hole asymmetry.

We thoroughly study the zero temperature properties of monoaxial chiral spin systems under a tilted magnetic field. The magnetic phase diagram includes two kinds of continuous phase transitions and one discontinuous phase transition. We clarify the properties of the phase transition in terms of the helical wave picture and the particle (soliton) picture and also in terms of the interaction between solitons. The interacting-soliton picture properly describes most of the discontinuous phase transition. In addition, we investigate several instabilities of the modulated structures such as an isolated soliton and the surface modulation, since their instabilities should be important to the changes of the magnetic properties. For this purpose, we perform the analyses based on the energy landscape as well as the excitation spectrum; the former approach gives an intuitive interpretation. We clarify the mechanisms of the instabilities through these analyses and draw the stability lines of solitons in the magnetic phase diagram., Comment: 26 pages, 19 figures

We show that the stability (existence/absence) and interaction (repulsion/attraction) of chiral solitons in monoaxial chiral magnets can be varied by tilting the direction of magnetic field. We, thereby, elucidate that the condensation of attractive chiral solitons causes the discontinuous phase transition predicted by a mean field calculation. Furthermore we theoretically demonstrate that the metastable field-polarized-state destabilizes through the surface instability, which is equivalent to the vanishing surface barrier for penetration of the solitons. We experimentally measure the magnetoresistance (MR) of micrometer-sized samples in the tilted fields in demagnetization-free configuration. We corroborate the scenario that hysteresis in MR is a sign for existence of the solitons, through agreement between our theory and experiments., 6+6 pages. the title is changed

We performed impedance spectroscopy and optical investigations below the reported mobility edge of two models systems (${X}_{n}$/${X}_{1.95\ensuremath{-}\ensuremath{\delta}}{\mathrm{Al}}_{1.95}{\mathrm{Si}}_{2.05}{\mathrm{O}}_{8.00}$; $X=\mathrm{K}$ or Rb, $n$ is the doping density of guest atoms) used to explore the metal-insulator transition in a deformable lattice. The bulk ac conductivity and imaginary dielectric permittivity of insulating samples near the mobility edge in both systems show a power-law behavior and variable-range hopping (VRH) conduction that is indeed related to the proposed polaron relaxation and conduction. We show that polaron relaxation and VRH occur simultaneously above a particular density of an interacting gas of small (bi)polarons, which is commensurate with the electron-lattice coupling strength and the intersite electron transfer energy. Based on our experimental results, we propose a rich phase diagram for the charge dynamics in a deformable lattice from the perspective of a Wigner lattice of small (bi)polarons.

Chiral solitons in monoaxial chiral magnets undergo a surface barrier, which is analogous to the Bean-Livingston barrier in type-II superconductors. The authors argue here that the surface barrier is the main mechanism for large hysteresis observed in magnetoresistance (MR) measurements on micrometer-sized samples of CrNb${}_{3}$S${}_{6}$. In the MR hysteresis, a conspicuous jump is observed at the field ${H}_{\mathrm{b}}$ for decreasing fields. Experimental results for ${H}_{\mathrm{b}}\ensuremath{\sim}0.4{H}_{\mathrm{c}}$ (with thermodynamic critical field ${H}_{\mathrm{c}}$) and the theoretical result ${H}_{\mathrm{b}}/{H}_{\mathrm{c}}=4/{\ensuremath{\pi}}^{2}$ are in good agreement with each other. The surface barrier is regarded as a common property of systems (superconductors, chiral magnets, and chiral liquid crystals) that exhibit continuous transitions of the nucleation type.

We study the impurity effects on bound states in a vortex core of a topological s-wave superconductor (SC). A previous study has revealed numerically that the impurity effects in a vortex in the topological s-wave SC are not so simple as those in a chiral p-wave SC; The impurity scattering rates depend on the Rashba spin orbit and Zeeman coupling as well as the type of vortex. We analytically solve the Andreev equation and derive the scattering rates in order to understand effects of those two coupling strengths and the type of vortex.

We numerically investigate the electronic structures around a vortex core in a bilayer superconducting system, with s-wave pairing, Rashba spin-orbit coupling and Zeeman magnetic field, with use of the quasiclassical Green's function method. The Bardeen-Cooper-Schrieffer (BCS) phase and the so-called pair-density wave (PDW) phase appear in the temperature-magnetic-field phase diagram in a bulk uniform system [Phys. Rev. B 86, 134514 (2012)]. In the low magnetic field perpendicular to the layers, the zero-energy vortex bound states in the BCS phase are split by the Zeeman magnetic field. On the other hand, the PDW state appears in the high magnetic field, and sign of the order parameter is opposite between the layers. We find that the vortex core suddenly shrinks and the zero-energy bound states appear by increasing the magnetic field through the BCS-PDW transition. We discuss the origin of the change in vortex core structure between the BCS and PDW states by clarifying the relation between the vortex bound states and the bulk energy spectra. In the high magnetic field region, the PDW state and vortex bound states are protected by the spin-orbit coupling. These characteristic behaviors in the PDW state can be observed by scanning tunneling microscopy/spectroscopy., 11 pages, 7 figures

The charging effect of a superconducting vortex core is very important for transport properties of superconducting vortices. The chiral p-wave superconductor, known as a topological superconductor (SC), has a Majorana fermion in a vortex core and the charging effect has been studied using microscopic Bogoliubov-de Gennes (BdG) theory. According to calculations based on the BdG theory, one type of the vortex is charged as well as the vortex of the s-wave SC, while the other is uncharged. We reproduce this interesting charging effect using an augmented quasiclassical theory in chiral p-wave SCs, by which we can deal with particle-hole asymmetry in the quasiclassical approximation., Comment: 6 pages, 2 figures

We develop a scheme of Gor'kov Green's functions to treat impurity effects on Caroli-de Gennes-Matricon (CdGM) mode in superconductors (SCs) by improving the Kopnin-Kravtsov scheme with respect to the coherence factors and applicability to various SCs. We can study the impurity effects keeping the discreteness of the energy spectrum in contrast to the quasiclassical theory. We can thus apply this scheme to the SCs with the small quasiclassical parameter $k_{\mathrm{F}}\xi_{0}$ (which is the product of the Fermi wavenumber $k_{\mathrm{F}}$ and the coherence length $\xi_{0}$ in pure SC at zero temperature) and/or superclean regime $\Delta_{\mathrm{mini}} \tau \gg1$ ($\Delta_{\mathrm{mini}}$ and $\tau$ denote, respectively, the level spacing of the CdGM mode called minigap and the relaxation time for CdGM mode and we take $\hbar =1$). We investigate the impurity effects as a white noise for a vortex in an s-wave SC and two types of vortices in a chiral p-wave SC, for various values of the quasiclassical parameters and impurity strengths (from moderately clean regime to superclean regime), and confirm the validity of this scheme., Comment: 8 pages, 6 figures

Potassium metal was loaded into porous crystals of potassium-form maximum aluminum zeolite P (K-MAP), and optical absorption, electrical resistivity, and magnetic susceptibility measurements were performed. The average loading density of potassium atoms per GIS cage of zeolite P, n, was systematically changed up to 1.09. Optical absorption bands are observed around 1.3–2 eV and grow to dominate the spectrum with increasing n. These absorption bands are assigned to the optical excitations of s-electrons confined in the zigzag channels of GIS cages. The temperature dependence of electrical resistivity ρ indicates insulating properties for \(n \lesssim 1.05\). At n = 1.09, ρ suddenly decreases by several orders of magnitude, and shows metallic properties indicating that an insulator-to-metal transition occurs between n = 1.05 and 1.09. The samples are basically nonmagnetic at any n. The nonmagnetic and insulating phase at \(n \lesssim 1.05\) is explained by the formation of small bipolarons in the spin-singl...

We study the impurity effects on bound states in a single vortex core of a topological s-wave superconductor (SC), which is a two-dimensional electron gas with Rashba spin-orbit coupling, Zeeman coupling and s-wave superconductivity. We calculate the Gor'kov Green's function in the presence of impurities in a way similar to Kopnin and Kravtsov (1976); We restrict the Hilbert space to that spanned by the eigenstates of the Bogoliubov-deGennes equation for a pure system, bounded near the vortex core. This topological s-wave SC has two types of vortices and each type is specified by the relative sign between the vorticity and Zeeman coupling. We find that the impurities affect bound states differently between the two types of vortices and bound states are more robust against impurities when vorticity and Zeeman coupling are opposite in signs.

Modern nanofabrication techniques have opened the possibility to create novel functional materials, whose properties transcend those of their constituent elements. In particular, tuning the magnetostatic interactions in geometrically frustrated arrangements of nanoelements called artificial spin ice1, 2 can lead to specific collective behaviour3, including emergent magnetic monopoles4, 5, charge screening6, 7 and transport8, 9, as well as magnonic response10, 11, 12. Here, we demonstrate a spin-ice-based active material in which energy is converted into unidirectional dynamics. Using X-ray photoemission electron microscopy we show that the collective rotation of the average magnetization proceeds in a unique sense during thermal relaxation. Our simulations demonstrate that this emergent chiral behaviour is driven by the topology of the magnetostatic field at the edges of the nanomagnet array, resulting in an asymmetric energy landscape. In addition, a bias field can be used to modify the sense of rotation of the average magnetization. This opens the possibility of implementing a magnetic Brownian ratchet13, 14, which may find applications in novel nanoscale devices, such as magnetic nanomotors, actuators, sensors or memory cells.