We have studied the BiCu${}_2$/Cu(111) surface alloy using low-temperature scanning tunneling microscopy and spectroscopy. We observed standing waves caused by scattering off defects and step edges. Different from previous studies on similar Rashba-type surfaces, we identified multiple scattering vectors that originate from various intraband as well as interband scattering processes. A detailed energy-dependent analysis of the standing-wave patterns enables a quantitative determination of band dispersions, including the Rashba splitting. The results are in good agreement with angle-resolved photoelectron spectroscopy (ARPES) data and demonstrate the usefulness of this strategy to determine the band structure of Rashba systems. The lack of other possible scattering channels will be discussed in terms of spin conservation and hybridization effects. The results open new possibilities to study spin-dependent scattering on complex spin-orbit coupled surfaces.

We study theoretically interaction between electrons in a quantum ring embedded in a microcavity and vacuum fluctuations of electromagnetic field in the cavity. It is shown that the vacuum fluctuations can split electron states of the ring with opposite angular momenta. As a consequence, the ground state of the electron system in the quantum ring can be associated with nonzero electric current. Since a ground-state current flows without dissipation, such a quantum ring gets a magnetic moment and can be treated as an artificial spin.

A major challenge in the search for multiferroic materials among transition metal compounds has been that ferroelectricity is primarily found in $d^0$ materials while magnetism is found in $d^n$ systems. Considering a well-known ferroelectric oxide, namely BaTiO${}_3$, the question we asked within a theoretical study was whether ferroelectric distortions disappeared for the slightest amount of doping. Surprisingly, in the case of V-doped BaTiO${}_3$, ferroelectricity was found to be stronger than in the undoped limit. Another surprise was that the presence of charged impurities rather than free carriers was found to be most detrimental to the presence of ferroelectric distortions. These ideas of the low doping limit were used to design alternative multiferroics.

We use scanning tunneling microscopy to map the surface structure, nanoscale electronic inhomogeneity, and vitreous vortex phase in the hole-doped superconductor Sr${}_{0.75}$K${}_{0.25}$Fe${}_2$As${}_2$ with $T_c=32$ K. We find that the low-$T$ cleaved surface is dominated by a half Sr/K termination with $1\times 2$ ordering and ubiquitous superconducting gap, while patches of gapless, unreconstructed As termination appear rarely. The superconducting gap varies by $\sigma /\overline{\Delta }=16\%$ on a $\sim$3 nm length scale, with average $2\overline{\Delta }/k_B{T}_c=3.6$ in the weak-coupling limit. The vortex core size provides a measure of the superconducting coherence length $\xi =2.3$ nm. We quantify the vortex lattice correlation length at 9 T in comparison to several iron-based superconductors. The comparison leads us to suggest the importance of dopant size mismatch as a cause of dopant clustering, electronic inhomogeneity, and strong vortex pinning.

Motivated by recent experiments on the possible supersolid behavior of ${}^4$He solids at low temperature, we consider a model of superfluidity in a defected solid containing a system spanning network of correlated linear dislocations, or planar grain boundaries. Using arguments based on the Harris criterion, as well as numerical simulations, we find that such correlated quenched disorder shifts the familiar superfluid lambda transition to a new disordered universality class in which the correlation length exponent $\nu \ge 1$. This results in the temperature derivatives for the superfluid density $d{\rho }_{\mathrm{s}}/dT$ and for the heat capacity $dc/dT$ remaining finite at the transition $T_{\mathrm{c}}$, and thus a less singular transition, profoundly different from the usual lambda transition.

It is argued on the basis of the BCS theory that the zero-$T$ penetration depth satisfies ${\lambda }^{-2}\left(0\right)\propto \sigma T_c$ ($\sigma$ is the normal state dc conductivity) not only in the extreme dirty limit ${\xi }_0/\ell \gg 1$, but in a broad range of scattering parameters down to ${\xi }_0/\ell \sim 1$ (${\xi }_0$ is the zero-$T$ BCS coherence length and $\ell$ is the mean free path). Hence, the scaling ${\lambda }^{-2}\left(0\right)\propto \sigma T_c$, suggested as a new universal property of superconductors [Dordevic, Basov, and Homes, Sci. Rep. 3, 1713 (2013)], finds a natural explanation within the BCS theory.

Magnetic fluctuations in the molecular-intercalated FeSe superconductor ${\mathrm{Li}}_x{\left({\mathrm{ND}}_2\right)}_y{\left({\mathrm{ND}}_3\right)}_{1-y}{\mathrm{Fe}}_2{\mathrm{Se}}_2$ ($T_{\mathrm{c}}=43\mathrm{K}$) have been measured by inelastic neutron scattering from a powder sample. The strongest magnetic scattering is observed at a wave vector $Q\approx 1.4$ Å${}^{-1}$, which is not consistent with the ($\pi ,0$) nesting wave vector that characterizes magnetic fluctuations in several other iron-based superconductors but is close to the ($\pi ,\pi /2$) position found for $A_x{\mathrm{Fe}}_{2-y}{\mathrm{Se}}_2$ systems. At the energies probed ($\sim 5k_{\mathrm{B}}{T}_{\mathrm{c}}$), the magnetic scattering increases in intensity with decreasing temperature below $T_{\mathrm{c}}$, consistent with the superconductivity-induced magnetic resonance found in other iron-based superconductors.

Based on micromagnetic theory, we have derived analytical expressions for the magnetic small-angle neutron scattering (SANS) cross section of a two-phase particle-matrix-type ferromagnet. The approach—valid close to magnetic saturation—provides access to several features of the spin structure, such as perturbing magnetic anisotropy and magnetostatic fields. Depending on the applied magnetic field and on the magnitude $H_p$ of the magnetic anisotropy field relative to the magnitude $\Delta M$ of the jump in the longitudinal magnetization at the particle-matrix interface, we observe a variety of angular anisotropies in the magnetic SANS cross section. In particular, the model explains the “clover-leaf”-shaped angular anisotropy, which was previously observed for several nanostructured magnetic materials, and provides access to the magnetic interaction parameters, such as the average exchange-stiffness constant. It is also shown that the ratio $H_p/\Delta M$ decisively determines the asymptotic power-law exponent and the range of spin-misalignment correlations.

Measurements of magnetic susceptibility, heat capacity, and thermal expansion are reported for single-crystalline CuSb${}_2$O${}_6$ in the temperature range $5<T<350$ K. The magnetic susceptibility exhibits a broad peak centered near 60 K that is typical of one-dimensional antiferromagnetic compounds. Long-range antiferromagnetic order at $T_N$ = 8.7 K is accompanied by an energy gap [$\Delta$ = 17.48(6) K]. This transition represents a crossover from one- to three-dimensional antiferromagnetic behavior. Both heat capacity and the thermal expansion coefficients exhibit distinct jumps at $T_N$, which are similar to those observed at the normal-superconducting phase transition in a superconductor. This behavior is quite unusual and is presumably associated with a spin-Peierls transition occurring as a result of three-dimensional phonons coupling with Jordan-Wigner-transformed fermions.

The SU(4) Heisenberg model can serve as a low-energy model of the Mott insulating state in materials where the spins and orbitals are highly symmetric or in systems of alkaline-earth atoms on an optical lattice. Recently, it has been argued that on the honeycomb lattice, the model exhibits a unique spin-orbital liquid phase with an algebraic decay of correlations [P. Corboz et al., Phys. Rev. X 2, 041013 (2012)]. Here we study the instability of the algebraic spin-orbital liquid toward the spontaneous formation of SU(4) singlet plaquettes (tetramerization). Using a variational Monte Carlo approach to evaluate the projected wave function of fermions with $\pi$-flux state, we find that the algebraic liquid is robust and that a small finite value of the next-nearest exchange is needed to induce tetramerization. We also studied the phase diagram of a model which interpolates between the nearest-neighbor Heisenberg model and a Hamiltonian for which the singlet-plaquette-product state is an exact ground state.

Here we show that spontaneous oxidation of MnO nanoparticles into MnO/Mn${}_3$O${}_4$ core/shell nanoparticles has the effect of a local pressure, decreasing the MnO cell parameter and increasing strain, resulting in the increase of the MnO antiferromagnetic/paramagnetic transition temperature $T_N$. These effects are more severe in smaller nanoparticles.

Magnetism of $A$-site-ordered perovskites, CaCu${}_3$Ti${}_4$O${}_{12}$, CaCu${}_3$Ge${}_4$O${}_{12}$, CaCu${}_3$Sn${}_4$O${}_{12}$, and CaCu${}_3$Zr${}_4$O${}_{12}$, is comprehensively studied by means of ab initio electronic structure calculations. The magnetic exchange coupling constants between Cu-site spins, $J_1$, $J_2$, and $J_3$, are estimated within an effective Heisenberg model, revealing relative importance of $J_3$ despite its long interaction length. The ground-state magnetic order is reasonably explained by the combination of (i) relatively weak ferromagnetic superexchange interaction ($J_1$) that works for all the oxides and (ii) antiferromagnetic long-range superexchange interaction ($J_3$) that works selectively for CaCu${}_3$Ti${}_4$O${}_{12}$ and CaCu${}_3$Zr${}_4$O${}_{12}$.

We use extensive series expansions at $T=0$ to investigate the phase diagram of a spin-1 Hamiltonian on the square lattice. The model includes bilinear and biquadratic interactions (the “$J$-$K$” model) and has been studied recently using a variety of other methods. We find a clear indication of the three-sublattice order conjectured recently by Toth et al. [Phys. Rev. B 85, 140403(R) (2012)]. We also compute the energy and order parameter in the quadrupolar phases.

We have completely determined the Fermi surface in KFe${}_2$As${}_2$ via de Haas–van Alphen (dHvA) measurements. Fundamental frequencies $\epsilon$, $\alpha$, $\zeta$, and $\beta$ are observed in KFe${}_2$As${}_2$. The first one is attributed to a hole cylinder near the $X$ point of the Brillouin zone while the others to hole cylinders at the $\Gamma$ point. We also observe magnetic breakdown frequencies between $\alpha$ and $\zeta$ and suggest a plausible explanation for them. The experimental frequencies show deviations from frequencies predicted by band-structure calculations. Large effective masses up to 19 $m_e$ for $B\parallel c$ have been found, $m_e$ being the free-electron mass. The carrier number and Sommerfeld coefficient of the specific heat are estimated to be 1.01–1.03 holes per formula unit and 82–94 mJmol${}^{-1}$ K${}^{-2}$, respectively, which are consistent with the chemical stoichiometry and a direct measure of 93 mJmol${}^{-1}$ K${}^{-2}$ [Fukazawa et al., J. Phys. Soc. Jpn. 80, SA118 (2011)]. The Sommerfeld coefficient is about nine times enhanced over a band value, suggesting the importance of low-energy spin and/or orbital fluctuations, and places KFe${}_2$As${}_2$ among strongly correlated metals. We have also performed dHvA measurements on Ba${}_{0.07}$K${}_{0.93}$Fe${}_2$As${}_2$ and have observed the $\alpha$ and $\beta$ frequencies.

We investigate exotic paired states of spin-imbalanced Fermi gases in anisotropic lattices, tuning the dimension between one and three. We calculate the finite temperature phase diagram of the system using real-space dynamical mean-field theory in combination with the quantum Monte Carlo method. We find that regardless of the intermediate dimensions examined, the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state survives to reach about one third of the BCS critical temperature of the spin-density balanced case. We show how the gapless nature of the state found is reflected in the local spectral function. While the FFLO state is found at a wide range of polarizations at low temperatures across the dimensional crossover, with increasing temperature we find out strongly dimensionality-dependent melting characteristics of shell structures related to harmonic confinement. Moreover, we show that intermediate dimension can help to stabilize an extremely uniform finite temperature FFLO state despite the presence of harmonic confinement.

The tendency to bifurcate can often be utilized to improve performance characteristics of amplifiers or even to build detectors. The Bloch oscillating transistor is such a device. Here, we show that bistable behavior can be approached by tuning the base current and that the critical value depends on the Josephson coupling energy $E_J$ of the device. We demonstrate current-gain enhancement for the device operating near the bifurcation point at small $E_J$. From our results for the current gains at various $E_J$, we determine the bifurcation threshold on the $E_J$-base current plane. The bifurcation threshold curve can be understood using the interplay of interband and intraband tunneling events.

The nature of orbital ordering and type of the Jahn-Teller (JT) distortion in the doped manganite La${}_{0.5}$Sr${}_{1.5}$MnO${}_4$ has been a topic of long standing debate, with both experiments and theory supporting opposite views. With the help of ab initio density functional and exact diagonalization studies we have investigated the energetics of the cooperative JT distortion and orbital ordering in this system. The density functional calculations yield a $x^2-1/y^2-1$ (rod-type) orbital order as well as a Jahn-Teller distortion where the octahedral axes on the plane are elongated along the chain direction and compressed perpendicular to it in the $ab$ plane (type I). The essential physics of the problem is captured by a single zigzag chain model, which was studied with a tight-binding Hamiltonian and exact diagonalization on a finite chain, which help unravel the reason for the rod-type orbital order. The main conclusions from the latter studies are that: (i) The Coulomb correlation effects are relatively unimportant due to the low electron density in the $e_g$ states, (ii) the kinetic energy term (hopping along the ferromagnetic zigzag chain) favors the rod-type orbital order, and (iii) a significant noncubic crystal field, proposed earlier and found here from our DFT calculations, helps stabilize the rod-type orbital order even further.

Bulk-sensitive hard x-ray photoemission spectroscopy (HAXPES) reveals for as-grown epitaxial films of half-metallic ferromagnetic CrO${}_2$(100) a pronounced screening feature in the Cr 2$p_{3/2}$ core level and an asymmetry in the O 1$s$ core level. This gives evidence of a finite, metal-type Fermi edge, which is surprisingly not observed in HAXPES. A spectral weight shift in HAXPES to below the Fermi energy is attributed to single-ion recoil effects due to high-energy photoelectrons. In conjunction with inverse PES the intrinsic correlated Mott-Hubbard-type electronic structure is unraveled, yielding an averaged Coulomb correlation energy $U_{\mathrm{av}}$ ≅ 3.2 eV.

The role of nuclear spin fluctuations in the dynamic polarization of nuclear spins by electrons is investigated in (In,Ga)As/GaAs quantum dots. The photoluminescence polarization under circularly polarized optical pumping in transverse magnetic fields (Hanle effect) is studied. A weak additional magnetic field parallel to the optical axis is used to control the efficiency of nuclear spin cooling and the sign of nuclear spin temperature. The shape of the Hanle curve is drastically modified when changing this control field, as observed earlier in bulk semiconductors and quantum wells. However, the standard nuclear spin cooling theory, operating with the mean nuclear magnetic field (Overhauser field), fails to describe the experimental Hanle curves in a certain range of control fields. This controversy is resolved by taking into account the nuclear spin fluctuations owed to the finite number of nuclei in the quantum dot. We propose a model considering cooling of the nuclear spin system by electron spins experiencing fast vector precession in the random Overhauser fields of nuclear spin fluctuations. The model allows us to accurately describe the measured Hanle curves and to evaluate the parameters of the electron-nuclear spin system of the studied quantum dots.

We report first-principles calculations that reveal atomic and electronic structures of interfaces consisting of Si(110) face and 3$C$-SiC (111) face in which the lattices match apparently but the atomic densities in both sides are different. We find two distinct interface structures in the total-energy difference of 4–5 meV per Å${}^2$ for both the Si-Si interface and the Si-C interface: The interface of sparse Si(110) and dense SiC(111) exhibits bistability. We find that the stable and metastable structures are associated with vacant channels along a particular direction which are stabilized by the existence of over-coordinated atoms (floating bonds). We reveal that the coexistence of the floating bonds and the dangling bonds (under-coordinated atoms) and then the electron transfer from the dangling- to the floating bonds are the microscopic mechanism of the stabilization of the interface. We also find that atomic scale undulation mainly confined in the Si region also exists to stabilize the interface. The calculated interface energies show that the Si-Si interface is energetically favorable compared with the Si-C interface. Calculated energy bands exhibit semiconducting characteristics for all the stable and metastable interfaces.