Magnetic impurities embedded in a metal interact via an effective Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling mediated by the conduction electrons, which is commonly assumed to be long ranged, with an algebraic decay in the interimpurity distance. However, they can also form a Kondo screened state that is oblivious to the presence of other impurities. We study the competition mechanisms between both effects on the square and cubic lattices by introducing an exact mapping onto an effective one-dimensional problem that we can solve with the density matrix renormalization group method. We show a dramatic departure from the conventional RKKY theory, that can be attributed to the dimensionality and different densities of states, as well as the quantum nature of the magnetic moments. In particular, for dimension $d>1$, Kondo physics dominates even at short distances, while the ferromagnetic RKKY state is energetically unfavorable. Our findings can have clear implications in the interpretation of experiments and for tailoring the magnetic properties of surfaces.

Using a graded heterobarrier placed along an electron channel, phonons emitted in Joule heating are recycled in situ by increasing the entropy of phonon-absorbing electrons. The asymmetric electric potential distribution created by alloy grading separates the phonon absorption and emission regions, and emission in the larger effective-mass region causes momentum relaxation with smaller electron kinetic energy loss. These lead to smaller overall phonon emission and simultaneous potential-gain and self-cooling effects. Larger potential is gained with lower current and higher optical-phonon temperature. The self-consistent Monte Carlo simulations complying with the lateral momentum conservation combined with the entropy analysis are applied to a GaAs:Al electron channel with a graded heterobarrier, and under ideal lateral thermal isolation from surroundings, the phonon recycling efficiency reaches 25% of the reversible limit at 350 K, and it increases with temperature. The lateral momentum contributes to the transmission across the barrier, so partially nonconserving lateral momentum electron scattering (rough interface) can improve efficiency.

Motivated by the observation that electrons in graphene, in the hydrodynamic regime of transport, can be treated as a two-dimensional ultrarelativistic gas with very low shear viscosity, we examine the existence of the Rayleigh-Bénard instability in a massless electron-hole plasma. First, we perform a linear stability analysis, derive the leading contributions to the relativistic Rayleigh number, and calculate the critical value above which the instability develops. By replacing typical values for graphene, such as thermal conductivity, shear viscosity, temperature, and sample sizes, we find that the instability might be experimentally observed in the near future. Additionally, we have performed simulations for vanishing reduced chemical potential and compare the measured critical Rayleigh number with the theoretical prediction, finding good agreement.

Understanding how topologically protected surface states behave at surfaces and interfaces requires knowledge of the atomic structure. Whether the (111) surfaces of the prototypical topological insulators ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ are Bi or chalcogen terminated is the subject of current controversies. We employ photoelectron diffraction and holography, combining the advantages and avoiding the disadvantages of the contesting techniques previously used. We find bulklike chalcogen termination with a very small surface relaxation $\left(<1\%\right)$ in agreement with density functional theory simulations. We prove the chalcogen termination for cleaved crystals and epitaxial films which shows the robustness of our conclusions.

A metastable phase of body-centered-tetragonal ruthenium (bct Ru) is identified to exhibit a large perpendicular magnetocrystalline anisotropy (PMCA), whose energy $E_{\text{MCA}}$ is as large as $150 \mu \mathrm{eV}/\mathrm{atom}$, which is two orders of magnitude greater than those of $3d$ magnetic metals. Further investigation over the range of tetragonal distortion suggests that the appearance of magnetism in the bct Ru is governed by the Jahn-Teller spit $e_g$ orbitals. Moreover, from band analysis, MCA is mainly determined by an interplay between two $e_g$ states, $d_{x^2-y^2}$ and $d_{z^2}$ states, as a result of level reversal associated with tetragonal distortion.

The impact of antisite disorder $x$ on the intrinsic Gilbert damping ${\alpha }_0$ in well-ordered $L{1}_0$ FePt films is investigated by time-resolved magneto-optical Kerr effect. The variation of $x$ mainly affects the electron scattering rate $1/{\tau }_{\mathrm{e}}$, while other leading parameters remain unchanged. The experimentally observed linear dependence of ${\alpha }_0$ on $1/{\tau }_{\mathrm{e}}$ indicates that spin relaxation is through electron interband transitions, as predicted by the spin-orbit coupling torque correlation model. Measurements at low temperature show that ${\alpha }_0$ remains unchanged with temperature even for FePt with very high chemical order, indicating that electron-phonon scattering is negligible. Moreover, as $x$ decreases, the perpendicular magnetic anisotropy increases, and the Landau $g$ factor exhibits a negative shift due to an increase in orbital momentum anisotropy. Our results will facilitate the design and exploration of magnetic alloys with large magnetic anisotropy and desirable damping properties.

Using dc and ac magnetometry, the pressure dependence of the magnetization of the three-dimensional antiferromagnetic coordination polymer $\text{Mn}{\left(\text{N}{\left(\text{CN}\right)}_2\right)}_2$ was studied up to 12 kbar and down to 8 K. The antiferromagnetic transition temperature, $T_{\mathrm{N}}$, increases dramatically with applied pressure $\left(P\right)$, where a change from $T_{\mathrm{N}}(P=\text{ambient})=16.0\mathrm{K}$ to $T_{\mathrm{N}}(P=12.1\text{kbar})=23.5\mathrm{K}$ was observed. In addition, a marked difference in the magnetic behavior is observed above and below 7.1 kbar. Specifically, for $P<7.1$ kbar, the differences between the field-cooled and zero-field-cooled magnetizations, the coercive field, and the remanent magnetization decrease with increasing pressure. However, for $P>7.1$ kbar, the behavior is inverted. Additionally, for $P>8.6$ kbar, minor hysteresis loops are observed. All of these effects are evidence of the increase of the superexchange interaction and the appearance of an enhanced exchange anisotropy with applied pressure.

The crystal structure, superconducting properties, and electronic structure of the novel superconducting 122-type antimonide ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$ have been studied by measurements of powder x-ray diffraction patterns, electrical resistivity, ac magnetic susceptibility, specific heat, and by ab initio calculations. The material crystallizes in a new monoclinic variant of the ${\mathrm{CaBe}}_2{\mathrm{Ge}}_2$-type structure, in which ${\mathrm{Pt}}_2{\mathrm{Sb}}_2$ layers that consist of ${\mathrm{PtSb}}_4$ tetrahedra, and ${\mathrm{Sb}}_2{\mathrm{Pt}}_2$ layers that consist of ${\mathrm{SbPt}}_4$ tetrahedra, are stacked alternatively, with Ba atoms located between them. Measurements of electrical resistivity, ac magnetic susceptibility, and specific heat revealed that ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$ is a superconducting material with a critical temperature of 1.8 K. The electronic heat capacity coefficient and the Debye temperature were $8.6\left(2\right)\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$ and 146(4) K, respectively. The upper critical field and the Ginzburg-Landau coherent length were determined to be 0.27 T and 35 nm, respectively. The calculations showed that the material has two three-dimensional Fermi surfaces (FSs) and two two-dimensional FSs, leading to anisotropic transport properties. The $d$ states of the Pt atoms in the ${\mathrm{Pt}}_2{\mathrm{Sb}}_2$ layers are the main contributors to the density of states at the Fermi level. A comparison between experimental and calculated results indicates that ${\mathrm{BaPt}}_2{\mathrm{Sb}}_2$ is a superconducting material with moderate coupling.

The theory of current transport in a narrow superconducting strip is revisited taking the effect of thermal fluctuations into account. The value of voltage drop across the sample is found as a function of temperature (close to the transition temperature, $T-T_{\mathrm{c}}$ $\ll T_{\mathrm{c}}$) and bias current $J<J_{\mathrm{c}}$ ($J_{\mathrm{c}}$ is the critical current calculated in the framework of the BCS approximation, neglecting thermal fluctuations). It is shown that careful analysis of vortices crossing the strip results in considerable increase of the activation energy.

We realize a device allowing for tunable and switchable coupling between two frequency-degenerate superconducting resonators mediated by an artificial atom. For the latter, we utilize a persistent current flux qubit. We characterize the tunable and switchable coupling in the frequency and time domains and find that the coupling between the relevant modes can be varied in a controlled way. Specifically, the coupling can be tuned by adjusting the flux through the qubit loop or by controlling the qubit population via a microwave drive. Our measurements allow us to find parameter regimes for optimal coupler performance and quantify the tunability range.

We have carried out in-plane resistivity measurements under a uniaxial pressure on ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ single crystals. A clear distinction of the in-plane resistivity ${\rho }_a$ and ${\rho }_b$ with the uniaxial pressure along the $b$ axis was discovered in the parent and underdoped regime with a doping level of up to about $x=0.025\pm 0.002.$ From the deviating point of ${\rho }_a$ and ${\rho }_b$, and the unique kinky structure of resistivity, together with the published data, we determined the temperatures for the nematic, structural, and antiferromagnetic transitions. It is clearly shown that the nematic electronic state vanishes simultaneously with the structural transition. The antiferromagnetic state disappears, however, at a lower doping level. Our results, in combination with the data on ${\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2\mathrm{As}}_2$, indicate a close relationship between nematicity and superconductivity.

We argue that surface spin and thermal conductivities of three-dimensional topological superconductors are universal and topologically quantized at low temperature. For a bulk winding number $\nu$, there are $\left|\nu \right|$ “colors” of surface Majorana fermions. Localization corrections to surface transport coefficients vanish due to time-reversal symmetry (TRS). We argue that Altshuler-Aronov interaction corrections vanish because TRS forbids color or spin Friedel oscillations. We confirm this within a perturbative expansion in the interactions, and to lowest order in a large-$\left|\nu \right|$ expansion. In both cases, we employ an asymptotically exact treatment of quenched disorder effects that exploits the chiral character unique to two-dimensional, time-reversal-invariant Majorana surface states.

Using the infinite density matrix renormalization group method on an infinite cylinder geometry, we characterize the $1/3$ fractional Chern insulator state in the Haldane honeycomb lattice model at $\nu =1/3$ filling of the lowest band and check its stability. We investigate the chiral and topological properties of this state through (i) its Hall conductivity, (ii) the topological entanglement entropy, (iii) the $U\left(1\right)$ charge spectral flow of the many-body entanglement spectrum, and (iv) the charge of the anyons. In contrast to numerical methods restricted to small finite sizes, the infinite cylinder geometry allows us to access and characterize directly the metal to fractional Chern insulator transition. We find indications it is first order and no evidence of other competing phases. Since our approach does not rely on any band or subspace projection, we are able to prove the stability of the fractional state in the presence of interactions exceeding the band gap, as has been suggested in the literature. As a by-product we discuss the signatures of Chern insulators within this technique.

We show that electrons in ordinary III–V semiconductor double wells with an in-plane modulating periodic potential and interwell spin-orbit interaction are tunable topological insulators (TIs). Here the essential TI ingredients, namely, band inversion and the opening of an overall bulk gap in the spectrum arise, respectively, from (i) the combined effect of the double-well even-odd state splitting ${\Delta }_{\mathrm{SAS}}$ together with the superlattice potential and (ii) the interband Rashba spin-orbit coupling $\eta$. We corroborate our exact diagonalization results with an analytical nearly-free-electron description that allows us to derive an effective Bernevig-Hughes-Zhang model. Interestingly, the gate-tunable mass gap $M$ drives a topological phase transition featuring a discontinuous Chern number at ${\Delta }_{\mathrm{SAS}}\sim 5.4\text{meV}$. Finally, we explicitly verify the bulk-edge correspondence by considering a strip configuration and determining not only the bulk bands in the nontopological and topological phases but also the edge states and their Dirac-like spectrum in the topological phase. The edge electronic densities exhibit peculiar spatial oscillations as they decay away into the bulk. For concreteness, we present our results for InAs-based wells with realistic parameters.

We investigate the influence of potassium adsorption and selenium vacancies in the surface layer on the electronic properties of the prototypical topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. These modifications of the surface give rise to oscillations in the charge density that extend deep into the crystal. They result in a long-ranged potential perpendicular to the surface (also referred to as band bending) and new states in the band structure that are reminiscent of the states of a two-dimensional electron gas. Very similar effects have been observed in several experiments. The reorganization of the charge deep inside the crystal as a reaction to the surface modification constitutes a remarkable property of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and is closely related to its layered structure. The emergence of the long-ranged potential as a direct consequence of the charge reorganization turns out to be a generic property of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. However, calculations without spin-orbit coupling show that the band bending is not related to the nontrivial topological character of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$.

By means of electronic transport, we study the transverse magnetic anisotropy of an individual ${\mathrm{Fe}}_4$ single-molecule magnet (SMM) embedded in a three-terminal junction. In particular, we determine in situ the transverse anisotropy of the molecule from the pronounced intensity modulations of the linear conductance, which are observed as a function of applied magnetic field. The proposed technique works at temperatures exceeding the energy scale of the tunnel splittings of the SMM. We deduce that the transverse anisotropy for a single ${\mathrm{Fe}}_4$ molecule captured in a junction is substantially larger than the bulk value.