The recent discovery of superconductivity in the quasi-one-dimensional compound ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$, which consists of double-walled tubes of ${\left[{\left({\mathrm{Cr}}_3{\mathrm{As}}_3\right)}^{2-}\right]}^{\infty }$ that run along the $c$ axis, has attracted immediate attention as a potential system for studying superconductors with reduced dimensionality. Here we report clear experimental evidence for the unconventional nature of the superconducting order parameter in ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$, by precisely measuring the temperature dependence of the change in the penetration depth $\mathrm{\Delta }\lambda \left(T\right)$ using a tunnel-diode oscillator. Linear behavior of $\mathrm{\Delta }\lambda \left(T\right)$ is observed for $T\ll T_c$, instead of the exponential behavior of conventional superconductors, indicating that there are line nodes in the superconducting gap. This is strong evidence for unconventional behavior and may provide key information for identifying the pairing state of this super-conductor.

Aluminum has been used prolifically as an impedance matching standard in the multimegabar regime (1 Mbar = 100 GPa), particularly in nuclear driven, early laser driven, and early magnetically driven flyer plate experiments. The accuracy of these impedance matching measurements depends upon the knowledge of both the Hugoniot and release or reshock response of aluminum. Here, we present the results of several adiabatic release measurements of aluminum from $\sim 400\text{–}1200$ GPa states along the principal Hugoniot using full density polymethylpentene (commonly known as TPX), and both $\sim 190$ and $\sim 110$ mg/cc silica aerogel standards. These data were analyzed within the framework of a simple, analytical model that was motivated by a first-principles molecular dynamics investigation into the release response of aluminum, as well as by a survey of the release response determined from several tabular equations of state for aluminum. Combined, this theoretical and experimental study provides a method to perform impedance matching calculations without the need to appeal to any tabular equation of state for aluminum. As an analytical model, this method allows for propagation of all uncertainty, including the random measurement uncertainties and the systematic uncertainties of the Hugoniot and release response of aluminum. This work establishes aluminum for use as a high-precision standard for impedance matching in the multimegabar regime.

Work, moments of work, and heat flux are studied for the generic case of a strongly driven two-level system immersed in a bosonic heat bath in domains of parameter space where perturbative treatments fail. This includes in particular the interplay between non-Markovian dynamics and moderate to strong external driving. Exact data are compared with predictions from weak-coupling approaches. Further, the role of system-bath correlations in the initial thermal state and their impact on the heat flux are addressed. The relevance of these results for current experimental activities on solid-state devices is discussed.

A high-pressure study of the lattice dynamics in the filled skutterudite ${\mathrm{Eu}}_{0.84}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ was carried out by means of x-ray powder diffraction and nuclear inelastic scattering. The anharmonicity of particular phonon modes was characterized by mode and element specific Grüneisen parameters. The large anharmonicity of the rattling optical mode that is hybridized with the acoustical phonons at ambient pressure is reduced at high pressure as the phonon modes decouple. This result suggests that anharmonic coupling between acoustic and optical phonon modes plays a central role in the reduced thermal conductivity.

Magnonic vortex crystals are studied via scanning transmission x-ray microscopy and ferromagnetic-resonance spectroscopy. We investigate a two-dimensional vortex crystal by imprinting waves with tunable wave vectors. The dispersion relation $\omega \left(k\right)$ is determined via ferromagnetic-resonance spectroscopy with a tunable frequency and wave vector for two vortex core polarization patterns that are adjusted by self-organized state formation prior to the measurement. We demonstrate that the band structure of the crystal is reprogrammed by tuning the vortex polarizations.

We report elastic and inelastic neutron-scattering measurements of the high-$T_{\mathrm{C}}$ ferromagnet ${\mathrm{Mn}}_{1+\delta }\mathrm{Sb}$. Measurements were performed on a large, $T_{\mathrm{C}}=434$ K, single crystal with an interstitial Mn content of $\delta \approx 0.13$. The neutron-diffraction results reveal that the interstitial Mn has a magnetic moment, and that it is aligned antiparallel to the main Mn moment. We perform density functional theory calculations including the interstitial Mn and find the interstitial to be magnetic, in agreement with the diffraction data. The inelastic neutron-scattering measurements reveal two features in the magnetic dynamics: (i) a spin-wave-like dispersion emanating from ferromagnetic Bragg positions $\left(\mathrm{H}\mathrm{K}2n\right)$, and (ii) a broad, nondispersive signal centered at forbidden Bragg positions $\left(\mathrm{H}\mathrm{K}2n+1\right)$. The inelastic spectrum cannot be modeled by simple linear spin-wave-theory calculations and appears to be significantly altered by the presence of the interstitial Mn ions. The results show that the influence of the interstitial Mn on the magnetic state in this system is more important than previously understood.

The $\mathrm{Ce}T{X}_3$ compounds belong to the intensively studied group of materials revealing such properties as valence fluctuations, heavy-fermion behavior, or pressure-induced superconductivity. So far, magnetic structure was investigated in only a few of these. We performed a series of neutron-diffraction experiments on a structurally well-defined ${\mathrm{CeCuAl}}_3$ single crystal. The ordered non-centro-symmetric tetragonal ${\mathrm{BaNiSn}}_3$-type (I4mm) crystal structure as well as the antiferromagnetic order below $T_{\mathrm{N}}\approx 2.7$ K were corroborated by our diffraction experiments. The amplitude modulated magnetic structure with in-plane magnetic moments described by the propagation vector $\mathbf{k}=(0.4,0.6,0)$ was revealed. The relation between structural parameters and magnetic structures in the family of isostructural $\mathrm{Ce}T{X}_3$ compounds is discussed.

Unusual behavior of reentrant spin-glass (RSG) compound $\mathrm{L}{\mathrm{u}}_2\mathrm{MnNi}{\mathrm{O}}_6$ has been investigated by magnetometry and neutron diffraction. The system possesses a ferromagnetic (FM) ordering below 40 K and undergoes a RSG transition at 20 K. Additionally, $\mathrm{L}{\mathrm{u}}_2\mathrm{MnNi}{\mathrm{O}}_6$ retains memory effect above the glassy transition till spins sustain ordering. A novel critical behavior with unusual critical exponents ($\beta =\sim 0.241$ and $\gamma \sim 1.142$) is observed that indicates a canting in the spin structure below the ferromagnetic transition ($T_C$). A comprehensive analysis of temperature-dependent neutron diffraction data and first-principles calculations divulge that a structural distortion induced by an octahedral tilting results in a canted spin structure below $T_C$.

We study the effect of a single impurity on the transport properties of a one-dimensional quantum liquid highly excited away from its ground state by a sudden quench of the bulk interaction. In particular we compute the time-dependent dc current to leading order in the impurity potential, using bosonization and starting both from the limit of a uniform system and from the limit of two decoupled semi-infinite systems. Our results reveal that the nonequilibrium excitation of bulk modes induced by the global quench has important effects on the conductor-insulator quantum phase transition, turning it into a crossover for any small quench amplitude and destroying the exact duality between the conducting and insulating fixed points. In addition, the current displays a faster decay towards the steady state than the equilibrium case, a signature of quench-induced decoherence.

We study systems of multiple interacting quantum impurities deposited on a metallic surface in a three-dimensional host. For the real-space two-impurity problem, using numerical renormalization group calculations, a rich range of behavior is shown to arise due to the interplay between Kondo physics and effective Ruderman-Kittel-Kasuya-Yosida interactions—provided the impurity separation is small. Such calculations allow identification of the minimum impurity separation required for a description in terms of independent impurities, and thereby the onset of the “dilute-impurity limit” in many-impurity systems. A “dilute-cluster” limit is also identified in systems with higher impurity density, where interimpurity interactions are important only within independent clusters. We calculate the quasiparticle interference due to two and many impurities, and explore the consequences of the independent impurity and cluster paradigms. Our results provide a framework to investigate the effects of disorder due to interacting impurities at experimentally relevant surface coverages.

We studied the electronic structures of antiferromagnetic (AFM) GdBiPt with propagating vectors $\overrightarrow{Q_1}=(0,0,\pi$) (A-type) and $\overrightarrow{Q_2}=(\pi ,\pi ,\pi$) (G-type) by performing first-principles calculation based on density-functional theory with modified Becke and Johnson local-density approximation potentials plus Hubbard U (MBJLDA+U). With the total energy calculation, the G-type AFM spin-ordered state is relatively more stable than the A-type AFM spin-ordered state, although the difference in total energy is minute. Our band-structure calculation predicts that the A-type AFM state is topologically nontrivial due to a single $s$-character band inversion at the $\mathrm{\Gamma }$ point, which is similar to the band inversions in half-Heusler topological insulator candidates and bulk HgTe semiconductors, while the G-type AFM state is topologically trivial due to the absence of $s/p$ band inversion. With a realistic tight-binding model calculation with 20 bands coupled to an AFM Zeeman field, GdBiPt with A-type AFM spin order presents a metallic surface state on the terminations with AFM aligned Gd ions, and this surface state is independent of the strength of the AFM Zeeman field, i.e., this surface state will be preserved in a nonmagnetic case. Upon terminating the ferromagnetic spin-aligned Gd ions, the surface state is dependent on the strength of the Zeeman field, and the metallic surface can recover when the Zeeman field approaches zero.

The heat capacity, magnetic, and electrical transport properties of the binary intermetallic compound ${\mathrm{Y}}_9{\mathrm{Co}}_7$ are re-investigated through careful low-temperature measurements performed on one and the same high-quality polycrystalline sample. Our results indicate a local coexistence of itinerant ferromagnetism and bulk superconductivity in ${\mathrm{Y}}_9{\mathrm{Co}}_7$ below $T_{\mathrm{sc}}=2.95$ K, as opposite to a previous conviction that superconductivity occurs in the paramagnetic phase embedded in a basically normal magnetic environment. Since the clean-limit condition is satisfied for a pure sample of ${\mathrm{Y}}_9{\mathrm{Co}}_7$, the question whether magnetic fluctuations contribute in the formation of Cooper pairs in this sole $d$-band ferromagnetic superconductor remains open.

Stacks of InAs/GaAs submonolayer depositions forming small In-rich islands are interesting for efficient optoelectronic applications due to the high areal density of localization centers and their fast carrier relaxation. The electronic confinement of charge carriers in InAs/GaAs submonolayers can be influenced by adding Sb during growth. Eight-band $k·p$ simulations show that electrons and holes experience a different localization depending on where the Sb is incorporated into the submonolayer stacks. Samples grown with metalorganic vapor-phase epitaxy show sharp interfaces between InAs(Sb)/GaAs submonolayers and the surrounding matrix material in transmission electron microscopy and x-ray diffraction measurements. Cross-sectional scanning tunneling microscopy shows the formation of In-rich agglomerations as well as a slight clustering of Sb atoms. Temperature-dependent photoluminescence and spatially resolved cathodoluminescence show evidence for strong electronic confinement whose depth is controlled by the Sb amount.

The monolayer of black phosphorus, or “phosphorene,” has recently emerged as a two-dimensional semiconductor with intriguing highly anisotropic transport properties. Existing calculations of its intrinsic phonon-limited electronic transport properties so far rely on the deformation potential approximation, which is in general not directly applicable to anisotropic materials since the deformation along one specific direction can scatter electrons traveling in all directions. We perform a first-principles calculation of the electron-phonon interaction in phosphorene based on density functional perturbation theory and Wannier interpolation. Our calculation reveals that (1) the high anisotropy provides extra phase space for electron-phonon scattering, and (2) optical phonons have appreciable contributions. Both effects cannot be captured by the deformation potential calculations. Our simulation predicts carrier mobilities $\sim 170\mathrm{c}{\mathrm{m}}^2/\mathrm{V}\mathrm{s}$ for both electrons and holes at $300\mathrm{K}$, and a thermoelectric figure of merit $zT$ of up to 0.14 in $p$-type impurity-free phosphorene at $500\mathrm{K}$.

Vicinal Si(111) surfaces are known to undergo faceting when the temperature is lowered below the $(1\times 1)$ to $(7\times 7)$ phase transition temperature. Depending on the cutoff angle value and direction with respect to the crystallographic axis, various facets, together with low Miller index terraces, are formed. Here, we report the formation of regularly distributed steps over macroscopic sample regions of the bare Si(553) surface. The surface morphology is studied with scanning tunneling microscopy and reflection high energy electron diffraction techniques. The (111) terraces of 2.88 nm in width, which are separated by double atomic height steps, reveal an unusual reconstruction. However, the electronic structure determined with angle resolved photoemission spectroscopy shows bands very similar to those observed for the $\mathrm{Si}\left(111\right)\text{−}(7\times 7)$ surface.

The charge and spin patterns of a quantum dot embedded into a spin-orbit coupled quantum wire subject to a magnetic field are investigated. A Luttinger liquid theory is developed, taking into account open boundaries and a finite magnetic field. In the quasihelical regime, when spin-orbit effects dominate over the Zeeman interaction, peculiar states develop at the Fermi surface of the dot. Anomalous Friedel oscillations with twice the expected wavelength develop in the total tunneling rate from a scanning tunneling microscope tip, accompanied by a peculiar oscillation of spin-resolved transport properties. Both effects are analyzed in detail. The stability against electron interactions and magnetic field is investigated. We also discuss how signatures of such states survive in the total charge and spin densities.

The problem of electrostatic screening of a charged line by undoped or weakly doped graphene is treated beyond the linear-response theory. The induced electron density is found to be approximately doping independent, $n\left(x\right)\sim x^{-2}{log}^{2}x$, at intermediate distances $x$ from the charged line. At larger $x$, twin $p\text{−}n$ junctions may form if the external perturbation is repulsive for graphene charge carriers. The effect of such inhomogeneities on conductance and quantum capacitance of graphene is calculated. The results are relevant for transport properties of graphene grain boundaries and for local electrostatic control of graphene with ultrathin gates.

We investigate the Andreev transport through double-quantum-dot Cooper pair splitters with ferromagnetic leads. The analysis is performed with the aid of the real-time diagrammatic technique in the sequential tunneling regime. We study the dependence of the Andreev current, the differential conductance, and the tunnel magnetoresistance on various parameters of the model in both the linear and nonlinear response regimes. In particular, we analyze the spin-resolved transport in the crossed Andreev reflection regime, where a blockade of the current occurs due to enhanced occupation of the triplet state. We show that in the triplet blockade, finite intradot correlations can lead to considerable leakage current due to direct Andreev reflection processes. Furthermore, we find additional regimes of current suppression resulting from enhanced occupation of singlet states, which decreases the rate of crossed Andreev reflection. We also study how the splitting of Andreev bound states, triggered by either dot level detuning, finite hopping between the dots, or finite magnetic field, affects the Andreev current. While in the first two cases the number of Andreev bound states is doubled, whereas transport properties are qualitatively similar, in the case of finite magnetic field further level splitting occurs, leading to a nontrivial behavior of spin-resolved transport characteristics, and especially that of tunneling magnetoresistance. Finally, we discuss the entanglement fidelity between split Cooper pair electrons and show that by tuning the device parameters, fidelity can reach unity.

Recent experiments showed that the surface of a three-dimensional topological insulator develops gaps in the Floquet-Bloch band spectrum when illuminated with a circularly polarized laser. These Floquet-Bloch bands are characterized by nontrivial Chern numbers which only depend on the helicity of the polarization of the radiation field. Here we propose a setup consisting of a pair of counterrotating lasers, and show that one-dimensional chiral states emerge at the interface between the two lasers. These interface states turn out to be spin polarized and may trigger interesting applications in the field of optoelectronics and spintronics.