The half-Heusler filled tetrahedral structures (FTSs) are zinc-blende-like compounds, where an additional atom is filling its previously empty interstitial site. The FTSs having 18 valence electrons per formula unit are an emerging family of functional materials, whose intrinsic doping trends underlying a wide range of electronic functionalities are yet to be understood. Interestingly, even pristine compounds without any attempt at impurity/chemical doping exhibit intriguing trends in the free carriers they exhibit. Applying the first principles theory of doping to a few prototype compounds in the $A^{\mathrm{IV}}{B}^{\mathrm{X}}{C}^{\mathrm{IV}}$ and $A^{\mathrm{IV}}{B}^{\mathrm{IX}}{C}^{\mathrm{V}}$ groups, we describe the key ingredients controlling the materials’ propensity for both intrinsic and extrinsic doping: (a) The spontaneous deviations from 1:1:1 stoichiometry reflect predictable thermodynamic stability of specific competing phases. (b) Bulk ABC compounds containing $3d$ elements in the $B$ position (ZrNiSn and ZrCoSb) are predicted to be naturally $3d$ rich. The $B=3d$ interstitials are the prevailing shallow donors, whereas the potential acceptors (e.g., Zr vacancy and Sn-on-Zr antisite) are ineffective electron killers, resulting in an overall uncompensated $n$-type character, even without any chemical doping. In these materials, the band edges are “natural impurity bands” due to non-Daltonian off-stoichiometry, such as $B$ interstitials, not intrinsic bulk controlled states as in a perfect crystal. (c) Bulk ABC compounds containing $5d$ elements in the $B$ position (ZrPtSn, ZrIrSb, and TaIrGe) are predicted to be naturally $C$ rich and $A$ poor. This promotes the hole-producing $C$-on-$A$ antisite defects rather than $B$-interstitial donors. The resultant $p$-type character (without chemical doping) therein is “latent” for $C=\mathrm{Sn}$ and Sb; however, as the $C$-on-$A$ hole-producing acceptors are rather deep and $p$ typeness is manifest only at high temperature or via impurity doping. In contrast, in TaIrGe $(B=\mathrm{Ir},5d)$, the prevailing hole-producing Ge-on-Ta antisite ($C$-on-$A$) is shallow, making it a real $p$-type compound. This general physical picture establishes the basic trends of carriers in this group of materials.

We report comprehensive studies of the single crystal growth and electrical transport properties for various samples of TaAs, the first experimentally confirmed inversion symmetry-breaking Weyl semimetal. The transport parameters for different samples are obtained through the fitting of the two-band model and the analysis of Shubnikov–de Haas oscillations. We find that the ratio factor of transport lifetime to quantum lifetime is intensively enhanced when the Fermi level approaches the Weyl node. This result is consistent with the side-jump interpretation derived from a chirality-protected shift in the scattering process for a Weyl semimetal.

Ever since the first experimental investigations of organic field effect transistors (OFETs) the dimensionality of charge transport has alternately been described as two dimensional (2D) and three dimensional (3D). More recently, researchers have turned to an analytical analysis of the temperature-dependent transfer characteristics to classify the dimensionality as either 2D or 3D as well as to determine the disorder of the system, thereby greatly simplifying dimensionality investigations. We applied said analytical analysis to the experimental results of our OFETs comprising molecularly well-defined polymeric layers as the active material as well as to results obtained from kinetic Monte Carlo simulations and found that it was not able to correctly distinguish between 2D and 3D transports or give meaningful values for the disorder and should only be used for quasiquantitative and comparative analysis. We conclude to show that the dimensionality of charge transport in OFETs is a function of the interplay between transistor physics and morphology of the organic material.

We generalize the diffusive model for spin injection and detection in nonlocal spin structures to account for spin precession under an applied magnetic field in an anisotropic medium, for which the spin lifetime is not unique and depends on the spin orientation. We demonstrate that the spin precession (Hanle) line shape is strongly dependent on the degree of anisotropy and on the orientation of the magnetic field. In particular, we show that the anisotropy of the spin lifetime can be extracted from the measured spin signal, after dephasing in an oblique magnetic field, by using an analytical formula with a single fitting parameter. Alternatively, after identifying the fingerprints associated with the anisotropy, we propose a simple scaling of the Hanle line shapes at specific magnetic field orientations that results in a universal curve only in the isotropic case. The deviation from the universal curve can be used as a complementary means of quantifying the anisotropy by direct comparison with the solution of our generalized model. Finally, we applied our model to graphene devices and find that the spin relaxation for graphene on silicon oxide is isotropic within our experimental resolution.

The localization properties of the highly excited states of a system projected into a single Landau level are analyzed. An analysis of level spacing ratios for finite size systems shows a clear crossover from extended (GUE) to localized (Poisson) statistics, indicating a many-body localization transition. However, the location of this transition depends very strongly on system size, and appears to scale to infinite disorder in the thermodynamic limit. This result does not depend on the properties of the ground state (such as whether the ground state exhibits topological order), as expected for a transition of highly excited eigenstates. We therefore conclude that many-body localization does not exist in these systems. Our results demonstrate that a subthermodynamic number of single particle effectively extended states is sufficient to cause all many-body states to become extended.

We report high-field magnetization, high-frequency electron spin resonance (ESR), and ${}^{77}\mathrm{Se}$ nuclear magnetic resonance (NMR) measurements on the linear spin tetramer system ${\mathrm{CuSeO}}_3$, consisting of strongly interacting Cu(1) dimers and weakly coupled Cu(2) spins. The magnetization exhibits anisotropic half-step magnetization plateaus at ${\mu }_{0}H=45$ T, depending on a crystallographic orientation. A temperature dependence of the ESR linewidth $\mathrm{\Delta }{H}_{\mathrm{pp}}$ in a paramagnetic phase points towards the significance of anisotropic exchange interactions. Below $T_{\mathrm{N}}=9–10$ K long-range magnetic order is evidenced by the observation of a critical divergence of both $\mathrm{\Delta }{H}_{\mathrm{pp}}\left(T\right)$ and the nuclear spin-lattice relaxation rate $1/T_1$. In addition, we identify a magnetic anomaly at $T^{*}=6.0\left(5\right)$ K below $T_{\mathrm{N}}$, which is caused by a spin reorientation. The nuclear spin-spin relaxation rate $1/T_2$ unveils the development of site-specific spin correlations. The intriguing magnetism of ${\mathrm{CuSeO}}_3$ is discussed in terms of the energy hierarchy of Cu(1) and Cu(2) spins in concert with additional intertetramer interactions.

The phase diagram of numerous materials of technological importance features high-symmetry high-temperature phases that exhibit phonon instabilities. Leading examples include shape-memory alloys, as well as ferroelectric, refractory, and structural materials. The thermodynamics of these phases have proven challenging to handle by atomistic computational thermodynamic techniques due to the occurrence of constant anharmonicity-driven hopping between local low-symmetry distortions, while maintaining a high-symmetry time-averaged structure. To compute the free energy in such phases, we propose to explore the system's potential-energy surface by discrete sampling of local minima by means of a lattice gas Monte Carlo approach and by continuous sampling by means of a lattice dynamics approach in the vicinity of each local minimum. Given the proximity of the local minima, it is necessary to carefully partition phase space by using a Voronoi tessellation to constrain the domain of integration of the partition function in order to avoid double counting artifacts and enable an accurate harmonic treatment near each local minima. We consider the bcc phase of titanium as a prototypical example to illustrate our approach.

We predict large magnetoresistance (MR) and spin transfer torque (STT) in antiferromagnetic $\mathrm{Fe}\left|\mathrm{MgO}\right|\mathrm{FeMn}|\mathrm{Cu}$ tunnel junctions based on first-principles scattering theory. MR as large as $\sim 100\%$ is found in one junction. Magnetic dynamic simulations show that STT acting on the antiferromagnetic order parameter dominates the spin dynamics, and an electronic bias of order ${10}^{-1}\mathrm{mV}$ and current density of order ${10}^5{\mathrm{Acm}}^{-2}$ can switches a junction of three-layer MgO, they are about one order smaller than that in $\mathrm{Fe}\left|\mathrm{MgO}\right|\mathrm{Fe}$ junction with the same barrier thickness, respectively. The multiple scattering in the antiferromagnetic region is considered to be responsible for the enhanced spin torque and smaller switching current density.

The low-energy electronic structure of $\mathrm{LaFeAs}{\mathrm{O}}_{1-x}{\mathrm{H}}_x$ $\left(0.0\le x\le 0.60\right)$, a system which exhibits two superconducting domes and antiferromagnetic orders in its phase diagram, is investigated by utilizing laser photoemission spectroscopy. From the precise temperature-dependent measurement of the spectra near the Fermi level, we find a suppression of the density of states with cooling, namely a pseudogap formation, for all doping range. The pseudogap gets suppressed on doping through the first superconducting dome regime to the higher-$x$ region, whereas it tends to recover when further increasing $x$ toward the second antiferromagnetic ordered phase. The systematic doping dependence indicates the different origins of pseudogaps in the low- and high-$x$ regions, possibly related to the respective antiferromagnetic ground states residing at both ends of the phase diagram.

Systems with strong spin-orbit coupling support a number of new phases of matter and novel phenomena. This work focuses on the interplay of spin-orbit coupling and interactions in yielding correlated phenomena in two-dimensional transition-metal dichalcogenides. In particular we explore the physics of Kondo screening resulting from the lack of centrosymmetry, large spin splitting, and spin valley locking in hole-doped systems. The key ingredients are (i) valley-dependent spin-momentum locking perpendicular to the two-dimensional crystal, (ii) a single nondegenerate Fermi surface per valley, and (iii) nontrivial Berry curvature associated with the low-energy bands. The resulting Kondo resonance has a finite-triplet component and nontrivial momentum space structure which facilitates new approaches to both probing and manipulating the correlated state. Using a variational wave function and the numerical renormalization group approaches we study the nature of the Kondo resonance both in the absence and presence of circularly polarized light. The latter induces an imbalance in the population of the two valleys leading to novel magnetic phenomena in the correlated state.

We present an investigation of the magnetic structure for iron-based superconductors (FeSCs) when inversion symmetry is broken, such as in substrate-supported monolayers or in the presence of a $c$-axis electric field. We perform group-, mean-field-, and density-functional-theoretic analyses on a model system of monolayer iron selenide (FeSe) on a strontium titanate $[{\mathrm{SrTiO}}_3$(001)] substrate. Our group- and mean-field-theoretic calculations are more generally applicable to thin films of the rest of the 11 (e.g., FeSe) family of iron-based superconductors, as well as to thin films of the 111 (e.g., LiFeAs) and 1111 (e.g., LaOFeAs) families, as these all belong to the same space group. We find that in systems with a collinear antiferromagnetic phase in bulk, when inversion symmetry is broken, the transition is instead into a “spin vortex crystal” phase and that a further phase transition can occur at a lower temperature in some circumstances. The spin vortex crystal is a $C_4$-symmetric magnetic phase which is related to this parent $C_2$-symmetric collinear antiferromagnetic (stripe) phase which is ubiquitous among the iron-based superconductors.

${\mathrm{Li}}_2{\mathrm{RuO}}_3$ is known to crystallize in either $C2/m$ or $P{2}_1/m$ structures at room temperature. We report the first single crystal growth of ${\mathrm{Li}}_2{\mathrm{RuO}}_3$ and Na substituted crystals $\left({\mathrm{Li}}_{0.95}{\mathrm{Na}}_{0.05}\right){}_2{\mathrm{RuO}}_3$ crystallizing in the $P{2}_1/m$ structure where a first order magnetostructural transition is observed at high temperatures. Using high temperature $(T\le 1000$ K) magnetic susceptibility $\chi$ measurements we study the magnetic anisotropy across the magnetostructural transition. Our results show that for ${\mathrm{Li}}_2{\mathrm{RuO}}_3$ the magnetic and structural transitions most likely occur at slightly different temperatures. The structural transition which is first-order-like occurs first $(T_{\mathrm{onset}}\approx 570$ K) and drives the magnetic transition $(T\approx 540$ K). Rather surprisingly, just $5\%$ Na substitution for Li affects the magnetostructural transition in an interesting way. The first order transition temperature remains at $\approx 540$ K, the magnetic anisotropy is reversed, and the Ru-Ru dimerization pattern changes from two short and four long Ru-Ru bonds per honeycomb in an armchair pattern for ${\mathrm{Li}}_2{\mathrm{RuO}}_3$ to four short and two long bonds per honeycomb in $\left({\mathrm{Li}}_{0.95}{\mathrm{Na}}_{0.05}\right){}_2{\mathrm{RuO}}_3$ which can be viewed as two interpenetrating armchair patterns.

Topological order has become a new paradigm to distinguish ground states of interacting many-body systems without conventional long-range order. Here, we discuss possible extensions of this concept to density matrices describing statistical ensembles. For a large class of quasithermal states, which can be realized as thermal states of some quasilocal Hamiltonian, we generalize earlier definitions of density-matrix topology to generic many-body systems with strong correlations. We point out that the robustness of topological order, defined as a pattern of long-range entanglement, depends crucially on the perturbations under consideration. While it is intrinsically protected against local perturbations of arbitrary strength in an infinite closed quantum system, purely local perturbations can destroy topological order in open systems coupled to baths if the coupling is sufficiently strong. We discuss our classification scheme using the finite-temperature quantum Hall states and point out that the classical Hall effect can be understood as a finite-temperature topological phase.

The electrical control of single spin qubits based on semiconductor quantum dots is of great interest for scalable quantum computing since electric fields provide an alternative mechanism for qubit control compared with magnetic fields and can also be easier to produce. Here we outline the mechanism for a drastic enhancement in the electrically-driven spin rotation frequency for silicon quantum dot qubits in the presence of a step at a heterointerface. The enhancement is due to the strong coupling between the ground and excited states which occurs when the electron wave function overcomes the potential barrier induced by the interface step. We theoretically calculate single qubit gate times $t_{\pi }$ of 170 ns for a quantum dot confined at a silicon/silicon-dioxide interface. The engineering of such steps could be used to achieve fast electrical rotation and entanglement of spin qubits despite the weak spin-orbit coupling in silicon.

We present energy-momentum mapping of the surface Dirac photocurrent in the topological insulator ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ by means of time- and angle-resolved two-photon photoemission spectroscopy combined with polarization-variable midinfrared pulsed laser excitation. It is demonstrated that a direct optical transition from the occupied to the unoccupied part of the surface Dirac cone permits the linear and circular photogalvanic effect, which thereby enables us to coherently control the electric surface photocurrent by laser polarization. Moreover, the photocurrent mapping directly visualizes ultrafast current dynamics in the Dirac cone as a function of time. We unravel the ultrafast intraband relaxation dynamics of the inelastic scattering and momentum scattering separately. Our observations pave the way for coherent optical control over surface Dirac electrons in topological insulators.

In Ti-intercalated self-doped $1T\text{−}{\mathrm{TiSe}}_2$ crystals, the charge density wave (CDW) superstructure induces two nonequivalent sites for Ti dopants. Recently, it has been shown that increasing Ti doping dramatically influences the CDW by breaking it into phase-shifted domains. Here, we report scanning tunneling microscopy and spectroscopy experiments that reveal a dopant-site dependence of the CDW gap. Supported by density functional theory, we demonstrate that the loss of the long-range phase coherence introduces an imbalance in the intercalated-Ti site distribution and restrains the CDW gap closure. This local resilient behavior of the $1T\text{−}{\mathrm{TiSe}}_2$ CDW reveals an entangled mechanism between CDW, periodic lattice distortion, and induced nonequivalent defects.

The correlation between the charge density wave (CDW) and $f$ local moments is observed in ${\mathrm{GdNiC}}_2$ by means of x-ray diffraction in a magnetic field. Various kinds of electronic states exist in the magnetic field. The intensity of the CDW peak changes in the successive transitions and the commensurate-incommensurate transition of the CDW takes place as well. The successive transitions are explained in terms of a cooperative effect of the Peierls instability and the spin Friedel oscillation, in which the antiferromagnetic order of the $f$ local moments is coupled to the spin density wave coexisting with the CDW of the conduction electron.

Quantum phase transitions reflect singular changes taking place in a many-body ground state; however, computing and analyzing large-scale critical wave functions constitutes a formidable challenge. Physical insights into the sub-Ohmic spin-boson model are provided by the coherent-state expansion (CSE), which represents the wave function by a linear combination of classically displaced configurations. We find that the distribution of low-energy displacements displays an emergent symmetry in the absence of spontaneous symmetry breaking while experiencing strong fluctuations of the order parameter near the quantum critical point. Quantum criticality provides two strong fingerprints in critical low-energy modes: an algebraic decay of the average displacement and a constant universal average squeezing amplitude. These observations, confirmed by extensive variational matrix-product-state (VMPS) simulations and field theory arguments, offer precious clues into the microscopics of critical many-body states in quantum impurity models.

Concentration of light into a nanospot is essential for the heat assisted magnetic recording, biomedical imaging, sensing, and nanolasing. We propose a novel all-dielectric optical field concentrator, which focuses the light, pumped through the waveguide, into a hot nanospot, which is much smaller than the wavelength. The dissipative loss, which is characteristic to a plasmonic nanoantenna, is absent in the dielectric concentrator. Therefore, the detrimental thermal effects almost vanish, which gives an opportunity to use the concentrator for the heat-assisted magnetic recording. The electric field is much enhanced in the proposed new device at the vertex of the dielectric beak, which is attached to the dielectric resonator. The resonator in turn is pumped through the special waveguide. The electric field enhancement and concentration is achieved by longitudinal polarization of the beak vertex, which is exposed to em electric field generated by the pumped resonator. The spatial scale of the hot spot, where the field concentrates, is determined by the curvature of the vertex and can be of few nanometers. We take as a design concept the cylindrical waveguide, the spherical resonator, and the elliptic beak. The rectangular, 2.5-dimensional design of the light concentrator is also considered.

We have experimentally and numerically studied the directional birefringence of $X$-band microwaves by magnetochiral (MCh) effects of a single metamolecule under dc magnetic fields at room temperature. Phase and amplitude transmission coefficients from top and bottom, i.e., $S$ parameters of $S_{21}$ and $S_{12}$, are measured for the single metamolecule consisting of a copper chiral structure and ferrite cylinder in a waveguide. By applying a dc magnetic field, we observe a difference between $S_{21}$ and $S_{12}$, which is an emergence of the MCh effects with simultaneous space-inversion and time-reversal symmetry breaking. Numerical calculation based on a finite element method reproduces well the experimental results of the MCh effects. The MCh effect can be enhanced by using the magnetic resonance of the ferrite cylinder. Notably, numerical calculation predicts that the MCh effect is extremely enhanced by interacting magnetic resonance with a specific resonant structural optical activity, leading to a giant MCh effect. The giant MCh effect observed in the present study originates from the one-way transparency caused by the Fano resonance in the metamolecule.