We construct minimum effective models to investigate the pairing symmetry in the newly discovered quasi-one-dimensional superconductor ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$. We show that a minimum three-band model based on the $d_{z^2}, d_{xy}$, and $d_{x^2-y^2}$ orbitals of one Cr sublattice can capture the band structures near Fermi surfaces. In both weak and strong coupling limits, the standard random phase approximation and mean-field solutions consistently yield the triplet $p_z$-wave pairing as the leading pairing symmetry for physically realistic parameters. The triplet pairing is driven by the ferromagnetic fluctuations within the sublattice. The gap function of the pairing state possesses line gap nodes on the $k_z=0$ plane on the Fermi surfaces.

Electronic transport properties of the antiferromagnetic Mott insulator $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ have been investigated under extremely high electric biases. Using nanoscale contacts, we apply electric fields up to a few MV/m to a single crystal of $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ and observe a continuous reduction in the material's resistivity with increasing bias, characterized by a reduction in the transport activation energy by as much as $16\%$. Temperature-dependent resistivity measurements provide a means to unambiguously retrieve the bias dependence of the activation energy from the Arrhenius plots at different biases. We further demonstrate the feasibility of reversible resistive switching induced by the electric bias, which is of interest for the emerging field of antiferromagnetic spintronics. Our findings demonstrate the potential of electrical means for tuning electronic properties in $5d$ transition-metal oxides and suggest a promising path towards development of next-generation functional devices.

We study a system of interacting spinless fermions in one dimension that, in the absence of interactions, reduces to the Kitaev chain [Kitaev, Phys. Usp. 44, 131 (2001)]. In the noninteracting case, a signal of topological order appears as zero-energy modes localized near the edges. We show that the exact ground states can be obtained analytically even in the presence of nearest-neighbor repulsive interactions when the on-site (chemical) potential is tuned to a particular function of the other parameters. As with the noninteracting case, the obtained ground states are twofold degenerate and differ in fermionic parity. We prove the uniqueness of the obtained ground states and show that they can be continuously deformed to the ground states of the noninteracting Kitaev chain without gap closing. We also demonstrate explicitly that there exists a set of operators each of which maps one of the ground states to the other with opposite fermionic parity. These operators can be thought of as an interacting generalization of Majorana edge zero modes.

Dynamic nuclear spin polarization (DNP) mediated by paramagnetic point defects in semiconductors is a key resource for both initializing nuclear quantum memories and producing nuclear hyperpolarization. DNP is therefore an important process in the field of quantum-information processing, sensitivity-enhanced nuclear magnetic resonance, and nuclear-spin-based spintronics. DNP based on optical pumping of point defects has been demonstrated by using the electron spin of nitrogen-vacancy (NV) center in diamond, and more recently, by using divacancy and related defect spins in hexagonal silicon carbide (SiC). Here, we describe a general model for these optical DNP processes that allows the effects of many microscopic processes to be integrated. Applying this theory, we gain a deeper insight into dynamic nuclear spin polarization and the physics of diamond and SiC defects. Our results are in good agreement with experimental observations and provide a detailed and unified understanding. In particular, our findings show that the defect electron spin coherence times and excited state lifetimes are crucial factors in the entire DNP process.

We present a microscopic theory of optical initialization, control, and detection for a single electron spin in a quantum dot embedded into a zero-dimensional microcavity. The strong coupling regime of the trion and the cavity mode is addressed. We demonstrate that efficient spin orientation by a single circularly polarized pulse is possible in relatively weak transverse magnetic fields. The possibilities for spin control by additional circularly polarized pulse are analyzed. Under optimal conditions the Kerr and Faraday rotation angles induced by the spin polarized electron may reach tens of degrees.

We report the subpicosecond initialization of a single heavy hole spin in a self-assembled quantum dot with $>98.5\%$ fidelity and without external magnetic field. Using an optically addressable charge and spin storage device we tailor the relative electron and hole tunneling escape time scales from the dot and simultaneously achieve high-fidelity initialization, long hole storage times, and high-efficiency readout via a photocurrent signal. We measure electric-field-dependent Rabi oscillations of the neutral and charged exciton transitions in the ultrafast tunneling regime and demonstrate that tunneling-induced dephasing (TID) of excitonic Rabi rotations is the major source for the intensity damping of Rabi oscillations in the low Rabi frequency, low temperature regime. Our results are in very good quantitative agreement with quantum-optical simulations revealing that TID can be used to precisely measure tunneling escape times and extract changes in the Coulomb binding energies for different charge configurations of the quantum dot. Finally, we demonstrate that for subpicosecond electron tunneling escape, TID of a coherently driven exciton transition facilitates ultrafast hole spin initialization with near-unity fidelity.

Very recently the topological Weyl semimetal (WSM) state was predicted in the noncentrosymmetric compounds NbP, NbAs, TaP, and TaAs and soon led to photoemission and transport experiments to verify the presumed topological properties such as Fermi arcs (unclosed Fermi surfaces) and the chiral anomaly. In this work we have performed fully ab initio calculations of the surface band structures of these four WSM materials and revealed the Fermi arcs with spin-momentum-locked spin texture. On the (001) polar surface, the shape of the Fermi surface depends sensitively on the surface terminations (cations or anions), although they exhibit the same topology with arcs. The anion (P or As) terminated surfaces are found to fit recent photoemission measurements well. Such surface potential dependence indicates that the shape of the Fermi surface can be sensitively manipulated by depositing guest species (such as K atoms), as we demonstrate. On the polar surface of a WSM without inversion symmetry, Rashba-type spin polarization naturally exists in the surface states and leads to strong spin texture. By tracing the spin polarization of the Fermi surface, one can distinguish Fermi arcs from trivial Fermi circles. The four compounds NbP, NbAs, TaP, and TaAs present an increasing amplitude of spin-orbit coupling (SOC) in band structures. By comparing their surface states, we reveal the evolution of topological Fermi arcs from the spin-degenerate Fermi circle to spin-split arcs when the SOC increases from zero to a finite value. Our work presents a comprehensive understanding of the topological surface states of WSMs, which will especially be helpful for future spin-revolved photoemission and transport experiments.

Magnetization dynamics of a nanomagnet, when strongly coupled with a topological insulator (TI) via the proximity interaction, is examined theoretically in the presence of electrical current on the TI surface under realistic transport conditions. Due to the spin-momentum interlock, the magnetic state and TI electron transport depend significantly on each other. Such an interdependence leads to a variety of nonlinear dynamical responses in all transport regimes including the scattering dominant diffusive cases. Generation of the anomalous Hall current, in particular, is found to be a key to the unique features that have not been observed previously. For instance, the anomalous Hall current can result in antiparallel alignment of the final magnetization state in reference to the effective driving magnetic field by inducing an extra term that counters the damping effect. Similarly the calculation also reveals steady oscillation of the magnetization under a broad range of conditions, offering a robust mechanism for highly efficient magnetization reversal and/or spin wave excitation under a dc bias.

We model optical absorption of monolayer and bilayer graphene on hexagonal boron nitride for the case of closely aligned crystal lattices. We show that perturbations with different spatial symmetry can lead to similar absorption spectra. We suggest that a study of the absorption spectra as a function of the doping for an almost completely full first miniband is necessary to extract meaningful information about the moiré characteristics from optical absorption measurements and to distinguish between various theoretical proposals for the physically realistic interaction. Also, for bilayer graphene, the ability to compare spectra for the opposite signs of electric-field-induced interlayer asymmetry might provide additional information about the moiré parameters.

We demonstrate an on-demand hole spin qubit initialization scheme meeting four key requirements of quantum information processing: fast initialization (1/e ∼ 100 ps), high fidelity ($F>99\%$), long qubit lifetime ($2T_h>T_2^{*}\simeq 10\mathrm{ns}$), and compatibility with optical coherent control schemes. This is achieved by rapidly ionizing an exciton in an InGaAs quantum dot with very low fine-structure splitting at zero magnetic field. Furthermore, we show that the hole spin fidelity of an arbitrary quantum dot can be increased by optical Stark effect tuning of the fine-structure splitting close to zero.

The recently proposed diagrammatic expansion (DE) technique for the full Gutzwiller wave function (GWF) is applied to the Anderson lattice model. This approach allows for a systematic evaluation of the expectation values with full Gutzwiller wave function in finite-dimensional systems. It introduces results extending in an essential manner those obtained by means of the standard Gutzwiller approximation (GA), which is variationally exact only in infinite dimensions. Within the DE-GWF approach we discuss the principal paramagnetic properties and their relevance to heavy-fermion systems. We demonstrate the formation of an effective, narrow $f$ band originating from atomic $f$-electron states and subsequently interpret this behavior as a direct itineracy of $f$ electrons; it represents a combined effect of both the hybridization and the correlations induced by the Coulomb repulsive interaction. Such a feature is absent on the level of GA, which is equivalent to the zeroth order of our expansion. Formation of the hybridization- and electron-concentration-dependent narrow $f$ band rationalizes the common assumption of such dispersion of $f$ levels in the phenomenological modeling of the band structure of ${\mathrm{CeCoIn}}_5$. Moreover, it is shown that the emerging $f$-electron direct itineracy leads in a natural manner to three physically distinct regimes within a single model that are frequently discussed for $4f$- or $5f$-electron compounds as separate model situations. We identify these regimes as (i) the mixed-valence regime, (ii) Kondo/almost-Kondo insulating regime, and (iii) the Kondo-lattice limit when the $f$-electron occupancy is very close to the $f$-state half filling, $\langle {\widehat{n}}_f\rangle \to 1$. The nonstandard features of the emerging correlated quantum liquid state are stressed.

Motivated by model calculations, for the heavy fermion superconductor $\mathrm{CeP}{\mathrm{t}}_3\mathrm{Si}$ predicting phonon anomalies because of antisymmetric spin-orbit coupling, we performed a detailed experimental study of the lattice dynamical properties of $\mathrm{CeP}{\mathrm{t}}_3\mathrm{Si}$. In particular, we investigated the dispersion of transverse acoustic and low energy optic phonon branches along the [110] direction using inelastic neutron scattering. In these branches we found deviations from our ab initio lattice dynamical calculations, which overall give a good description of the phonon dispersion in $\mathrm{CeP}{\mathrm{t}}_3\mathrm{Si}$. However, the agreement for the [110] transverse modes can be improved if we neglect the Ce $4f$ states, done in an additional calculation. We conclude that the lattice dynamics of $\mathrm{CeP}{\mathrm{t}}_3\mathrm{Si}$ are conventional and that the observed deviations are not related to effects of antisymmetric spin-orbit coupling. More likely, ab initio calculations overestimate the exchange between different phonon branches, particularly in the presence of $4f$ electron states. Our results imply that the ASOC plays less a role in noncentrosymmetric superconductors than commonly believed.

Exploring magnetic semiconductors is one of the most important questions for spintronic applications. Although various solutions, such as dilute magnetic semiconductors, have been proposed, a practical spintronic device working at room temperature has not been realized. The key to address this issue is to find magnetic materials with both room-temperature magnetic ordering and large spin polarization around the Fermi energy level. Here, we predict a new concept of asymmetric antiferromagnetic (AFM) semiconductors (AAFMSs) with both features. The high temperature magnetic ordering originates from the AFM coupling between different transition metal ions with strong super-exchange interaction, whereas the large spin polarization around the Fermi energy level owes to $d$ orbital mismatch among these ions. Through first-principles calculations, a family of double perovskites $A_2\mathrm{Cr}M{\mathrm{O}}_6$ ($A=\mathrm{Ca},\mathrm{Sr},\mathrm{Ba}$, and $M=\mathrm{Ru},\mathrm{Os}$) are predicted to be AAFMSs. This paper provides a way for developing spintronic devices working at room temperature.

Carrier-induced ferromagnetism in magnetic III-V semiconductors has opened up several opportunities for spintronic device applications as well as for fundamental studies of a material system in which itinerant carriers interact with the localized spins of magnetic impurities. In order to understand the hole mediated ferromagnetism, probing the band structure in these material systems is crucial. Here we present magnetic circular dichroism (MCD) studies on MOVPE grown InMnSb and InMnAs, both with the Curie temperatures above 300 K. The measurements were performed on samples with different Mn contents with the excitation energy tuned from 0.92–1.42 eV and external magnetic fields up to 31 T. The large $g$ factors in these systems allow us to measure the MCD at relatively high temperatures (190 K). These measurements are compared with MCD calculations based on an eight-band Pidgeon-Brown model, which is generalized to include the coupling between the electron/hole and the Mn spin in a ferromagnetic state. Comparison of the observed MCD with the theoretical calculations provides a direct method to probe the band structure including the temperature dependence of the spin-orbit split-off gap and $g$ factors, and to estimate the $sp-d$ coupling constants.

We report an experimental technique to measure and manipulate the arrival-time and energy distributions of electrons emitted from a semiconductor electron pump, operated as both a single-electron source and a two-electron source. Using an energy-selective detector whose transmission we control on picosecond time scales, we can measure directly the electron arrival-time distribution and we determine the upper bound to the distribution width to be 30 ps. We study the effects of modifying the shape of the voltage wave form that drives the electron pump, and show that our results can be explained by a tunneling model of the emission mechanism. This information was in turn used to control the emission-time difference and energy gap between a pair of electrons.

Contacting a bilayer ferromagnet with a singlet even-frequency superconductor allows for the realization of an effective triplet odd-frequency superconductor. In this work, we investigate the Josephson effect between superconductors with different symmetries (e.g., odd versus even frequency). In particular, we study the supercurrent flowing between two triplet odd-frequency superconducting leads through a weak singlet even-frequency superconductor. We show that the peculiar temperature dependence of the critical current below the superconducting transition of the weak superconductor is a signature of the competition between odd/odd-frequency and odd/even-frequency Josephson couplings.