Over the past decade, the open frame (“cagey”) structure of $\mathrm{CoS}{\mathrm{b}}_3$ skutterudite has invited intensive filling studies with various rare-earth elements for delivering state-of-the-art midtemperature thermoelectric performance. To rationalize previously reported experimental results and provide new insight into the underexplored roles of La fillers and Sb vacancies, ab initio density functional theory studies, along with semiclassical Boltzmann transport theory calculations, are performed for pristine $\mathrm{CoS}{\mathrm{b}}_3$ of different lattice settings and La-filled $\mathrm{CoS}{\mathrm{b}}_3$ with and without Sb's mono- and divacancy defects. The effects of spin-orbit coupling (SOC), partial La filling, Sb vacancy defects, and spin polarization on the electronic and thermoelectric properties are systematically examined. The SOC shows minor effects on the electronic and thermoelectric properties of $\mathrm{CoS}{\mathrm{b}}_3$. The peculiar quasi-Dirac band in the pristine $\mathrm{CoS}{\mathrm{b}}_3$ largely survives La filling but not Sb vacancies, which instead introduce dispersive bands in the band gap region. The non-spin-polarized and spin-polarized solutions of La-filled $\mathrm{CoS}{\mathrm{b}}_3$ are nearly degenerate. Importantly, the band structure, density of states, and Fermi surface of the latter are significantly spin polarized, giving rise to spin-dependent thermoelectric properties. Seebeck coefficients directly calculated as a function of chemical potential are interpreted in connection with the electronic structures. Temperature-dependent Seebeck coefficients derived for the experimentally studied materials agree well with available experimental data. Seebeck coefficients obtained as a function of charge carrier concentration corroborate the thermoelectrically favorable role at high filling fractions played by the Fermi electron pockets associated with the degenerate valleys in the conduction bands, and also point toward a similar role of the Fermi hole pockets associated with the degenerate hills in the valence bands. These results serve to advance the understanding of $\mathrm{CoS}{\mathrm{b}}_3$ skutterudite, a class of materials with important fundamental and application implications for thermoelectrics and spintronics.

We study an exotic state which is localized only at an intersection of edges of a topological material. This “edge-of-edge” state is shown to exist generically. We construct explicitly generic edge-of-edge states in five-dimensional Weyl semimetals and their dimensional reductions, such as four-dimensional topological insulators of class A and three-dimensional chiral topological insulators of class AIII. The existence of the edge-of-edge state is due to a topological charge of the edge states. The notion of the Berry connection is generalized to include the space of all possible boundary conditions, where Chern-Simons forms are shown to be nontrivial.

The search for novel two-dimensional giant Rashba semiconductors is a crucial step in the development of the forthcoming nanospintronic technology. Using first-principles calculations, we study a stable two-dimensional crystal phase of BiSb having buckled honeycomb lattice geometry, which is yet unexplored. The phonon, room temperature molecular dynamics, and elastic constant calculations verify the dynamical and mechanical stability of the monolayer at 0 K and at room temperature. The calculated electronic band structure reveals the direct band gap semiconducting nature of a BiSb monolayer with the presence of a highly mobile two-dimensional electron gas (2DEG) near the Fermi level. Inclusion of spin-orbit coupling yields the giant Rashba spin-splitting of a 2DEG near the Fermi level. The calculated Rashba energy and Rashba splitting constant are 13 meV and 2.3 eVÅ, respectively, which are amongst the largest yet known Rashba spin splitting parameters in 2D materials. We demonstrate that the strength of the Rashba spin splitting can be significantly tuned by applying in-plane biaxial strain on the BiSb monolayer. The presence of the giant Rashba spin splitting together with the large electronic band gap (1.6 eV) makes this system of peculiar interest for optoelectronics applications. Furthermore, we study the electronic properties of BiSb/AlN heterostructures having a lattice mismatch of 1.3% at the interface. Our results suggest that a BiSb monolayer and BiSb/AlN heterostructure systems could be potentially used to develop highly efficient spin field-effect transistors, optoelectronics, and nanospintronic devices. Thus, this comprehensive study of two-dimensional BiSb systems can expand the range of possible applications in future spintronic technology.

Potassium is introduced into the crystalline herringbone structure of pentacene searching for a compound showing metallic electronic transport properties and, hopefully, superconductivity at small enough temperatures. Several possible structures for stoichiometric KPentacene (1:1), ${\mathrm{K}}_2\mathrm{Pentacene}$ (2:1), and ${\mathrm{K}}_3\mathrm{Pentacene}$ (3:1) compounds are theoretically investigated. Detailed densities of states for all of them are presented. As a more prominent result, a new monoclinic structure has been stabilized for the potassium richer material that could correspond to the recently synthesized superconducting phase of ${\mathrm{K}}_3\mathrm{Pentacene}$. Although energetically unfavorable, it is the only metallic candidate found to date.

The magnetization processes of the spin-3/2 antiferromagnet ${\mathrm{LiInCr}}_4{\mathrm{O}}_8$ comprising a “breathing” pyrochlore lattice, which is an alternating array of small and large tetrahedra, are studied under ultrahigh magnetic fields of up to 130 T using state-of-the-art pulsed magnets. A half magnetization plateau is observed above 90 T to 130 T, suggesting that ${\mathrm{LiInCr}}_4{\mathrm{O}}_8$ has a strong spin-lattice coupling, similar to conventional chromium spinel oxides. The magnetization of ${\mathrm{LiGa}}_{0.125}{\mathrm{In}}_{0.875}{\mathrm{Cr}}_4{\mathrm{O}}_8$, in which the structural and magnetic transitions at low temperatures have been completely suppressed, shows a sudden increase above 13 T, indicating that a spin gap of 2.2 meV exists between a tetramer singlet ground state and an excited state with total spin 1, with the latter being stabilized by the application of a magnetic field. The breathing pyrochlore antiferromagnet is found to be a unique frustrated system with strong spin-lattice coupling and bond alternation.

We propose a semiclassical description of the low-energy properties of quantum spin ice in the strong Ising limit. Within the framework of a semiclassical, perturbative Villain expansion, that can be truncated at arbitrary order, we give an analytic and quantitative treatment of the deconfining phase. We find that photon-photon interactions significantly renormalize the speed of light and split the two transverse photon polarizations at intermediate wave vectors. We calculate the photon velocity and the ground-state energy to first and second order in perturbation theory, respectively. Both are in good agreement with recent numerical simulations. We further compute the classical energy of the vison excitation.

We calculate the spectrum of the Andreev bound states in a one-dimensional superconductor with a strong Rashba spin-orbit coupling. We focus on the fate of the zero-energy Andreev modes in the presence of time reversal symmetry breaking perturbations, both at the boundary and in the bulk. It is shown that the zero modes are destroyed by time reversal symmetry breaking fluctuations, even if the mean-field state of the system is time-reversal invariant and topologically nontrivial.

The use of artificial atoms as an active lasing medium opens a way to construct novel sources of nonclassical radiation. An example is the creation of photon-number squeezed light. Here, we present a design of a laser consisting of multiple Cooper-pair transistors coupled to a microwave resonator. Over a broad range of experimentally realizable parameters, this laser creates photon-number squeezed microwave radiation, characterized by a Fano factor $F\ll 1$, at a very high resonator photon number. We investigate the impact of gate-charge disorder in a Cooper-pair transistor and show that the system can create squeezed strong microwave fields even in the presence of maximum disorder.

We present measurements of superconducting transition temperature, resistivity, magnetoresistivity, and temperature dependence of the upper critical field of ${\mathrm{Ta}}_4{\mathrm{Pd}}_3{\mathrm{Te}}_{16}$ under pressures up to 16.4 kbar. All measured properties have an anomaly at $\sim 2–4$ kbar pressure range; in particular there is a maximum in $T_c$ and upper critical field, $H_{c2}\left(0\right)$, and minimum in low temperature, normal state resistivity. Qualitatively, the data can be explained considering the density of state at the Fermi level as a dominant parameter.

Rohart et al. [Phys. Rev. B 93, 214412 (2016)] recently reported calculations indicating two mechanisms for the annihilation of a skyrmion in a Co monolayer on Pt(111). The path calculations were based on a method presented in the Supplemental Material of their article. It is pointed out here that their method does not in general converge on minimum energy paths unlike the geodesic nudged elastic band method which is discussed for comparison. As a result, the maximum energy along the two paths presented does not provide an accurate estimate of the activation energy for the transition and the fact that two different paths were obtained should not be taken as an indication that the skyrmion annihilation can occur by two different mechanisms.

Majorana bound states have been recently observed at the boundaries of one-dimensional topological superconductors. Yet, controlling the localization of the Majorana states, which is essential to the realization of any topological quantum device, is an ongoing challenge. To this end, we introduce a mechanism which can break a topologically homogeneous state via the formation of topological domains, and which can be exploited to control the position of Majorana states. We found, in fact, that in the presence of amplitude-modulated fields, contiguous magnetic domains can become topologically inequivalent and, as a consequence, Majorana states can be pinned to the domain walls of the magnetic structure. The formation of topological domains and the position of Majorana states can be externally controlled by tuning an applied field (e.g., magnetic or gate).

A newly discovered family of high-$T_{\mathrm{c}}$ Fe-based superconductors, $A_{e}A{\mathrm{Fe}}_4{\mathrm{As}}_4$ ($A_e=\text{Ca}$, Sr, Eu and $A=\text{K}$, Rb, Cs), offers further opportunities to understand unconventional superconductivity in these materials. In this Rapid Communication, we report on the superconducting and magnetic properties of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, studied using muon spectroscopy. Zero-field muon spin relaxation studies carried out on the ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ superconductor do not show any detectable magnetic anomaly at $T_{\mathrm{c}}$ or below, implying that time-reversal symmetry is preserved in the superconducting ground state. The temperature dependence of the superfluid density of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ is found to be compatible with a two-gap $s+s$-wave model with gap values of 8.6(4) and 2.5(3) meV, similar to the other Fe-based superconductors. The presence of two superconducting energy gaps is consistent with theoretical and other experimental studies on this material. The value of the penetration depth at $T=0$ K has been determined as $289\left(22\right)$ nm.

Observations of multiferroicity and giant improper ferroelectricity in ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$ invigorated the research on this material. Delicate structural changes have been reported to cause ferroelectricity in the magnetically ordered state of ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$ as well as high-temperature crystallographic transformations. Through high-resolution synchrotron x-ray diffraction experiments we show for the first time the large negative thermal expansion (NTE) behavior of the cubic phase of ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$. A large NTE coefficient ${\alpha }_L$ for a temperature window of around 60 K is inferred for the cubic phase in the phase coexistence region during the high-temperature structural transition. It is explained using quasirigid unit modes. We also observe a large phase coexistence region of about 100 K across the high-temperature first-order structural transition, which is different from previous reports.

Thermal tensor networks constitute an efficient and versatile representation for quantum lattice models at finite temperatures. By Trotter-Suzuki decomposition, one obtains a $D$+1 dimensional TTN for the $D$-dimensional quantum system and then employs efficient renormalizaton group (RG) contractions to obtain the thermodynamic properties with high precision. The linearized tensor renormalization group (LTRG) method, which can be used to contract TTN efficiently and calculate the thermodynamics, is briefly reviewed and then generalized to a bilayer form. We dub this bilayer algorithm as LTRG++ and explore its performance in both finite- and infinite-size systems, finding the numerical accuracy significantly improved compared to single-layer algorithm. Moreover, we show that the LTRG++ algorithm in an infinite-size system is in essence equivalent to transfer-matrix renormalization group method, while reformulated in a tensor network language. As an application of LTRG++, we simulate an extended fermionic Hubbard model numerically, where the phase separation phenomenon, ground-state phase diagram, as well as quantum criticality-enhanced magnetocaloric effects, are investigated.

We measured magnetization, specific heat, electron spin resonance, neutron diffraction, and inelastic neutron scattering of ${\mathrm{CrVMoO}}_7$ powder. An antiferromagnetically ordered state appears below $T_{\mathrm{N}}=26.5\pm 0.8$ K. We consider that the probable spin model for ${\mathrm{CrVMoO}}_7$ is an interacting antiferromagnetic spin-$\frac{3}{2}$ dimer model. We evaluated the intradimer interaction $J$ to be $25\pm 1$ K and the effective interdimer interaction $J_{\mathrm{eff}}$ to be $8.8\pm 1$ K. ${\mathrm{CrVMoO}}_7$ is a rare spin dimer compound that shows an antiferromagnetically ordered state at atmospheric pressure and zero magnetic field. The magnitude of ordered moments is $0.73\left(2\right){\mu }_{\mathrm{B}}$. It is much smaller than a classical value $\sim 3{\mu }_{\mathrm{B}}$. Longitudinal-mode magnetic excitations may be observable in single crystalline ${\mathrm{CrVMoO}}_7$.

The theoretical understanding of the nematic state of iron-based superconductors and especially of FeSe is still a puzzling problem. Although a number of experiments call for a prominent role of local correlations and place iron superconductors at the entrance of a Hund's metal state, the effect of the electronic correlations on the nematic state has been theoretically poorly investigated. In this work we study the nematic phase of iron superconductors accounting for local correlations, including the effect of the Hund's coupling. We show that Hund's physics strongly affects the nematic properties of the system. It severely constrains the precise nature of the feasible orbital-ordered state and induces a differentiation in the effective masses of the $zx/yz$ orbitals in the nematic phase. The latter effect leads to distinctive signatures in different experimental probes overlooked so far in the interpretation of experiments. As notable examples the splittings between $zx$ and $yz$ bands at $\mathrm{\Gamma }$ and $M$ points are modified, with important consequences for angle-resolved photoemission spectroscopy measurements.

We discuss thermal transport through a Josephson junction in a time-dependent situation. We write the spectral representation of the heat current pumped by a generic drive. This enables separation of the dissipative and reactive contributions, of which the latter does not contribute to long-time averages. We discuss the physical interpretation, and note that the condensate heat current identified by Maki and Griffin [Phys. Rev. Lett. 15, 921 (1965)] is purely reactive. The results enable a convenient description of heat exchanges in a Josephson system in the presence of an external drive, with possible applications for the implementation of cooling devices.

${}^{31}\mathrm{P}$ NMR measurements have been made on polycrystalline samples to study a metal-insulator (MI) transition and magnetic fluctuations in ${\mathrm{Ru}}_{1-x}{\mathrm{Rh}}_x\mathrm{P}$ which has metallic (M), pseudogap (PG), insulating (I), and superconducting (SC) phases. We find that RuP undergoes a crossover from the high-temperature (high-$T$) M phase to the PG phase with the pseudo spin-gap behavior probed by the nuclear spin-lattice relaxation rate at $T_{\mathrm{PG}}=330\mathrm{K}$. The first-order MI transition is observed to take place from the PG phase to the low-$T$ nonmagnetic I phase with the spin-gap energy of 1250 K at $T_{\mathrm{MI}}=270\mathrm{K}$. In the PG phase of ${\mathrm{Ru}}_{1-x}{\mathrm{Rh}}_x\mathrm{P}$ with $0\le x<0.45$, an analysis based on the modified Korringa relation, which is applicable to an itinerant paramagnet with weak electron correlation, shows that antiferromagnetic (AFM) fluctuations described in the random-phase approximation are enhanced in the low-$T$ and low-$x$ region. Around the PG-M phase boundary at $x_{\mathrm{c}}\sim 0.45$, there is the SC phase whose normal state has negligible electron-electron interaction. We discuss the MI transition, the crossover from the M phase to the PG phase, and the magnetic properties of each phase based on the band structure.