Tailoring and controlling magnetic properties is an important factor for materials design. Here, we present a case study for Ni-based Heusler compounds of the type ${\mathrm{Ni}}_{2}YZ$ with $Y=$ Mn, Fe, Co and $Z=$ B, Al, Ga, In, Si, Ge, Sn based on first-principles electronic structure calculations. These compounds are interesting since the materials properties can be quite easily tuned by composition and many of them possess a noncubic ground state being a prerequisite for a finite magnetocrystalline anisotropy (MAE). We discuss systematically the influence of doping at the $Y$ and $Z$ sublattices as well as the effect of lattice deformation on the MAE. We show that in case of ${\mathrm{Ni}}_2\mathrm{Co}Z$ the phase stability and the MAE can be improved using quaternary systems with elements from main group III and IV on the $Z$ sublattice whereas changing the $Y$ sublattice occupation by adding Fe does not lead to an increase of the MAE. Furthermore, we studied the influence of the lattice ratio on the MAE. Showing that small deviations can lead to a doubling of the MAE as in case of ${\mathrm{Ni}}_2\mathrm{FeGe}$. Even though we demonstrate this for a limited set of systems, the findings may carry over to other related systems.

Herein, we have systematically reported the synthesis, structure, magnetism, and electrical/thermal transport properties of $\mathrm{AsNC}{\mathrm{r}}_3$. Tetragonal-antiperovskite metallic $\mathrm{AsNC}{\mathrm{r}}_3$ exhibits a second-order ferromagnetic (FM)-paramagnetic (PM) transition around 243 K, where the resistivity and specific heat also show a related change. Based on the obtained parameters from basic physical properties, the nature of weak electron-electron correlation was confirmed by both the Kadowaki-Woods ratio and Wilson ratio. Moreover, the value of the Rhodes-Wolfarth ratio (1.312) is larger than 1, suggesting an itinerant ferromagnetism in $\mathrm{AsNC}{\mathrm{r}}_3$. To further clarify the magnetic interaction of the $\mathrm{AsNC}{\mathrm{r}}_3$ system, the study of critical behavior around the FM-PM transition has been performed. The critical exponents ($\beta$, $\gamma$, and $\delta$) of $\mathrm{AsNC}{\mathrm{r}}_3$ obtained from different methods are very close to the theoretical values of the mean-field model ($\beta =0.5$, $\gamma =1.0$, and $\delta =3.0$), indicating the existence of a long-range FM interaction. Besides, the exchange interaction distance $J\left(r\right)\sim r^{-4.3}$ for $\mathrm{AsNC}{\mathrm{r}}_3$ decreases more slowly than that of mean-field model $J\left(r\right)\sim r^{-4.5}$, further confirming the long-range magnetic coupling in our system. The strong hybridization between Cr-$3d$ and N-$2p$ states as well as between Cr-$3d$ and As-$4p$ states confirmed by our theoretical calculations results in the itinerant characterization of carriers and the long-range FM interaction in tetragonal-antiperovskite $\mathrm{AsNC}{\mathrm{r}}_3$.

First-principles calculations of ionic-liquid-gated (Li,Fe)OHFeSe suggest a metal-insulator transition at a nominal Li/Fe ratio of 75/25 in the (Li,Fe)OH layer. While doping increases Fermi energy, the formation of an antiferromagnetic bicollinear phase is the key for the transition. It is expected that this insulating phase also exists in other FeSe systems upon heavy electron doping, and its presence can hinder the increase of superconducting temperature. These results offer clues on how to optimize superconductivity amid its interplay with magnetic properties in FeSe systems.

Recent experimental evidences point to rotation symmetry-breaking superconductivity in doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, where the relevant order parameter belongs to a two-component odd-parity representation $E_u$ of the crystal point group. The $E_u$ channel admits two possible phases, the nematic phase, that well explains the reported rotation symmetry breaking, and the chiral phase, that breaks time-reversal symmetry. In weakly anisotropic three-dimensional (3D) systems the nematic phase is the stable one. We study the stability of the nematic phase versus the chiral phase as a function of the anisotropy of the Fermi surface and the thickness of the sample and show that by increasing the 2D character of the Fermi surface or by reducing the number of layers in thin slabs the chiral phases is favored. For the extreme 2D limit composed by a single layer of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ the chiral phase is always the stable one and the system hosts two chiral Majorana modes flowing at the boundary of the system.

Superconducting qubits are hampered by flux noise produced by surface spins from a variety of microscopic sources. Recent experiments indicated that hydrogen (H) atoms may be one of those sources. Using density functional theory calculations, we report that H atoms either embedded in, or adsorbed on, an $\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\left(0001\right)$ surface have sizable spin moments ranging from 0.81 to $0.87{\mu }_B$ with energy barriers for spin reorientation as low as $\sim 10\mathrm{mK}$. Furthermore, H adatoms on the surface attract gas molecules such as ${\mathrm{O}}_2$, producing new spin sources. We propose coating the surface with graphene to eliminate H-induced surface spins and to protect the surface from other adsorbates.

In an attempt to confirm the end of the $\frac{1}{3}$ magnetization plateau of volborthite single crystals, Faraday rotation measurements were taken in magnetic fields up to 182 T at low temperatures. Depending on whether or not the sample setting condition is free-standing, two types of $\frac{1}{3}$ plateaus have been observed: One ends around 100 T with a plateau width about $80–90$ T, and the other continues above 160 T, exhibiting an extremely wide plateau (more than 140 T). These contrasting differences are explained by the way in which the $\frac{1}{3}$-plateau phase changes with or without a lattice distortion associated with the crystal symmetry.

Here, we use angle-resolved photoemission spectroscopy to study superconductivity that emerges in two extreme cases, from a Fermi-liquid phase (LiFeAs) and an incoherent bad-metal phase (${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$). We find that although the electronic coherence can strongly reshape the single-particle spectral function in the superconducting state, it is decoupled from the maximum-superconducting-gap and $T_c$ ratio $2{\mathrm{\Delta }}_{max}/k_B{T}_c$, which shows a universal scaling that is valid for all iron-based superconductors (FeSCs). Our observation excludes pairing scenarios in the BCS and the BEC limit for FeSCs and calls for a universal strong-coupling pairing mechanism for the FeSCs.

Optical control of structure-driven magnetic order offers a platform for magneto-optical terahertz devices. We control the magnetic phases of $d^1$ Mott insulating titanates using nonlinear phononics to transiently perturb the atomic structure based on density functional theory (DFT) simulations and solutions to a lattice Hamiltonian including nonlinear multimode interactions. We show that magnetism is tuned by indirect excitation of a Raman-active phonon mode, which affects the amplitude of the ${\mathrm{TiO}}_6$ octahedral rotations that couple to static Ti-O Jahn-Teller distortions, through driven infrared-active modes of ${\mathrm{LaTiO}}_3$ and ${\mathrm{YTiO}}_3$. The mode excitation reduces the rotational angle, driving a magnetic phase transition from a ferromagnetic (FM) to $G$-type antiferromagnetic (AFM) state. A novel $A$-type AFM state hidden in the bulk equilibrium phase diagram emerges as a dynamically accessible optically induced phase under multimode excitations. Our work shows that nonlinear phononics can stabilize phases inaccessible to static chemical substitutions or lattice strains.

The observation of unusual metastable behavior in ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ remains an ongoing puzzle. When the magnetic field is increased at certain very slow rates, evenly spaced steps occur in the magnetization and other physical quantities as a function of magnetic field every 1.2 T. The ground state without steps is approached by slow relaxation processes over hours to days. It is a striking example of extremely slow dynamics arising from geometrical frustration in an otherwise clean and long-range ordered system, and its precise description remains controversial. Here we shed light on the mystery by reporting similar behavior in isostructural ${\mathrm{Ca}}_3{\mathrm{Co}}_{2-x}{\mathrm{Mn}}_x{\mathrm{O}}_6$, albeit at magnetic field sweep rates that are six orders of magnitude faster than that in ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$. We observe these steps not only in the magnetization, but also in the magnetostriction, electric polarization, and magnetocaloric effect. We present a study of metastable behavior in both compounds across seven orders of magnitude of magnetic field sweep rates using unique magnets at the National High Magnetic Field Laboratory. The metastable steps occur for intermediate ranges of magnetic field sweep rates, vanishing or evolving for the fastest and slowest sweep rates. Our data support that these metastable steps are an intrinsic feature of the three-dimensionally frustrated geometry of Ising spins in this structural family and thus narrow the number of applicable models.

We report the formation of a macroscopic coherent state emerging from colliding polariton fluids. Four lasers with random relative phases, arranged in a square, pump resonantly a planar microcavity, creating four coherent polariton fluids propagating toward each other. When the density (interactions) increases, the four fluids synchronize and the topological excitations (vortex or soliton) disappear to form a single quantum superfluid.

We report measurements of the superconducting properties of the two-dimensional (2D) gas that forms at the interface between ${\mathrm{LaAlO}}_3$ (LAO) and ${\mathrm{SrTiO}}_3$ (STO) in the (111) crystal orientation, a system that permits in situ tuning of carrier density and disorder by means of a back-gate voltage $V_g$. Like the (001) oriented LAO/STO interface, superconductivity at the (111) LAO/STO interface can be tuned by $V_g$. The 2D superconductivity in these (111) LAO/STO samples shows an in-plane anisotropy, being different along different interface crystal directions, and “remembers” the disorder landscape at which they are cooled through the superconducting transition. The low energy scale and other characteristics of this memory effect distinguish it from charge-trapping effects previously observed in (001) interface samples.

We report a combined infrared and angle-resolved photoemission study of the electronic response of $\mathrm{S}{\mathrm{r}}_3{\left(\mathrm{I}{\mathrm{r}}_{1-x}\mathrm{R}{\mathrm{u}}_x\right)}_2{\mathrm{O}}_7$ ($x=0$, 0.22, 0.34). The low-temperature optical conductivities of the three compounds exhibit the characteristic feature of the effective total angular momentum $J_{\mathrm{eff}}=1/2$ antiferromagnetic Mott state. As the temperature increases across the antiferromagnetic ordering temperature $T_{\mathrm{N}}$, the indirect gap gradually closes whereas the direct gap remains open. In the optical conductivity of $\mathrm{S}{\mathrm{r}}_3{\left(\mathrm{I}{\mathrm{r}}_{0.66}\mathrm{R}{\mathrm{u}}_{0.34}\right)}_2{\mathrm{O}}_7$ which shows a thermally driven insulator-metal transition at $T_{\mathrm{N}}$, a Drude-like response from itinerant carriers is registered in the paramagnetic phase. We observe in angle-resolved photoemission data of $\mathrm{S}{\mathrm{r}}_3{\left(\mathrm{I}{\mathrm{r}}_{0.66}\mathrm{R}{\mathrm{u}}_{0.34}\right)}_2{\mathrm{O}}_7$ that the valence band shifts continuously toward the Fermi energy with the weakening of the antiferromagnetic order and crosses the Fermi level in the paramagnetic phase. Our findings demonstrate that the temperature-induced metal-insulator transition of the $\mathrm{S}{\mathrm{r}}_3{\left(\mathrm{I}{\mathrm{r}}_{1-x}\mathrm{R}{\mathrm{u}}_x\right)}_2{\mathrm{O}}_7$ system should be attributed to a magnetically driven band shift.

We describe the coherent dynamics of electrical transport through a localized spin-dependent state, such as is associated with a defect spin, at the interface of a ferromagnet and a nonmagnetic material during ferromagnetic resonance. As the ferromagnet magnetic moment precesses, charge carriers are dynamically spin filtered by the localized state, leading to a dynamic spin accumulation on the defect. Local effective magnetic fields modify the precession of a spin on the defect, which also modifies the time-integrated total charge current through the defect. We thus identify a form of current-detected spin resonance that reveals the local magnetic environment of a carrier spin located at a defect, and thus potentially the defect's identity.

We compare three different notions of concurrence to measure the polarization entanglement of two-photon states generated by the biexciton cascade in a quantum dot embedded in a microcavity. The focus of the paper lies on the often-discussed situation of a dot with finite biexciton binding energy in a cavity tuned to the two-photon resonance. Apart from the time-dependent concurrence, which can be assigned to the two-photon density matrix at any point in time, we study single- and double-time integrated concurrences commonly used in the literature that are based on different quantum state reconstruction schemes. In terms of the photons detected in coincidence measurements, we argue that the single-time integrated concurrence can be thought of as the concurrence of photons simultaneously emitted from the cavity without resolving the common emission time, while the more widely studied double-time integrated concurrence refers to photons that are neither filtered with respect to the emission time of the first photon nor with respect to the delay time between the two emitted photons. Analytic and numerical calculations reveal that the single-time integrated concurrence indeed agrees well with the typical value of the time-dependent concurrence at long times, even when the interaction between the quantum dot and longitudinal acoustic phonons is accounted for. Thus the more easily measurable single-time integrated concurrence gives access to the physical information represented by the time-dependent concurrence. However, the double-time integrated concurrence shows a different behavior with respect to changes in the exciton fine structure splitting and even displays a completely different trend when the ratio between the cavity loss rate and the fine structure splitting is varied while keeping their product constant. This implies the nonequivalence of the physical information contained in the time-dependent and single-time integrated concurrence on the one hand and the double-time integrated concurrence on the other hand.

We consider theoretically the influence of crystalline fields on the electronic structure of graphene placed on a layered material with reduced symmetry and large spin-orbit coupling (SOC). We use a perturbative procedure combined with the Slater-Koster method to derive the low-energy effective Hamiltonian around the $K$ points and estimate the magnitude of the effective couplings. Two simple models for the envisaged graphene-substrate hybrid bilayer are considered, in which the relevant atomic orbitals hybridize with either top or hollow sites of the graphene honeycomb lattice. In both cases, the interlayer coupling to a crystal-field-split substrate is found to generate highly anisotropic proximity spin-orbit interactions, including in-plane “spin-valley” coupling. Interestingly, when an anisotropic intrinsic-type SOC becomes sizable, the bilayer system is effectively a quantum spin Hall insulator characterized by in-plane helical edge states robust against the Bychkov-Rashba effect. Finally, we discuss the type of substrate required to achieve anisotropic proximity-induced SOC and suggest possible candidates to further explore crystal-field effects in graphene-based heterostructures.

Using the constraints imposed by the crystalline symmetry of FeSe and the experimentally observed phenomenology, we analyze the possible pairing symmetry of the superconducting order parameter focusing on intercalated and monolayer FeSe compounds. Such analysis leads to three possible pairing symmetry states—$s$ wave, $d$ wave, and helical $p$ wave. Despite the differences in the pairing symmetry, each of these states is fully gapped with gap minimum centered above the normal state Fermi surface, in agreement with photoemission data of Zhang et al. [Phys. Rev. Lett. 117, 117001 (2016)]. The analysis provides additional insights into the possible pairing mechanism for each of these states, highlighting the detrimental role of the renormalized repulsive intraorbital Hubbard $U$ and interorbital $U^{\prime }$ and the beneficial role of the pair hopping $J^{\prime }$ and the Hund's $J$ terms, as well as the spin-orbit coupling in the effective low-energy Hamiltonian.