$\left(\mathrm{T}{\mathrm{m}}_{1-x}\mathrm{M}{\mathrm{n}}_x\right)\mathrm{Mn}{\mathrm{O}}_3$ solid solutions were synthesized at a high pressure of 6 GPa and a high temperature of about 1570–1670 K for 2 h for $x=0$, 0.1, 0.2, and 0.3. Magnetic, dielectric, and neutron diffraction measurements revealed that the introduction of magnetic ${\mathrm{Mn}}^{2+}$ cations into the A site leads to an incommensurate spin structure for $x=0.1$ and to a ferrimagnetic structure for $x\ge 0.2$. Commensurate magnetic structures have a much larger correlation length $(\sim 400\mathrm{nm}$ for $x=0,\sim 600\mathrm{nm}$ for $x=0.3)$ than the incommensurate magnetic structure $(\sim 12\mathrm{nm}$ for $x=0.1)$. The presence of ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Mn}}^{2+}$ (with different sizes) at the A site causes significant microstrain effects along the $a$ direction which are absent for $x=0$ and get stronger with increasing $x$. Magnetic ordering occurs at the Néel temperature $T_N=37\mathrm{K}\left(x=0.1\right)$ and at the ferrimagnetic Curie temperatures $T_C=75\mathrm{K}\left(x=0.2\right)$ and $T_C=104\mathrm{K}\left(x=0.3\right)$. Ordering of magnetic Mn moments triggers short-range order (for $x=0.1)$ and long-range order (for $x\ge 0.2)$ of the ${\mathrm{Tm}}^{3+}$ cations at the same temperature—an unusual situation in perovskite materials with a simple ${\mathrm{GdFeO}}_3$-type $Pnma$ structure. For $x=0.1$, long-range IC magnetic order [with propagation vector $\mathbf{k}=(k_0,0,0)$ and $k_0\approx 0.40]$ of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ cations at the B site coexists with short-range order of ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Mn}}^{2+}$ moments at the A site. Short-range order is induced at the Néel temperature $T_N=37\mathrm{K}$, increases towards an additional specific heat anomaly at $T=4\mathrm{K}$, and remains at lower temperature. The ferrimagnetic structure [with propagation vector $\mathbf{k}=(0,0,0)]$ consists of ferromagnetically ordered ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ cations at the B site which are coupled antiferromagnetically with ordered ${\mathrm{Mn}}^{2+}$ moments at the A site. ${\mathrm{Tm}}^{3+}$ moments adopt a zigzag magnetic structure which contains a macroscopic ferromagnetic moment that aligns with the direction of the ordered ${\mathrm{Mn}}^{2+}$ moments. Towards low temperature, the ordered ${\mathrm{Tm}}^{3+}$ moments strongly increase and overcome the saturated magnetic Mn moments at the B site, and this behavior results in the observation of magnetization reversal or negative magnetization phenomena with a compensation temperature of about 15 K at small magnetic fields in the $x=0.2$ and 0.3 samples. This is a classical mechanism of the magnetization reversal effects for ferrimagnets.

We present ${}^{75}\mathrm{As}$ nuclear magnetic and quadrupole resonance results (NMR, NQR) on a new set of $\mathrm{La}\mathrm{Fe}\mathrm{As}{\mathrm{O}}_{1-x}{\mathrm{F}}_x$ polycrystalline samples. Improved synthesis conditions led to more homogenized samples with better control of the fluorine content. The $\mathrm{structural}\equiv \mathrm{nematic}$, magnetic, and superconducting transition temperatures have been determined by NMR spin-lattice relaxation rate and AC susceptibility measurements. The so-determined phase diagram deviates from the published one, especially for low F-doping concentrations. However, if the doping level is determined from the NQR spectra, both phase diagrams can be reconciled. The absence of bulk coexistence of magnetism and superconductivity and a nanoscale separation into low-doping-like and high-doping-like regions have been confirmed. Additional frequency-dependent intensity, spin-spin, and spin-lattice relaxation rate measurements on underdoped samples at the boundary of magnetism and superconductivity indicate that orthorhombicity and magnetism originate from the low-doping-like regions, and superconductivity develops first in the high-doping-like regions.

ZrSiS-type compounds are Dirac semimetals and thus have been attracting considerable interest in recent years due to their topological electronic properties and possible technological applications. In particular, gapped Dirac nodes can possess large spin Berry curvatures and thus give rise to a large spin Hall effect (SHE) and a large spin Nernst effect (SNE), which may be used to generate pure spin current for spintronics and spin caloritronics without applied magnetic field or magnetic material. In this paper we study both SHE and SNE in $\mathrm{Zr}XY (X=\mathrm{Si},\mathrm{Ge};Y=\mathrm{S},\mathrm{Se},\mathrm{Te})$ based on ab initio relativistic band structure calculations. Our theoretical calculations reveal that some of these compounds exhibit large intrinsic spin Hall conductivity (SHC) and spin Nernst conductivity. For example, the calculated SHC of ZrSiTe is as large as $-755$ ($\hslash /e$) (S/cm). Since the electric conductivity of these Dirac semimetals is much smaller than that of platinum, which has the largest intrinsic SHC of $\sim 2200$ ($\hslash /e$) (S/cm), this indicates that they will have a larger spin Hall angle (and hence a more efficient charge-spin-current conversion) than that of platinum. Remarkably, we find that both the magnitude and sign of the SHE and SNE in these compounds can be significantly tuned by changing either the electric field direction or spin current direction and may also be optimized by slightly varying the Fermi level via chemical doping. Analysis of the calculated band-decomposed and $k$-resolved spin Berry curvatures shows that the large SHE and SNE as well as their remarkable tunabilities originate from the presence of many slightly spin-orbit coupling-gapped Dirac nodal points and lines near the Fermi level in these Dirac semimetals. Our findings thus suggest that ZrSiS-type compounds are promising candidates for spintronic and spin caloritronic devices and will certainly stimulate further experiments on these Dirac semimetals.

The nature of quasiparticles in two-dimensional quantum antiferromagnets at finite temperature remains an open question despite decades of theoretical work. In particular, it is not fully understood how long wavelength excitations contribute to significant broadening of the experimentally observable spectrum. Motivated by this problem, we consider the $XY$ model of easy-plane antiferromagnets, and compute the dynamic structure factor by direct summation of diagrams. In doing so, we find that noninteracting quasiparticles with infinite lifetimes can still lead to a broad response. This forms the basis for a new paradigm describing the interaction of experimental probes with a physical system, where broadening is due neither to the lifetime, nor to the emergence of fractional quasiparticles. Instead, strong fluctuations drive the probe to absorb and radiate an infinite number of arbitrarily low energy quasiparticles, leading us to draw parallels with the infrared catastrophe in quantum electrodynamics.

The interfacial Dzyaloshinskii-Moriya interaction (iDMI) is of great interest in thin-film magnetism because of its ability to stabilize chiral spin textures. It can be quantified by investigating the frequency nonreciprocity of oppositely propagating spin waves. However, as the iDMI is an interface interaction, the relative effect reduces when the films become thicker, making quantification more difficult. Here, we utilize all-electrical propagating spin-wave spectroscopy to disentangle multiple contributions to spin wave frequency nonreciprocity to determine the iDMI. This is done by investigating nonreciprocities across a wide range of magnetic layer thicknesses (from 4 to 26 nm) in Pt/Co/Ir, Pt/Co/Pt, and Ir/Co/Pt stacks. We find the expected sign change in the iDMI when inverting the stack order and a negligible iDMI for the symmetric Pt/Co/Pt. We additionally extract a difference in surface anisotropies and find a large contribution due to the formation of different crystalline phases of the Co, which is corroborated using nuclear magnetic resonance and high-resolution transmission-electron-microscopy measurements. These insights will open up avenues to investigate, quantify, and disentangle the fundamental mechanisms governing the iDMI, and pave a way toward engineered large spin-wave nonreciprocities for magnonic applications.

There is considerable current interest to explore electronic topology in strongly correlated metals, with heavy fermion systems providing a promising setting. Recently, a Weyl-Kondo semimetal phase has been concurrently discovered in theoretical and experimental studies. The theoretical work was carried out in a Kondo lattice model that is time-reversal invariant but inversion-symmetry breaking. In this paper, we show in some detail how nonsymmorphic space-group symmetry and strong correlations cooperate to form Weyl nodal excitations with highly reduced velocity and pin the resulting Weyl nodes to the Fermi energy. A tilted variation of the Weyl-Kondo solution is further analyzed here, following the recent consideration of such effect in the context of understanding a large spontaneous Hall effect in ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pd}}_3$ (Dzsaber et al., arXiv:1811.02819). We discuss the implications of our results for the enrichment of the global phase diagram of heavy fermion metals, and for the space-group symmetry enforcement of topological semimetals in other strongly correlated settings.

We investigate a chain of spinless fermions with nearest-neighbor interactions that are subject to a local loss process. We determine the time evolution of the system using matrix product state methods. We find that at intermediate times, a metastable state is formed, which has very different properties than usual equilibrium states. In particular, in a region around the loss, the filling is reduced, while Friedel oscillations with a period corresponding to the original filling continue to exist. The associated momentum distribution is emptied at all momenta by the loss process and the Fermi edge remains approximately at its original value. Even in the presence of strong interactions, where a redistribution by the scattering is naively expected, such a regime can exist over a long timescale. Additionally, we point out the existence of the interaction-dependent Zeno effect in such a system.

We numerically investigate the odd-parity states of the green exciton series in cuprous oxide. Taking into account the coupling to the yellow series and especially to the yellow continuum, the green excitons are quasibound resonances with a finite lifetime which cannot be described with Hermitian operators. To calculate their positions and linewidths, we use the method of complex-coordinate rotation, leading to a non-Hermitian complex eigenvalue problem. We find that the behavior of the dominant $P$ states is very well approximated by a modified Rydberg formula using a negative quantum defect. The corresponding linewidths induced by the coupling to the yellow continuum decrease with the third power of the principal quantum number.

Quantum point contact devices are indispensable tools for probing the edge structure of the fractional quantum Hall (FQH) states. Recent observations of quantized conductance plateaus accompanied by shot noise in such devices, as well as suppression of Mach-Zehnder interference, call for theoretical explanations. In this paper we develop a theory of FQH edge state transport through quantum point contacts, which allows for a generic Abelian edge structure and assumes strong equilibration between edge modes (incoherent transport regime). We find that conductance plateaus are found whenever the quantum point contact locally depletes the Hall bar to a stable region with a filling factor lower than that of the bulk and the resulting edge states equilibrate. The shot noise generated on these plateaus can be classified according to 13 possible combinations of edge charge and heat transport in the device. We also comment on a relation between the the emergence of quantized plateaus and the suppression of Mach-Zehnder interference. Besides explaining recent experimental findings, our results provide novel insights and perspectives on quantum point contact devices in the FQH regime.

Geometrical singularities in plasmonic metasurfaces were recently proposed for the enhancement of light-matter interactions, owing to their broadband light-harvesting properties and extreme plasmon confinement. However, the large plasmon momenta thus achieved lead to failure of local descriptions of the optical response of metals. Here we study a class of metasurfaces consisting of a periodic metal slab with a smooth modulation of its thickness. When the thinnest part shrinks, the two surfaces almost touch, forming a near-singular point. Using transformation optics, we show analytically how nonlocal effects, such as a blueshift of the resonance peaks and a reduced density of states, become important and cannot be ignored in this singular regime. The method developed in this paper is very general and can be used to model a variety of metasurfaces, providing valuable insight in the current context of ultrathin plasmonic structures.

Topological insulators (TIs) are materials that are insulating in the bulk but have zero band-gap surface states with linear dispersion and are protected by time-reversal symmetry. These unique characteristics could pave the way for many promising applications that include spintronic devices and quantum computations. It is important to understand and theoretically describe TIs as accurately as possible to predict properties. Quantum mechanical approaches, specifically first-principles density-functional-theory (DFT)-based methods, have been used extensively to model electronic properties of TIs. Here we provide a comprehensive assessment of a variety of DFT formalisms and how these capture the electronic structure of TIs. We concentrate on ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ as examples of prototypical TI materials. We find that the generalized gradient approximation (GGA) and kinetic density functional (metaGGA) increase the thickness of the TI slab, whereas we see the opposite behavior in DFT computations using LDA. Accounting for van der Waals (vdW) interactions overcomes the apparent over-relaxations and retraces the atomic positions toward the bulk. Based on a systematic computational study, we show that GGA with vdW treatment is an appropriate method for structural optimization. However, the vdW corrections recover the experimental bulk parameters, and do not influence the charge density implicitly. Thus, electronic structures derived from the base GGA functional, employing experimental lattice parameters, is sufficient.

Fractionalized quasiparticles—anyons—bear a special role in present-day physics. At the same time, they display properties of interest both foundational, with quantum numbers that transcend the spin-statistics laws, and applied, providing a cornerstone for decoherence-free quantum computation. The development of platforms for realization and manipulation of these objects, however, remains a challenge. Typically these entail the zero-temperature ground state of incompressible, gapped fluids. Here we establish a strikingly different approach: The development and probing of anyon physics in a gapless fluid. The platform of choice is a chiral, multichannel, multi-impurity realization of the Kondo effect. We discuss how, in the proper limit, anyons appear at magnetic impurities, protected by an asymptotic decoupling from the fluid and by the emerging Kondo length scale. We discuss possible experimental realization schemes using integer quantum Hall edges. The gapless and charged degrees of freedom that coexist with the anyons suggest the possibility of extracting quantum information data by transport and simple correlation functions. To show that this is the case, we generalize the fusion ansatz of Cardy's boundary conformal field theory, now in the presence of multiple localized perturbations. The generalized fusion ansatz captures the idea that multiple impurities share quantum information nonlocally, in a way formally identical to anyonic zero modes. We display several examples supporting and illustrating this generalization and the extraction of quantum information data out of two-point correlation functions. With the recent advances in mesoscopic realizations of multichannel Kondo devices, our results imply that exotic anyon physics may be closer to reach than presently imagined.

The Kondo insulator ${\mathrm{SmB}}_6$ has emerged as a primary candidate for exotic quantum phases, due to the predicted formation of strongly correlated, low-velocity topological surface states and corresponding high Fermi-level density of states. However, measurements of the surface-state velocity in ${\mathrm{SmB}}_6$ differ by orders of magnitude, depending on the experimental technique used. Here we reconcile two techniques, scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), by accounting for surface band bending on polar terminations. Using spatially resolved scanning tunneling spectroscopy, we measure a band shift of $\sim 20$ meV between full-Sm and half-Sm terminations, in qualitative agreement with our density-functional theory calculations of the surface charge density. Furthermore, we reproduce the apparent high-velocity surface states reported by ARPES by simulating their observed spectral function as an equal-weight average over the two band-shifted domains that we image by STM. Our results highlight the necessity of local measurements to address inhomogeneously terminated surfaces or fabrication techniques to achieve uniform termination for meaningful large-area surface measurements of polar crystals such as ${\mathrm{SmB}}_6$.

We analyze the evolution of the exciton recombination energy $E_{\mathrm{PL}}$ and its pressure coefficient $d{E}_{\mathrm{PL}}/dp$, with the laser power density (LPD) exciting excitonic photoluminescence. Two $\mathrm{I}{\mathrm{n}}_{0.17}\mathrm{G}{\mathrm{a}}_{0.83}\mathrm{N}/\mathrm{GaN}$ quantum well (QW) structures are used: sample A consisting of a 5.2-nm-wide single QW and sample B consisting of two 2.6-nm-wide QWs separated by a 0.78-nm-thick barrier. Changes of $E_{\mathrm{PL}}$ and $d{E}_{\mathrm{PL}}/dp$ with LPD in both structures cover a wide range of magnitude. They switch character in the vicinity of the indirect to direct exciton induced by increasing LPD. In sample B, at low excitation, a negative pressure coefficient of $E_{\mathrm{PL}}$ has been found. Moreover, pressure dependences of $E_{\mathrm{PL}}$ are different for the two samples under study. We examined pressure dependence of the threshold LPD for indirect-direct exciton switching in both samples. The reported observations originate to a high extent from screening of a built-in electric field by carriers externally introduced into the quantum wells. We also note that the universal relation between transition energies and their pressure coefficients (observed previously in InGaN/GaN quantum wells) does not hold for samples modified by screening.

We study both the temperature and band-gap dependence of the first- and third-order intraband conductivities of a Weyl semimetal (WSM) as well as the resultant critical field. By a semiclassical model we further explore how different chemical potentials, cone separations, field geometries, and finite relaxation times affect photoresponse. We determine that the band-gap variation is characterized by the Weyl cone separation and observe significant thermal enhancement of the nonlinear response. Universal behaviors of both the linear and nonlinear response have also been observed under certain conditions. Coupled with experimentally accessible critical field values of ${10}^4$–${10}^5$ V ${\mathrm{m}}^{-1}$, our results give strong impetus for the use of WSMs as tunable photo-mixers.

Using the tight-binding approach we study the electronic transport in a ${\mathrm{MoS}}_2$ zigzag ribbon with a spatially varying potential profile. Considering a ribbon with a smooth potential step in the Fermi energy regime where the transport is dominated by the edge modes, we find that the conductance exhibits sharp resonances due to the resonant transport through a n-p-n junction effectively created in the structure. We show that in a gated wire the current carried on the wire edges can be blocked despite the metallic band structure of the ribbon. For the Fermi energies corresponding to ${\mathrm{MoS}}_2$ bulk conduction band, we identify states of the semi-infinite wire that are polarized in the $K$, $K^{\prime }$, $Q$ valleys and exhibit the valley Hall effect distinctly visible in a nonuniform ribbon. Finally, we show that well-defined momenta of the valley polarized modes allow nearly complete valley polarization of the current in a locally gated ribbon.

Motivated by experimental and theoretical interest in realizing multipolar orders in $d$-orbital materials, we discuss the quantum magnetism of $J=2$ ions which can be realized in spin-orbit coupled oxides with $5d^2$ transition metal ions. Based on the crystal-field environment, we argue for a splitting of the $J=2$ multiplet, leading to a low-lying non-Kramers doublet which hosts quadrupolar and octupolar moments. We discuss a microscopic mechanism whereby the combined perturbative effects of orbital repulsion and antiferromagnetic Heisenberg spin interactions leads to ferro-octupolar coupling between neighboring sites, and stabilizes ferro-octupolar order for a face-centered cubic lattice. This same mechanism is also shown to disfavor quadrupolar ordering. We show that studying crystal field levels via Raman scattering in a magnetic field provides a probe of octupolar order. We study spin dynamics in the ferro-octupolar state using a slave-boson approach, uncovering a gapped and dispersive magnetic exciton. For sufficiently strong magnetic exchange, the dispersive exciton can condense, leading to conventional type-I antiferromagnetic (AFM) order which can preempt octupolar order. Our proposal for ferrooctupolar order, with specific results in the context of a model Hamiltonian, provides a comprehensive understanding of thermodynamics, $\mu \mathrm{SR}$, x-ray diffraction, and inelastic neutron-scattering measurements on a range of cubic $5d^2$ double perovskite materials including ${\mathrm{Ba}}_2{\mathrm{ZnOsO}}_6$, ${\mathrm{Ba}}_2{\mathrm{CaOsO}}_6$, and ${\mathrm{Ba}}_2{\mathrm{MgOsO}}_6$. Our proposal for exciton condensation leading to type-I AFM order may be relevant to materials such as ${\mathrm{Sr}}_2{\mathrm{MgOsO}}_6$.

Magnetoresistance, ac and dc magnetization, and photoluminescence (PL) experiments have been performed on the $p$-type ${\mathrm{Zn}}_{0.99}{\mathrm{Mn}}_{0.01}\mathrm{Te}$ and ${\mathrm{Zn}}_{0.97}{\mathrm{Mn}}_{0.03}\mathrm{Te}$ alloys strongly doped with phosphorus (P). The investigated samples exhibit spin glass behavior with ferromagnetic regions near the freezing temperature $T_f=1.9$ K and colossal negative magnetoresistance (negMR) of $2.2\times {10}^3$ times at 4.2 K for $0\le B\le \pm 6$ T. The negMR curves contain occurrences of hysteresis providing evidence for the memory effect. Looking for possible ferromagnetic interactions responsible for the spin glass behavior at low hole densities, we observe in PL spectra that the P doping quenches internal recombination in ${\mathrm{Mn}}^{2+}$ ions. This strongly indicates a charge transfer between ${\mathrm{Mn}}^{2+}$ and ${\mathrm{P}}^{2-}$ ions leading to the creation of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{P}}^{3-}$ ions. The creation of ${\mathrm{Mn}}^{3+}$ is confirmed by an observation of strong PL enhancement at excitation energies lower than the band gap and PL study at various temperatures. The resulting simultaneous existence of the mixed valence ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Mn}}^{3+}$ ions can lead to double exchange mechanism of ferromagnetic interaction. Also, ${\mathrm{Mn}}^{3+}$ ions can exhibit a superexchange interaction leading to local ferromagnetic phases.

We study the electronic and magnetic properties of ferroelectric ${\mathrm{CaMnTi}}_2{\mathrm{O}}_6$ within density functional theory using the generalized gradient approximation (GGA) with the consideration of strong Coulomb correlations $\left(\mathrm{GGA}+U\right)$ in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band structure method. The x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism (XMCD) at the Mn, Ti $L_{2,3}$, and O $K$ edges have been investigated theoretically. The calculated results are in good agreement with experimental data. The core-hole effect in the final state as well as the effect of the electric quadrupole $E_2$ and magnetic dipole $M_1$ transitions have been investigated. The core-hole effect has improved the agreement with the experimental XAS and XMCD spectra at the Ti and Mn $L_{2,3}$ edges.