Low-energy electronic structures in $A$MnBi${}_2$ ($A$=alkaline earths) are investigated using a first-principles calculation and a tight binding method. An anisotropic Dirac dispersion is induced by the checkerboard arrangement of $A$ atoms above and below the Bi square net in $A$MnBi${}_2$. SrMnBi${}_2$ and CaMnBi${}_2$ have a different kind of Dirac dispersion due to the different stacking of nearby $A$ layers, where each Sr (Ca) of one side appears at the coincident (staggered) $xy$ position of the same element at the other side. Using the tight binding analysis, we reveal the chirality of the anisotropic Dirac electrons as well as the sizable spin-orbit coupling effect in the Bi square net. We suggest that the Bi square net provides a platform for the interplay between anisotropic Dirac electrons and the neighboring environment such as magnetism and structural changes.

We examine the effect of interface disorder in suppressing superconductivity in coherently grown ultrathin YBa${}_2$Cu${}_3$O${}_7$ (YBCO) layers on SrTiO${}_3$ (STO) in YBCO/STO superlattices. The termination plane of the STO is TiO${}_2$ and the CuO chains are missing at the interface. Disorder (steps) at the STO interface cause alterations of the stacking sequence of the intracell YBCO atomic layers. Stacking faults give rise to antiphase boundaries which break the continuity of the CuO${}_2$ planes and depress superconductivity. We show that superconductivity is directly controlled by interface disorder outlining the importance of pair breaking and localization by disorder in ultrathin layers.

Recent terahertz conductivity measurements observed low-power-law frequency dependence of optical conduction within the Mott gap of the kagome lattice spin-liquid candidate Herbertsmithite. We investigate mechanisms for this observed sub-gap conductivity for two possible scenarios in which the ground state is described by (1) a U(1) Dirac spin liquid with emergent fermionic spinons or (2) a nearly critical $Z_2$ spin liquid in the vicinity of a continuous quantum phase transition to magnetic order. We identify new mechanisms for optical absorption via magnetoelastic effects and spin-orbit coupling. In addition, for the Dirac spin liquid scenario, we establish an explicit microscopic origin for previously proposed absorption mechanisms based on slave-particle effective field theory descriptions.

We investigated an antiferromagnet UCo${}_2$Si${}_2$ by use of magnetization and ultrasound measurements in pulsed magnetic fields up to 60 T. It is found that the crystal UCo${}_2$Si${}_2$, which has the antiferromagnet type-I magnetic structure in zero field below $T_{\mathrm{N}}$ $=$ 83 K, undergoes the metamagnetic phase transition to ferrimagnetic structure $++-$ type similarly to the UNi${}_2$Si${}_2$ with substitution of 10$\%$–15$\%$ Ni by Pd or UPd${}_2$Si${}_2$. Therefore, similar phase transitions take place in the compounds with expected essentially different strength of the 5$f$-$d$ electron hybridization. In UCo${}_2$Si${}_2$, the transition occurs when the magnetic field is applied along the $c$ axis at 45 T (at 1.5 K). The transition is extremely sharp and exhibits a small but non-negligible hysteresis. With increasing temperature, it becomes broader and vanishes at $T_{\mathrm{N}}$. In our ultrasound measurements, the metamagnetic transition appears as anomalies in the sound velocity and attenuation. Our analysis suggests that the low-temperature changes in the sound velocity and attenuation predominantly are determined by an exchange renormalization caused by the sound waves.

Magnetometry, electrical transport, and neutron scattering measurements were performed on single crystals of the Fe${}^{4+}$-containing perovskite-related phase Sr${}_3$Fe${}_2$O${}_{7-\delta }$ as a function of oxygen content. Although both the crystal structure and electron configuration of this compound are closely similar to those of well-studied ruthenates and manganates, it exhibits very different physical properties. The fully oxygenated compound ($\delta =0$) exhibits a charge-disproportionation transition at $T_D=340$ K, and an antiferromagnetic transition at $T_N=115$ K. For temperatures $T\le T_D$, the material is a small-gap insulator; the antiferromagnetic order is incommensurate, which implies competing exchange interactions between the Fe${}^{4+}$ moments. The fully deoxygenated compound ($\delta =1$) is highly insulating, and its Fe${}^{3+}$ moments exhibit commensurate antiferromagnetic order below $T_N\sim 600$ K. Compounds with intermediate $\delta$ exhibit different order with lower $T_N$, likely as a consequence of frustrated exchange interactions between Fe${}^{3+}$ and Fe${}^{4+}$ sublattices. A previous proposal that the magnetic transition temperature reaches zero is not supported.

Second harmonic generation (SHG) and linear absorbance spectra of MnTiO${}_3$ were measured and the 1.88, 2.41, 2.63, and 3.06 eV SHG features were identified as $d-d$ optical transitions from ${}^6{A}_{1g}$ to ${}^4{T}_{1g}{(}^{4}G)$, ${}^4{T}_{2g}{(}^{4}G)$, ${\{}^4{E}_g$,${}^4{A}_{1g}{(}^{4}G)\}$, and ${}^4{E}_g{(}^{4}D)$, respectively. These zero-phonon transitions were used to calculate the octahedral crystal field splitting energy (${\Delta }_0$) and the Racah parameters $B$ and $C$. The SHG spectra showed significant distinctions between the antiferromagnetic and the paramagnetic or spin-flop phases. The polarization dependence of the SHG in the spin-flop phase provided optical evidence that the spins canted from the $c$ axis toward the $a$ axis. These distinctions between the three magnetic phases could be useful for mapping 180${}^{\circ }$ antiferromagnetic domains in MnTiO${}_3$.

The temperature dependence of an isolated quantum vortex, embedded in an otherwise homogeneous fermionic superfluid of infinite extent, is determined via the Bogoliubov–de Gennes (BdG) equations across the BCS-BEC crossover. Emphasis is given to the BCS side of this crossover, where it is physically relevant to extend this study up to the critical temperature for the loss of the superfluid phase, such that the size of the vortex increases without bound. To this end, two techniques are introduced. The first one solves the BdG equations with “free boundary conditions,” which allows one to determine with high accuracy how the vortex profile matches its asymptotic value at a large distance from the center, thus avoiding a common practice of constraining the vortex in a cylinder with infinite walls. The second one improves on the regularization procedure of the self-consistent gap equation when the interparticle interaction is of the contact type, and permits us to considerably reduce the time needed for its numerical integration by drawing elements from the derivation of the Gross-Pitaevskii equation for composite bosons starting from the BdG equations.

The disordered antiferromagnet PbFe${}_{1/2}$Nb${}_{1/2}$O${}_3$ is investigated in a wide temperature range by combining Mössbauer spectroscopy and neutron diffraction experiments. It is demonstrated that the magnetic ground state is a microscopic coexistence of antiferromagnetic and spin-glass orders. This speromagnetlike phase features frozen-in short-range fluctuations of the Fe${}^{3+}$ magnetic moments that are transverse to the long-range ordered antiferromagnetic spin component.

The emergence of a ferromagnetic component in ${\text{LaMnO}}_3$ with low Cr-for-Mn substitution has been studied by x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn and Cr $K$ edges. The local magnetic moment strengths for Mn and Cr are proportional to each other and follow the macroscopic magnetization. The net ferromagnetic components of ${\text{Cr}}^{3+}$ and ${\text{Mn}}^{3+}$ are found antiferromagnetically coupled. Unlike hole doping by La site substitution, the inclusion of ${\text{Cr}}^{3+}$ ions up to $x=0.15$ does not decrease the Jahn-Teller (JT) distortion and consequently does not significantly affect the orbital ordering. This demonstrates that the emergence of the ferromagnetism is not related to JT weakening and likely arises from a complex orbital mixing.

Ab initio calculations are employed to determine the lower and upper bounds of the interfacial energy and work of separation of a fcc-Ag/bcc-Fe interface. The strain-free interfacial energy of the coherent interface is taken as the lower bound and the interfacial energy of the commensurate incoherent interface as the upper bound of the interfacial energy of a realistic semicoherent interface. The latter is estimated by applying an averaging scheme based on the interfacial energies obtained for the coherent interfaces. Similar calculations are performed for determining the bounds of the work of separation. We justify the use of the averaging scheme by carrying out large supercell calculations for a semicoherent interface. For a Fe(110)/Ag(111) semicoherent interface, we show that taking either Fe or Ag as the underlying lattice, our averaging scheme can yield a reasonable estimation of the work of separation of the semicoherent interface. However, when taking Ag as the underlying lattice, the averaged interfacial energy of the semicoherent interface is significantly underestimated due to the magnetism. The structure and magnetism at the coherent and semicoherent interfaces are discussed.

Ultrafast dynamics of holes in a solid is usually hard to observe directly because the optical transition includes both electron and hole dynamics. In this paper we show an approach combining femtosecond luminescence and photoemission spectroscopy, successfully applied to graphite. Ultrafast infrared luminescence is observed in graphite and ascribed to recombination luminescence, though graphite is a semimetal. The dynamics of holes is deduced by taking a ratio between the time-resolved luminescence and photoemission data recorded on the same sample, and shown to be very close to that of electrons.

Based on the first-principles electronic structure calculations, we analyze the chemical and magnetic mechanisms stabilizing the cubic phase in Fe${}_2$-based Heusler materials, which were predicted to be tetragonal when being chemically ordered. In agreement with recent experimental data, we found that these compounds crystallize within the so-called “inverted” cubic Heusler structure perturbed by a certain portion of the intrinsic chemical disorder. Understanding of these mechanisms is a necessary step to guide the successful future synthesis of the stable Fe${}_2$-based tetragonal phases, which are promising candidates for the rare-earth-free permanent magnets.

The spin and orbital ordering in Dy${}_{1-x}$Tb${}_x$VO${}_3$ ($x=0$ and 0.2) was studied by measuring x-ray powder diffraction, magnetization, specific heat, and neutron single-crystal diffraction. The results show that G-OO/C-AF and C-OO/G-AF phases coexist in Dy${}_{0.8}$Tb${}_{0.20}$VO${}_3$ in the temperature range 2–60 K, and the volume fraction of each phase is temperature and field dependent. The ordering of Dy moments at $T$* = 12 K induces a transition from G-OO/C-AF to a C-OO/G-AF phase. Magnetic fields suppress the long-range order of Dy moments and thus the C-OO/G-AF phase below $T$*. The polarized moments induced at the Dy sublattice by external magnetic fields couple to the V 3d moments, and this coupling favors the G-OO/C-AF state. Also discussed is the effect of the Dy-V magnetic interaction and local structure distortion on the spin and orbital ordering in Dy${}_{1-x}$Tb${}_x$VO${}_3$.

Magnetically ordered crystals often show unique magneto-optical phenomena that are not found without magnetic order. For example, MnF${}_2$, a two-sublattice antiferromagnet with different sublattice symmetries shows magnetic linear dichroism (MLD), which is odd with respect to the external magnetic field in the Faraday geometry. In particular, the magnon sideband of an exciton with a particular orbital function exhibits MLD, and the commonly found magnetic circular dichroism (MCD) is almost absent. In this study, we clarify the microscopic origin of this characteristic feature. We found that the symmetry of the exciton orbital determines the polarization eigenstate of the light to create the exciton magnon pair. These eigenstates can even be two cross-linear polarizations, depending in which sublattice the exciton is created. In this case, only MLD is allowed and MCD is forbidden.

Metal-organic frameworks (MOFs) can exhibit many interesting properties such as multiferroic behavior, dipolar glass, gas storage, and protonic conductivity. Here we report that dimethylammonium copper formate (DMACuF) [(CH${}_3$)${}_2$NH${}_2$]Cu(HCOO)${}_3$, a cation templated nonporous MOF with perovskite topology, exhibits strong one-dimensional (1D) antiferromagnetism with a Néel temperature, $T_N$, of 5.2 K. These conclusions are derived from detailed magnetic susceptibility, heat capacity, dielectric constant, and high-frequency electron paramagnetic resonance measurements as well as density functional theory (DFT) calculations. The magnetic susceptibility exhibits a broad maximum at ∼50 K, suggesting low-dimensional magnetism; heat capacity measurements show a Néel temperature of 5.2 K. The magnetization versus field data at 1.8 K shows a spin-flop transition at $H_{sf}$ ∼ 1.7 T. The ratio $T_N$/$J=6.5\times {10}^{-2}$, where $J$ is the near-neighbor exchange constant (77.4 K), and the small value (2 K) of the interchain coupling suggests that DMACuF is close to an ideal 1D magnet. In this three-dimensional crystal lattice, the 1D magnetic behavior is made possible by the Jahn-Teller distortion of the 3$d^9$ Cu${}^{2+}$ ions. Temperature dependence of the electron paramagnetic resonance field and the linewidth exhibits critical broadening for temperatures below 50 K, following a behavior quite characteristic of 1D spin systems. DFT calculations show that [(CH${}_3$)${}_2$NH${}_2$]Cu(HCOO)${}_3$ has a magnetic structure in which 1D antiferromagnetic chains parallel to the $c$ direction are weakly coupled ferromagnetically, supporting the thermomagnetic and EPR results. Dielectric measurements under applied magnetic fields of 0−7 T reveal a kink at the $T_N$, a clear indication of magnetostriction behavior.

We study the dynamics and relaxation of elementary excitations (magnons) in the spin nematic (quadrupole ordered) phase of $S=1$ magnets. We develop a general phenomenological theory of spin dynamics and relaxation for spin-1 systems. The results of the phenomenological approach are compared to those obtained by microscopic calculations for the specific $S=1$ model with isotropic bilinear and biquadratic exchange interactions. This model exhibits a rich behavior depending on the ratio of bilinear and biquadratic exchange constants, including several points with an enhanced symmetry. It is shown that symmetry plays an important role in relaxation. Particularly, at the SU(3) ferromagnetic point the magnon damping $\Gamma$ depends on its wave vector $k$ as $\Gamma \propto k^4$, while a deviation from the high-symmetry point changes the behavior of the leading term to $\Gamma \propto k^2$. We point out a similarity between the behavior of magnon relaxation in spin nematics to that in an isotropic ferromagnet.

We calculate the spectrum of the amplitude mode, the analog of the Higgs mode in high-energy physics, for the $d$-density wave state proposed to describe the pseudogap phase of the high-$T_c$ cuprates. Even though the state breaks translational symmetry by a lattice spacing and is described by a particle-hole singlet order parameter at the wave vector $q=Q=(\pi ,\pi )$, remarkably, we find that the amplitude mode spectrum can have peaks at both $q=(0,0)$ and $q=Q=(\pi ,\pi )$; we shall set lattice spacing to unity. In general, the spectrum is nonuniversal and, depending on the microscopic parameters, can have one or two peaks in the Brillouin zone, signifying the existence of two kinds of magnetic excitations. Our theory sheds important light on how multiple inelastic neutron peaks at different wave vectors can, in principle, arise even with an order parameter that condenses at $Q=(\pi ,\pi )$.