We have investigated the magnetic structure and ferroelectricity in $\mathrm{RbFe}\left(\mathrm{MoO}{}_4\right){}_2$ via first-principles calculations. Phenomenological analyses have shown that ferroelectricity may arise due to both the triangular chirality of the magnetic structure and through coupling between the magnetic helicity and the ferroaxial structural distortion. Indeed, it was recently proposed that the structural distortion plays a key role in stabilizing the chiral magnetic structure itself. We have determined the relative contribution of the two mechanisms via ab initio calculations. Whilst the structural axiality does induce the magnetic helix by modulating the symmetric exchange interactions, the electric polarization is largely due to the in-plane spin triangular chirality, with both electronic and ionic contributions being of relativistic origin. At the microscopic level, we interpret the polarization as a secondary steric consequence of the inverse Dzyaloshinskii-Moriya mechanism and accordingly explain why the ferroaxial component of the electric polarization must be small.

Superconducting ${\mathrm{Pr}}_2$${\mathrm{CuO}}_x,x\simeq 4$ (PCO), films with $T^{\prime }$ structure and a $T_c$ of 27 K have been investigated by various optical methods in a wide frequency $\left(\mathrm{7–55 000}{\mathrm{cm}}^{-1}\right)$ and temperature (2–300 K) range. The optical spectra do not reveal any indication of a normal-state gap formation. A Drude-like peak centered at zero frequency dominates the optical conductivity below 150 K. At higher temperatures, it shifts to finite frequencies. The detailed analysis of the low-frequency conductivity reveals that the Drude peak and a far-infrared (FIR) peak centered at about $300{\mathrm{cm}}^{-1}$ persist at all temperatures. The FIR-peak spectral weight is found to grow at the expense of the Drude spectral weight with increasing temperature. The temperature dependence of the penetration depth follows a behavior typical for $d$-wave superconductors. The absolute value of the penetration depth for zero temperature is $1.6\mu \text{m}$, indicating a rather low density of the superconducting condensate.

We consider a torque acting on an electric dipole placed in anisotropic uniaxial metamaterial. The metamaterial is modeled as a cubic lattice of uniaxial polarizable particles. The torque arises from the response of the anisotropic structure polarized by the field of the test dipole. We calculate the self-induced torque taking into account the effects of frequency and spatial dispersion and also the effect of losses. We demonstrate that the maximal absolute values of the torque are achieved in the transition region between elliptic and hyperbolic dispersion regimes of the metamaterial.

The changes of physical properties occurring at a first-order-like transition among a wide family of $(\mathrm{Pr}{}_{1-y}$$L{n}_y){}_{1-x}\mathrm{Ca}{}_x\mathrm{CoO}{}_3$ compounds (Ln being a lanthanide) are controversially discussed as a transition of $\mathrm{Co}{}^{3+}$ ions from states of low spin (LS) to intermediate spin (IS), or from LS to a mixture of LS and high spin (HS). The debate also includes the nature of the spin state of $\mathrm{Co}{}^{4+}$, as well as the degree of valence shift between $\mathrm{Pr}{}^{3+}$ and $\mathrm{Pr}{}^{4+}$ which accompanies this transition. In the present paper, we investigated the evolution of spin and valence states of Co ions and the valence state of Pr ions in $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}\mathrm{CoO}{}_3$, at high temperature up to 750 K and under pressure up to 36.5 GPa. For this purpose, the data of three spectroscopic techniques were combined: high-resolution Co $K$-edge partial-fluorescence-yield x-ray absorption spectra, Pr $L_2$-edge x-ray absorption spectra, and Co $K$β x-ray emission spectra. The spectral weight transfer of the pre-edge peaks of the Co $K$-edge spectra reveals a continuous redistribution of $3d$ electrons between the $t_{2g}$ and $e_g$ levels of Co ions, reflecting a gradual increase of the average Co spin state upon heating. This is further confirmed by Co $K$β x-ray emission spectra, which show an increase of HS $\mathrm{Co}{}^{3+}$ population when increasing temperature from 300 to 750 K. Applying high pressure at 300 K, the Co $K$β x-ray emission spectra indicate a relatively sharp increase of LS $\mathrm{Co}{}^{3+}$ population below 4 GPa. Pr $L_2$-edge x-ray absorption spectra of $\left(\mathrm{Pr}{}_{0.7}\mathrm{Sm}{}_{0.3}\right){}_{0.7}\mathrm{Ca}{}_{0.3}\mathrm{CoO}{}_3$ exhibit a valence-state transition from $\mathrm{Pr}{}^{3+}$ to $\mathrm{Pr}{}^{3.26+}$, occurring predominantly upon applying pressure in the range ∼4 to ∼12 GPa. Owing to the charge balance, increasing pressure in this regime induces complex valence and spin-state transitions within the Co ions. Moreover, at high enough pressure, it is found that the $\mathrm{Co}{}^{4+}$ can also undergo a spin-state transition (from IS to LS), in addition to that affecting the $\mathrm{Co}{}^{3+}$ (HS to LS).

The popularity of graphene—a pseudospin-$\frac{1}{2}$ two-dimensional Dirac-Weyl material—has prompted the search for related materials and the characterization of their properties. In this work, the magneto-optical conductivity is calculated for systems that obey the general pseudospin-$s$ two-dimensional Dirac-Weyl Hamiltonian, with particular focus on $s=\{\frac{1}{2},1,\frac{3}{2},2\}$. This generalizes calculations that have been made for $s=\frac{1}{2}$ and follows previous work on the optical response of these systems in zero field. In the presence of a magnetic field, Landau levels condense out of the $2s+1$ energy bands. As the chemical potential in a system is shifted, patterns arise in the appearance and disappearance of certain peaks within the optical spectra. These patterns are markedly different for each case considered, creating unique signatures in the magneto-optics. The general structure of each spectrum and how they compare is discussed.

We study the effect of long-range disorder created by charge impurities on the carrier density distribution of graphene-based heterostructures. We consider heterostructures formed by two graphenic sheets [either single-layer graphene (SLG) or bilayer graphene (BLG)] separated by a dielectric film. We present results for symmetric heterostructures SLG-SLG and BLG-BLG, and hybrid ones BLG-SLG. As for isolated layers, we find that the presence of charged impurities induces strong carrier density inhomogeneities, especially at low dopings where the density landscape breaks up in electron-hole puddles. We provide quantitative results for the strength of the carrier density inhomogeneities and for the screened disorder potential for a large range of experimentally relevant conditions. For heterostructures in which BLG is present, we also present results for the band gap induced by the perpendicular electric field generated self-consistently by the disorder potential and by the distribution of charges in the heterostructure. For SLG-SLG heterostructures, we discuss the relevance of our results for the understanding of the recently observed metal-insulator transition in each of the graphene layers forming the heterostructure. Moreover, we calculate the correlation between the density profiles in the two graphenic layers and show that for standard experimental conditions, the two profiles are well correlated.

Nonequilibrium Green's functions calculations based on density functional theory show a direct link between the initial stages of ${\mathrm{H}}_2$ dissociation on a gold atomic wire and the electronic current supported by the gold wire. The simulations reveal that for biases below the stability threshold of the wire, the minimum-energy path for ${\mathrm{H}}_2$ dissociation is not affected. However, the electronic current presents a dramatic drop when the molecule initiates its dissociation. This current drop is traced back to quantum interference between electron paths when the molecule starts interacting with the gold wire.

Optical reflectivity spectra of ${\mathrm{SrCu}}_2$$\left({\mathrm{BO}}_3\right)$${}_2$ revealed a feature at 1.5 eV assigned as the energy gap for the charge-transfer excitation. Changes in the optical reflectivity induced by temperature and applied magnetic field were compared to population statistics to establish a correlation between the optical changes and magnetic excitations on the dimers. A Curie-like analysis of the optical data demonstrated that the Weiss constant and spin-gap energy obtained from an analysis of the magnetic susceptibility could also be used to describe reflectivity changes at the band edge. Differences between the temperature- and magnetic field-induced changes to the optical data were also identified and interpreted as potentially indicative of a multitriplet or cooperative interaction between dimer spin excitations and band-edge charge carriers.

Frustrated magnets can exhibit many novel forms of order when exposed to high magnetic fields. Much less, however, is known about materials where frustration occurs in the presence of itinerant electrons. Here we report thermodynamic and transport measurements on micron-size single crystals of the triangular-lattice metallic antiferromagnet $2H\text{−}{\mathrm{AgNiO}}_2$, in magnetic fields of up to 90 T and temperatures down to 0.35 K. We observe a cascade of magnetic phase transitions at 13.5, 20, 28, and 39 T in fields applied along the easy axis, and we combine magnetic torque, specific heat, and transport data to construct the field-temperature phase diagram. The low-field experimental data are compared with theoretical calculations for a frustrated easy-axis Heisenberg model based on realistic parameters for the localized moments of ${\mathrm{AgNiO}}_2$. Deviations from this model's predictions are attributed to the role played by the itinerant electrons.

We study the phases of correlated charge-density waves that form in a high magnetic field in two parallel graphene flakes separated by a thin insulator. The predicted phases include the square and hexagonal charge-density-wave bubbles, and a quasi-one-dimensional stripe phase. We find that the transition temperature for such phases is within the experimentally accessible range and that formation of interlayer-correlated states produces a negative compressibility contribution to the differential capacitance of this system.

We report the results of low-temperature thermal and magnetic measurements on ${\mathrm{Ba}}_3\mathrm{Co}{\mathrm{Nb}}_2{\mathrm{O}}_9$ powder, described as a uniform triangular-lattice antiferromagnet (TLAF) with a fictitious spin-1/2. ${\mathrm{Ba}}_3\mathrm{Co}{\mathrm{Nb}}_2{\mathrm{O}}_9$ is found to undergo two-step antiferromagnetic transitions at $T_{\mathrm{N}1}=1.39\mathrm{K}$ and $T_{\mathrm{N}2}=1.13\mathrm{K}$. As the magnetic field is increased, both $T_{\mathrm{N}1}$ and $T_{\mathrm{N}2}$ monotonically decrease. The magnetic field vs temperature phase diagram indicates that the exchange interactions are nearly of the Heisenberg type with weak easy-axis anisotropy and that the exchange interaction between triangular lattices is crucial, in contrast to the case of the quasi-two-dimensional TLAF ${\mathrm{Ba}}_3\mathrm{Co}{\mathrm{Sb}}_2{\mathrm{O}}_9$ [Susuki et al., Phys. Rev. Lett. 110, 267201 (2013)].

We study the magnetostatic interaction of submicron $\mathrm{Ni}{}_{81}$$\mathrm{Fe}{}_{19}$ rectangles arranged in a linear chain by measuring the anisotropic magnetoresistance (AMR) of a single rectangle. The rectangles have a lateral aspect ratio of 2 : 1 and are lined up with the long axis oriented side by side varying the interelement distance down to $60\mathrm{n}\mathrm{m}$. The energy density of the Landau state is determined from the hard-axis magnetization reversal for a field applied along the chain direction. As a second approach identical energy densities are deduced from the switching field (Landau to quasisingle domain state) like in the case of a Stoner-Wohlfarth particle. The results show that the impact of the magnetostatic interaction on the energy density of the Landau state in remanence is negligibly small $(<1\mathrm{k}$$\mathrm{J}$$/$$\mathrm{m}$${}^3$$)$. The magnetostatic interaction between field-distorted Landau states, however, is the same as for rectangles in a single domain state and is therefore governed by the compensation of surface charges at the rim. By studying rectangles with only one neighbor, the important role of symmetry on the magnetostatic interaction is shown.

Josephson junctions made of closely spaced conventional superconductors on the surface of three-dimensional topological insulators have been proposed to host Andreev bound states (ABSs), which can include Majorana fermions. Here, we present an extensive study of the supercurrent carried by low-energy ABSs in $\mathrm{Nb}/{\mathrm{Bi}}_2{\mathrm{Se}}_3/\mathrm{Nb}$ Josephson junctions in various superconducting quantum interference devices as we modulate the carrier density in the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ barriers through electrostatic top gates. As previously reported, we find a precipitous drop in the Josephson current at a critical value of the voltage applied to the top gate. This drop has been attributed to a transition where the topologically trivial two-dimensional electron gas (2DEG) at the surface is nearly depleted, causing a shift in the spatial location and a change in nature of the helical surface states. We present measurements that support this picture by revealing qualitative changes in the temperature and magnetic field dependence of the critical current across this transition. In particular, we observe pronounced fluctuations in the critical current near total depletion of the 2DEG that demonstrate the dynamical nature of the supercurrent transport through topological low-energy ABSs.

The thermal boundary conductance across solid-solid interfaces can be affected by the physical properties of the solid boundary. Atomic composition, disorder, and bonding between materials can result in large deviations in the phonon scattering mechanisms contributing to thermal boundary conductance. Theoretical and computational studies have suggested that the mixing of atoms around an interface can lead to an increase in thermal boundary conductance by creating a region with an average vibrational spectra of the two materials forming the interface. In this paper, we experimentally demonstrate that ion irradiation and subsequent modification of atoms at solid surfaces can increase the thermal boundary conductance across solid interfaces due to a change in the acoustic impedance of the surface. We measure the thermal boundary conductance between thin aluminum films and silicon substrates with native silicon dioxide layers that have been subjected to proton irradiation and post-irradiation surface cleaning procedures. The thermal boundary conductance across the Al/native oxide/Si interfacial region increases with an increase in proton dose. Supported with statistical simulations, we hypothesize that ion beam mixing of the native oxide and silicon substrate within $\sim 2.2\mathrm{nm}$ of the silicon surface results in the observed increase in thermal boundary conductance. This ion mixing leads to the spatial gradation of the silicon native oxide into the silicon substrate, which alters the acoustic impedance and vibrational characteristics at the interface of the aluminum film and native oxide/silicon substrate. We confirm this assertion with picosecond acoustic analyses. Our results demonstrate that under specific conditions, a “more disordered and defected” interfacial region can have a lower resistance than a more “perfect” interface.

Using low temperature Raman spectra on two-dimensional epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$ thin films, the presence of electron-phonon (e-p) and spin-phonon (s-p) couplings is established in the vicinity of Verwey transition ($T_{\mathrm{V}}$). The e-p coupling is linked to $A_{1g}$ and $T_{2g}$ Raman modes and s-p to the $T_{2g}$ mode, below isotropic point ($T_{\mathrm{K}}$). The parabolic dependence of the characteristic spin-correlation eigenstates on the magnetization, ${\Delta \omega }_{s-\mathrm{phn}}\propto \langle S_i|S_J\rangle \propto {\left[\frac{M\left(T\right)}{M_S}\right]}^2$ observed between $T_{\mathrm{K}}$ and $T_{\mathrm{V}}$ demonstrates the presence of s-p coupling. This work provides experimental evidence of the onset of correlation between these couplings and charge-orbital ordering below $T_{\mathrm{K}}$ to govern the Verwey transition in ${\mathrm{Fe}}_3{\mathrm{O}}_4$ films.

We investigate the crucial metallization pressure of the $Cmca\text{−}12$ phase of solid hydrogen (H) using many-body perturbation theory within the $GW$ approximation. We consider the effects of self-consistency, plasmon-pole models, and the vertex correction on the quasiparticle band gap $\left(E_{\mathrm{g}}\right)$. Our calculations show that self-consistency leads to an increase in $E_{\mathrm{g}}$ by 0.33 eV over the one-shot $G_0{W}_0$ approach. Because of error cancellation between the effects of self-consistency and the vertex correction, the simplest $G_0{W}_0$ method underestimates $E_{\mathrm{g}}$ by only 0.16 eV compared with the prediction of the more accurate $GW\Gamma$ approach. Employing the plasmon-pole models underestimates $E_{\mathrm{g}}$ by 0.1–0.2 eV compared to the full-frequency numerical integration results. We thus predict a metallization pressure around 280 GPa, instead of 260 GPa predicted previously. Furthermore, we compute the optical absorption including the electron-hole interaction by solving the Bethe-Salpeter equation (BSE). The resulting absorption spectra demonstrate substantial redshifts and enhancement of absorption peaks compared to the calculated spectra neglecting excitonic effects. We find that the exciton binding energy decreases with increasing pressure from 66 meV at 100 GPa to 12 meV at 200 GPa due to the enhanced electronic screening as solid H approaches metallization. Because optical measurements are so important in identifying the structure of solid H, our BSE results should improve agreement between theory and experiment.

The absence of nesting between electron and hole pockets in LiFeAs with $T_{\mathrm{c}}=18$ K attracts great attention, as an important hint to understand the pairing mechanism of Fe-based superconductors. Here, we study the five-orbital model of LiFeAs based on the recently developed orbital-spin fluctuation theories. It is found that the experimentally observed gap structure of LiFeAs, which is a “fingerprint” of the pairing mechanism, is quantitatively reproduced in terms of the orbital-fluctuation-mediated $s_{++}$-wave state. Specifically, the largest gap observed on the two small hole pockets composed of ($d_{xz},d_{yz}$) orbitals can be explained, and this is a hallmark of the orbital-fluctuation-mediated superconductivity. The $s_{++}$-wave gap structure becomes more anisotropic in the presence of weak spin fluctuations. As the spin fluctuations increase, we obtain the “hole-$s_{\pm }$-wave state,” in which only the gap of the large hole pocket made of the $d_{xy}$ orbital is sign reversed, due to the cooperation of orbital and spin fluctuations. This gap structure with “sign reversal between hole pockets” is similar to that recently reported in $(\mathrm{Ba},\mathrm{K}){\mathrm{Fe}}_2{\mathrm{As}}_2$.

In the recently discovered topological crystalline insulators SnTe and ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x$(Te, Se), crystal symmetry and electronic topology intertwine to create topological surface states with many interesting features including Lifshitz transition, Van-Hove singularity, and fermion mass generation. These surface states are protected by mirror symmetry with respect to the (110) plane. In this work we present a comprehensive study of the effects of different mirror-symmetry-breaking perturbations on the (001) surface band structure. Pristine (001) surface states have four branches of Dirac fermions at low energy. We show that ferroelectric-type structural distortion generates a mass and gaps out some or all of these Dirac points, while strain shifts Dirac points in the Brillouin zone. An in-plane magnetic field leaves the surface state gapless, but introduces asymmetry between Dirac points. Finally, an out-of-plane magnetic field leads to discrete Landau levels. We show that the Landau level spectrum has an unusual pattern of degeneracy and interesting features due to the unique underlying band structure. This suggests that Landau level spectroscopy can detect and distinguish between different mechanisms of symmetry breaking in topological crystalline insulators.

We explore possible ways to manipulate the intrinsic edge magnetism in a hexagonal graphene nanoflake with zigzag edges, using density functional theory supplemented with on-site Coulomb interaction. The effect of carrier doping, chemical modification at the edge, and finite temperature on the edge magnetism has been studied. The magnetic phase diagram with varied carrier doping and on-site Coulomb interaction is found to be complex. In addition to the intrinsic antiferromagnetic solution, as predicted for charge neutral hexagonal nanoflakes, fully polarized ferromagnetic and mixed phase solutions are obtained depending on the doped carrier concentration and on-site Coulomb interaction. The complexity arises due to the competing nature of local Coulomb interaction and carrier doping, favoring antiferromagnetic and ferromagnetic coupling, respectively. Chemical modification of the edge atoms by hydrogen leads to partial quenching of local moments, giving rise to a richer phase diagram consisting of antiferromagnetic, ferromagnetic, mixed, and nonmagnetic phases. We further report the influence of temperature on the long-range magnetic ordering at the edge using ab initio molecular dynamics. In agreement with the recent experimental observations, we find that temperature can also alter the magnetic state of the neutral nanoflake, which is otherwise antiferromagnetic at zero temperature. These findings will have important implications in controlling magnetism in graphene-based low dimensional structures for technological purpose, and in understanding varied experimental reports.