The ferromagnetic zircon-type phase of ${\mathrm{GdCrO}}_4$ presents high values for the magnetocaloric (MC) parameters. This compound has large isothermal entropy changes $\mathrm{\Delta }{S}_T$ under the magnetic field action in a wide temperature range, from 5 to 35 K, reaching a maximum $|\mathrm{\Delta }{S}_T|=29.0\pm 0.1\mathrm{J}/\mathrm{kg}\mathrm{K}$ at 22 K, for a field increment $\mathrm{\Delta }B=9$ T. It orders ferromagnetically at $T_{\mathrm{C}}=21.3\mathrm{K}$ via the Cr-Cr exchange interaction and shows a second transition at 4.8 K due to the ordering of the Gd sublattice. The large MC effect is enhanced by the polarization of the ${\mathrm{Gd}}^{3+}$ ions by the ${\mathrm{Cr}}^{5+}$ ones via a weaker Gd-Cr interaction. This effect is an interesting feature to be considered in the search for new compounds with a high MC effect in the range of liquid hydrogen or natural gas, regarding the liquefaction of gases by magnetization-demagnetization cycles. This paper contains experimental measurements of magnetization, heat capacity, and direct determinations of the MC effect. The magnetic contribution to the heat capacity $C_m$ has been obtained after subtracting the lattice component. Approximate values for the exchange constants $J_1$ (Cr-Cr) and $J_3$ (Gd-Cr) have been deduced from $C_m$.

We propose a projector-based renormalization framework to study exciton-polariton Bose-Einstein condensation in a microcavity matter-light system. Treating Coulomb interaction and electron-hole/photon coupling effects on an equal footing, we analyze the ground-state properties of the exciton-polariton model according to the detuning and the excitation density. We demonstrate that the condensate by its nature shows a crossover from an excitonic insulator (of Bose-Einstein, respectively, BCS type) to a polariton and finally photonic condensed state as the excitation density increases at large detuning. If the detuning is weak, polariton or photonic phases dominate. While in both cases a notable renormalization of the quasiparticle band structure occurs that strongly affects the coherent part of the excitonic luminescence, the incoherent wave-vector-resolved luminescence spectrum develops a flat bottom only for small detuning.

The origins of up-up-down-down ($\uparrow \uparrow \downarrow \downarrow$) magnetic structure and ferroelectricity discovered in multiferroic ${\mathrm{Lu}}_2{\mathrm{CoMnO}}_6$ have been studied on the basis of first-principles calculations. The energy of $\uparrow \uparrow \downarrow \downarrow$ spin order is very close to that of ferromagnetic order, which is caused by the weak out-of-plane nearest-neighbor exchange coupling and the exchange-striction effect. However, the ground state turns out to be of ferromagnetic order, and can be reversed if the out-of-plane bond angle decreases. We propose that the presence of $\upuparrows \downdownarrows$ spin order is related to the Co-Mn antisite disorder in the polycrystalline sample. The origin of ferroelectricity is independent of the charge order, and the exchange-striction mechanism induces the ferroelectricity through the bond-bending rather than the bond-stretching effect present in ${\mathrm{Ca}}_3{\mathrm{CoMnO}}_6$.

We investigate the topological surface states in heterostructures formed from a three-dimensional topological insulator (TI) and a two-dimensional insulating thin film, using first-principles calculations and the tight-binding method. Utilizing a single Bi or Sb bilayer on top of the topological insulators $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3, \mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_3, \mathrm{B}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_2\mathrm{Se}$, and $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$, we find that the surface states evolve in very peculiar but predictable ways. We show that strong hybridization between the bilayer and TI substrates causes the topological surface states to migrate to the top bilayer. It is found that the difference in the work function of constituent layers, which determines the band alignment and the strength of hybridization, governs the character of newly emerged Dirac states.

It is known that the Shubnikov–de Haas oscillations can be observed in the Hall resistivity, although their amplitude is much weaker than the amplitude of the diagonal resistivity oscillations. Employing a model of two-dimensional massive Dirac fermions that exhibits anomalous Hall effect, we demonstrate that the amplitude of the Shubnikov–de Haas oscillations of the anomalous Hall conductivity is the same as that of the diagonal conductivity. We argue that the oscillations of the anomalous Hall conductivity can be observed by studying the valley Hall effect in graphene superlattices and the spin Hall effect in the low-buckled Dirac materials.

Mode-dependent phonon and electron transport properties in Al, Ag, and Au are predicted using density functional theory and lattice dynamics calculations. The predicted thermal conductivities, electrical conductivities, electron-phonon coupling coefficients, and electron-phonon mass enhancement parameters are in agreement with experimental measurements. At a temperature of 100 K, the phonon contribution to the total thermal conductivity of Al is 5% in bulk and increases to 15% for a 50 nm thick film. In all three metals, phonons with mean free paths between 1 and 10 nm are the dominate contributors to the thermal conductivity at a temperature of 300 K, while the relevant electron mean free paths are 10–100 nm. Despite similar atomic masses, the phonon thermal conductivity of Al is an order of magnitude smaller than that of silicon due to a larger three-phonon phase space and stronger anharmonicity. These results will impact the interpretation of thermoreflectance experiments that can resolve carrier-level contributions to thermal conductivity.

Almost a decade ago, transformation optics established a geometrical perspective to describe the interaction of light with structured matter, enhancing our understanding and control of light. However, despite their huge technological relevance in applications such as optical circuitry, optical detection, and actuation, guided electromagnetic waves along dielectric waveguides have not yet benefited from the flexibility and conceptual simplicity of transformation optics. Indeed, transformation optics inherently imposes metamaterials not only inside the waveguide's core but also in the surrounding substrate and cladding. Here we restore the two-dimensional nature of guided electromagnetic waves by introducing a thickness variation on an anisotropic dielectric core according to alternative two-dimensional equivalence relations. Our waveguides require metamaterials only inside the core with the additional advantage that the metamaterials need not be magnetic and, hence, our purely dielectric waveguides are low loss. We verify the versatility of our theory with full wave simulations of three crucial functionalities: beam bending, beam splitting, and lensing. Our method opens up the toolbox of transformation optics to a plethora of waveguide-based devices.

Due to the strong spin-orbit coupling, silicene is a topological insulator and can open a relatively large energy gap at the Dirac point. Moreover, the applied bias can drive silicene from a topological insulator into an ordinary insulator. Here, we examine the adsorbate effect on the electronic properties of silicene. The calculated local density of states around the adsorbates clearly reveal that the induced localized states contain the band topology information, which can be used to distinguish whether the system is a topological insulator or not. We also explore the impact of randomly distributed adsorbates with a low concentration on the electron structures and the transport properties of silicene, and find that the edge mode backscattering is significantly enhanced when the energies of the incoming modes from leads match that of the in-gap localized states.

Three-dimensional (3D) GaN is a III-V compound semiconductor with potential optoelectronic applications. In this paper, starting from 3D GaN in wurtzite and zinc-blende structures, we investigated the mechanical, electronic, and optical properties of the 2D single-layer honeycomb structure of GaN ($g-\mathrm{GaN}$) and its bilayer, trilayer, and multilayer van der Waals solids using density-functional theory. Based on high-temperature ab initio molecular-dynamics calculations, we first showed that $g-\mathrm{GaN}$ can remain stable at high temperature. Then we performed a comparative study to reveal how the physical properties vary with dimensionality. While 3D GaN is a direct-band-gap semiconductor, $g-\mathrm{GaN}$ in two dimensions has a relatively wider indirect band gap. Moreover, 2D $g-\mathrm{GaN}$ displays a higher Poisson ratio and slightly less charge transfer from cation to anion. In two dimensions, the optical-absorption spectra of 3D crystalline phases are modified dramatically, and their absorption onset energy is blueshifted. We also showed that the physical properties predicted for freestanding $g-\mathrm{GaN}$ are preserved when $g-\mathrm{GaN}$ is grown on metallic as well as semiconducting substrates. In particular, 3D layered blue phosphorus, being nearly lattice-matched to $g-\mathrm{GaN}$, is found to be an excellent substrate for growing $g-\mathrm{GaN}$. Bilayer, trilayer, and van der Waals crystals can be constructed by a special stacking sequence of $g-\mathrm{GaN}$, and they can display electronic and optical properties that can be controlled by the number of $g-\mathrm{GaN}$ layers. In particular, their fundamental band gap decreases and changes from indirect to direct with an increasing number of $g-\mathrm{GaN}$ layers.

Vacancy defects are known to play an important role in the structural and chemical properties of metallic and semiconductor nanoparticles. Here, we investigate the likelihood of vacancy formation at the surface, in the subsurfaces, and in the interior of a model system of Al nanocrystals. The depth dependence of the vacancy formation energy (VFE) in 14–17 layered low-indexed surfaces of aluminium is studied using LDA, PBE, and PBEsol exchange-correlation functionals. Within a depth of two subsurface layers, the functionals make a transition from a similar description of surfaces to the differences in VFEs observed in bulk Al. The VFE converges to the bulk value within 0.01 eV beyond a maximum depth of 3–6 atomic layers, depending on the crystallographic surface plane. We find that the different convergence behaviors are related to the relaxations of atomic planes, normal to the surface, which in turn depend on the packing density of these surfaces. For the (111) subsurfaces, surprisingly, the defect formation energies are found to be higher than that of bulk Al, which is related to the hindered relaxations in its close-packed atomic planes. Although our results predict considerably lower VFE for the topmost layers of all the surfaces, the likelihood of forming a vacancy in the immediate subsurfaces of multifaceted Al nanoparticles is predicted to be lower than in bulk Al, which is in contrast to expectation.

Ferroelectric transitions of a hexagonal multiferroic, $\mathrm{ErMn}{\mathrm{O}}_3$, are studied by x-ray scattering techniques. An isosymmetry transition, similar to that previously observed for $\mathrm{YMn}{\mathrm{O}}_3$, approximately 300 K below the well-known ferroic transition temperature, is investigated. The partially glassy behavior of the isosymmetry transition is identified by the appearance of quasielastic scattering lines in high-energy-resolution scans. The glassy behavior is further supported by the increased interlayer decorrelation of $(\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ ordering below the isosymmetry transition. The transition behavior is considered for possible hidden sluggish modes and two-step phase transitions theoretically predicted for the stacked triangular antiferromagnets. The in-plane azimuthal (orientational) ordering behaviors were also compared to the theoretical predictions. Coherent x-ray speckle measurements show unambiguously that the domain sizes decrease anomalously near both the isosymmetry and ferroic transitions. However, domain boundary fluctuations increase monotonically with an Arrhenius form with an activation energy of 0.54(5) eV through both transitions.

We report systematic measurements of anomalous Hall effect (AHE) in $\mathrm{M}{\mathrm{n}}_{1.5}\mathrm{Ga}/\mathrm{Ta}$ and $\mathrm{M}{\mathrm{n}}_{1.5}\mathrm{Ga}/\mathrm{Pt}$ bilayers modified by strong spin-orbit coupling (SOC) of the heavy metals. The results are compared with the AHE in $\mathrm{M}{\mathrm{n}}_{1.5}\mathrm{Ga}/\mathrm{Cu}$ with weak SOC. We have used a scaling involving multiple competing scattering mechanisms to distinguish different contributions to the modified AHE. The scaling analysis reveals that the origin of the modified AHE in $\mathrm{M}{\mathrm{n}}_{1.5}\mathrm{Ga}/\mathrm{Cu}$ is different from those in $\mathrm{M}{\mathrm{n}}_{1.5}\mathrm{Ga}/\mathrm{Ta}$ and $\mathrm{M}{\mathrm{n}}_{1.5}\mathrm{Ga}/\mathrm{Pt}$. The direct observation of spin-orbit torque induced magnetization switching confirms that the result is a combination of the AHE of $\mathrm{M}{\mathrm{n}}_{1.5}\mathrm{Ga}$ and spin Hall effect of the heavy metals.

The topological crystalline insulator tin telluride is known to host superconductivity when doped with indium (${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$), and for low indium content ($x=0.04$) it is known that the topological surface states are preserved. Here we present the growth, characterization, and angle resolved photoemission spectroscopy analysis of samples with much heavier In doping (up to $x\approx 0.4$), a regime where the superconducting temperature is increased nearly fourfold. We demonstrate that despite strong $p$-type doping, Dirac-like surface states persist.

We report the dependence of magnetic properties on the crystalline disorder in ${\mathrm{Li}}_2{\mathrm{RuO}}_3$ with Ru honeycomb lattice. This oxide exhibits unconventional Ru-dimer transition below $T_{\text{d}}\sim 540$ K. We demonstrate that the cell parameters, related to the coherence of the dimer formation, are strongly dependent on the synthesis procedure. We show that the magnetic behavior at the dimer transition is closely related to the lattice parameters. In particular, we revealed that samples with well-ordered dimers exhibit a first-order magnetic transition with the onset exceeding 550 K, higher than that reported previously. We discuss possible dimer configurations leading to this magnetolattice coupling.

When a system is driven across a quantum critical point at a constant rate, its evolution must become nonadiabatic as the relaxation time $\tau$ diverges at the critical point. According to the Kibble-Zurek mechanism (KZM), the emerging post-transition excited state is characterized by a finite correlation length $\widehat{\xi }$ set at the time $\widehat{t}=\widehat{\tau }$ when the critical slowing down makes it impossible for the system to relax to the equilibrium defined by changing parameters. This observation naturally suggests a dynamical scaling similar to renormalization familiar from the equilibrium critical phenomena. We provide evidence for such KZM-inspired spatiotemporal scaling by investigating an exact solution of the transverse field quantum Ising chain in the thermodynamic limit.

Quantum anomalies, breakdown of classical symmetries by quantum effects, provide a sharp definition of symmetry protected topological phases. In particular, they can diagnose interaction effects on the noninteracting classification of fermionic symmetry protected topological phases. In this paper, we identify quantum anomalies in two kinds of (3+1)d fermionic symmetry protected topological phases: (i) topological insulators protected by CP (charge conjugation $\times$ reflection) and electromagnetic $\mathrm{U}\left(1\right)$ symmetries, and (ii) topological superconductors protected by reflection symmetry. For the first example, which is related to, by CPT-theorem, time-reversal symmetric topological insulators, we show that the CP-projected partition function of the surface theory is not invariant under large $\mathrm{U}\left(1\right)$ gauge transformations, but picks up an anomalous sign, signaling a ${\mathbb{Z}}_2$ topological classification. Similarly, for the second example, which is related to, by CPT-theorem, class DIII topological superconductors, we discuss the invariance/noninvariance of the partition function of the surface theory, defined on the three-torus and its descendants generated by the orientifold projection, under large diffeomorphisms (coordinate transformations). The connection to the collapse of the noninteracting classification by an integer ($\mathbb{Z}$) to ${\mathbb{Z}}_{16}$, in the presence of interactions, is discussed.

We generalize the theory of thermoelectrics to include coherent electron systems under adiabatic ac driving, accounting for quantum pumping of charge and heat, as well as for the work exchanged between the electron system and driving potentials. We derive the relevant response coefficients in the adiabatic regime and show that they obey generalized Onsager reciprocity relations. We analyze the consequences of our generalized thermoelectric framework for quantum motors, generators, heat engines, and heat pumps, characterizing them in terms of efficiencies and figures of merit. We illustrate these concepts in a model for a quantum pump.

The classical ground states of the SU(4) Heisenberg model on the face-centered-cubic lattice constitute a highly degenerate manifold. We explicitly construct all the classical ground states of the model. To describe quantum fluctuations above these classical states, we apply linear flavor-wave theory. At zero temperature, the bosonic flavor waves select the simplest of these SU(4) symmetry-breaking states, the four-sublattice-ordered state defined by the cubic unit cell of the fcc lattice. Due to geometrical constraints, flavor waves interact along specific planes only, thus rendering the system effectively two dimensional and forbidding ordering at finite temperatures. We argue that longer-range interactions generated by quantum fluctuations can shift the transition to finite temperatures.

This is the first of two papers that introduce a general expression for the tracer diffusivity in complex, periodic energy landscapes with $M$ distinct hop rates in one-, two-, and three-dimensional diluted systems (low-coverage, single-tracer limit). The present report focuses on the analysis of diffusion in systems where the end sites of the hops are located symmetrically with respect to the hop origins (symmetric hops), as encountered in many ideal surfaces and bulk materials. For diffusion in two dimensions, a number of formulas are presented for complex combinations of the different hops in systems with triangular, rectangular, and square symmetry. The formulas provide values in excellent agreement with kinetic Monte Carlo simulations, concluding that the diffusion coefficient can be directly determined from the proposed expressions without performing the simulations. Based on the diffusion barriers obtained from first-principles calculations and a physically meaningful estimate of the attempt frequencies, the proposed formulas are used to analyze the diffusion of Cu, Ag, and Rb adatoms on the surface and within the van der Waals (vdW) gap of a model topological insulator, ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. Considering the possibility of adsorbate intercalation from the terraces to the vdW gaps at morphological steps, we infer that, at low coverage and room temperature, (i) a majority of the Rb atoms bounce back at the steps and remain on the terraces, (ii) Cu atoms mostly intercalate into the vdW gap, the remaining fraction staying at the steps, and (iii) Ag atoms essentially accumulate at the steps and gradually intercalate into the vdW gap. These conclusions are in good qualitative agreement with previous experiments. The companion report (M. A. Gosálvez et al., Phys. Rev. B, submitted] extends the present study to the description of systems that contain asymmetric hops.