We show that the surface tension of fluid near the critical point may be correctly described by taking into consideration the microscopic structure of the system using a ${\varphi }^4$ field theory. We revise the theory of the surface tension near criticality to take into account a microscopic structure of the fluid. Focusing on the case of the Lennard-Jones fluid, we express the surface tension in terms of the compressibility of the reference hard-core system and its derivatives with respect to density. We demonstrate that the obtained analytical microscopic expression for the surface tension near the critical point is in a good agreement with numerical experiments, which emphasizes the impact of microscopic structure on the critical behavior of the surface tension in fluids. Our analysis provides a basis for studying the surface tension in small-volume systems important in many technological applications.

Spin orbit assisted Mott insulators such as sodium iridate (${\mathrm{Na}}_2{\mathrm{IrO}}_3$) have been an important subject of study in recent years. In these materials, the interplay of electronic correlations, spin-orbit coupling, crystal field effects, and a honeycomb arrangement of ions bring exciting ground states, predicted in the frame of the Kitaev model. The insulating character of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ has hampered its integration to an electronic device, desirable for applications, such as the manipulation of quasiparticles interesting for topological quantum computing. Here we show through electronic transport measurements supported by angle-resolved photoemission spectroscopy (ARPES) experiments, that electronic transport in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is ruled by variable range hopping and it is strongly dependent on the magnetic ordering transition known for bulk ${\mathrm{Na}}_2{\mathrm{IrO}}_3$, as well as on external electric fields. Electronic transport measurements allow us to deduce a value for the localization length and the density of states in our ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ thin crystal devices, and offer an alternative approach to study insulating 2D-materials.

Novel fermions with a pseudospin-1 structure can be realized as emergent quasiparticles in condensed matter systems. Here, we investigate its unusual properties during the Andreev reflection at a normal-metal/ superconductor (NS) interface. We show that distinct from the previously studied pseudospin-1/2 and two-dimensional electron gas models, the pseudospin-1 fermions exhibit a strongly enhanced Andreev reflection probability, and remarkably, can be further tuned to approach perfect Andreev reflection with unit efficiency for all incident angles, exhibiting a previously unknown super-Andreev reflection effect. The super-Andreev reflection leads to perfect transparency of the NS interface that strongly promotes charge injection into the superconductor and directly manifests as a differential conductance peak which can be readily probed in experiment. Additionally, we find that sizable longitudinal shifts exist in the normal and Andreev reflections of pseudospin-1 fermions. Distinct from the pseudospin-1/2 case, the shift is always in the forward direction in the subgap regime, regardless of whether the reflection is of retro- or specular type.

When the sizes of photonic nanoparticles are much smaller than the excitation wavelength, their optical response can be efficiently described with a series of polarizability tensors. Here, we propose a universal method to extract the different components of the response tensors associated with small plasmonic or dielectric particles. We demonstrate that the optical response can be faithfully approximated, as long as the effective dipole is not induced by retardation effects, hence do not depend on the phase of the illumination. We show that the conventional approximation breaks down for a phase-driven dipolar response, such as optical magnetic resonances in dielectric nanostructures. To describe such retardation induced dipole resonances in intermediate-size dielectric nanostructures, we introduce “pseudopolarizabilities” including first-order phase effects, which we demonstrate at the example of magnetic dipole resonances in dielectric spheres and ellipsoids. Our method paves the way for fast simulations of large and inhomogeneous metasurfaces.

Van der Waals heterostructures, created by putting graphene on other two-dimensional semiconducting materials, have become an effective strategy to enhance the physical properties and extend the possible applications of two-dimensional (2D) materials. Motivated by the successful synthesis of a graphene/${\mathrm{PbI}}_2$ heterostructure in a recent experiment [Nat. Commun. 11, 823 (2020)], here we use first-principles calculations to construct and investigate the electronic properties and interface characteristics of graphene/${\mathrm{PbI}}_2$ heterostructure. We find that the weak forces occurring at the interface keep heterostructures stable and maintain the intrinsic properties of the constituent graphene and ${\mathrm{PbI}}_2$ monolayers. At the equilibrium interlayer distance of 3.48 Å, the graphene/${\mathrm{PbI}}_2$ heterostructure forms an $n$-type Schottky contact. More interestingly, the Schottky barrier height and contact types in the graphene/${\mathrm{PbI}}_2$ heterostructure can be adjusted by electric field and interlayer coupling. The graphene/${\mathrm{PbI}}_2$ heterostructure can transform from a $n$-type Schottky contact to a $p$-type one or to Ohmic contact by applying electric field or by adjusting interlayer distance. The controllable electronic properties and contact types in graphene/${\mathrm{PbI}}_2$ heterostructure make it a promising candidate for designing and improving the performance of high-efficiency Schottky nanodevices.

The notion of spontaneous symmetry breaking has been used to describe phase transitions in a variety of physical systems. In crystalline solids, the breaking of certain symmetries, such as mirror symmetry, is difficult to detect unambiguously. Using $1T\text{−}{\mathrm{TaS}}_2$, we demonstrate here that rotational-anisotropy second harmonic generation (RA-SHG) is not only a sensitive technique for the detection of broken mirror symmetry, but also that it can differentiate between mirror symmetry-broken structures of opposite planar chirality. We also show that our analysis is applicable to a wide class of different materials with mirror symmetry-breaking transitions. Lastly, we find evidence for bulk mirror symmetry-breaking in the incommensurate charge density wave phase of $1T\text{−}{\mathrm{TaS}}_2$. Our results pave the way for RA-SHG to probe candidate materials where broken mirror symmetry may play a pivotal role.

We consider the time evolution of the gaps of the entanglement spectrum for a block of consecutive sites in finite transverse field Ising chains after sudden quenches of the magnetic field. We provide numerical evidence that, whenever we quench at or across the quantum critical point, the time evolution of the ratios of these gaps allows us to obtain universal information. They encode the low-lying gaps of the conformal spectrum of the Ising boundary conformal field theory describing the spatial bipartition within the imaginary time path integral approach to global quenches at the quantum critical point.

In recent years, there have been numerous examples of twisted bilayer systems that host remarkable physical properties that are not found in their untwisted counterparts. Motivated by this, we study the properties of twisted bilayers of the Kitaev honeycomb model in the Abelian spin-liquid phase. We show that for strong, short-ranged, interlayer interactions, a superlattice of non-Abelian defects forms in the twisted bilayer system. These non-Abelian defects are wormhole-like genons that allow anyons from one layer to tunnel to the other layer. We find that when a magnetic field is applied to the system, the low-energy dynamics of the twisted bilayer system can be mapped onto four quantum Ising models arising from the degrees of freedom localized on the genon defects. At small twist angles the Ising models are in a trivial paramagnetic phase, and at large twist angles they are in a ferromagnetic phase.

ZrTe was recently predicted to be a new type of three-dimensional topological semimetal (TSM) hosting triply degenerate point nodes close to Fermi energy. Here experimental results of magnetotransport and magnetization on single crystals of ZrTe are reported. The bulk electronic structure of the material is studied by quantum oscillations in both resistivity and magnetization, combined with first-principles calculations. ZrTe is a multiband system with both electron and hole charge carriers. Several small three-dimensional Fermi surfaces with light effective masses are observed. The magnetization at low temperature with field along the $C_3$ symmetry axis is paramagnetic and has a $H^{0.5}$ dependence at high field, in contrast to the diamagnetic response observed for in-plane magnetization. The compound provides a platform to explore exotic quantum phenomena related to the three-component fermionic quasiparticles distinct from other known TSMs.

${\mathrm{AlFe}}_2{\mathrm{B}}_2$, a layered ferromagnet with a Curie temperature near room temperature ($T_c\sim 270$ K), has attracted growing attention recently in condensed-matter physics and materials science. Utilizing angle-resolved photoemission spectroscopy and first-principles calculations, we provide evidence of a linear band and a saddlelike flat band in the vicinity of the Fermi energy ($E_F$) in ${\mathrm{AlFe}}_2{\mathrm{B}}_2$, consisting of slabs of ${\mathrm{Fe}}_2{\mathrm{B}}_2$ atoms separated by layers of Al. The anomalous scattering rate of the linear band varies linearly with the kinetic energy up to 120 meV, showing non-Fermi-liquid behavior consistent with T-linear dependence of resistivity. The flat band with $d_{xy}$ orbital character and with strong $k_y$ dispersion may be associated with spin orientation along the $a$ axis in layered ${\mathrm{AlFe}}_2{\mathrm{B}}_2$. Electronic and magnetic correlation can be modulated by tuning the flat band near $E_F$ in layered ternary borides. Our findings have important implications to exploit emergent physics and quantum phases in $3d$ transition metals.

We present a path integral formalism for expressing matrix elements of the density matrix of a quantum many-body system between any two coherent states in terms of standard Matsubara action with periodic (antiperiodic) boundary conditions on bosonic (fermionic) fields. We show that this enables us to express several entanglement measures for bosonic and fermionic many-body systems described by a Gaussian action in terms of the Matsubara Green function. We apply this formalism to compute various entanglement measures for the two-dimensional Bose-Hubbard model in the strong-coupling regime, both in the presence and absence of Abelian and non-Abelian synthetic gauge fields, within a strong-coupling mean-field theory. In addition, our method provides an alternative formalism for addressing time evolution of quantum many-body systems, with Gaussian actions, driven out of equilibrium without the use of Keldysh technique. We demonstrate this by deriving analytical expressions of the return probability and the counting statistics of several operators for a class of integrable models represented by free Dirac fermions subjected to a periodic drive in terms of the elements of their Floquet Hamiltonians. We provide a detailed comparison of our method with the earlier, related, techniques used for similar computations, discuss the significance of our results, and chart out other systems where our formalism can be used.

Topological properties are associated with nonlocal entanglement and global properties. On the other hand, in some topological systems, it has been shown that quasilocal operators detect topological transitions. We show in this work that in the case of noninteracting long-range symmetry-protected topological models, energy bonds signal topological transitions. Interestingly, we also find that they display some signatures at the Berezinskii-Kosterlitz-Thouless transition that occurs in a spin chain with first and second neighbor interactions, if we consider the first excited state instead of the ground state of the system. Moreover, we show that the ground-state topological transition in the spin Kitaev model in a honeycomb lattice, which displays topological long-range order, is also detected by the energy bond correlator. Despite the model being interacting, in the ground state it reduces to a noninteracting model. Even for the spin-liquid phase of the two-dimensional Heisenberg model with first and second neighbor interactions, where the system has true long-range entanglement and topological order, the local bonds do signal the topological phase transition.

We propose two measurement-based schemes to cool a nonlinear mechanical resonator down to energies close to that of its ground state. The protocols rely on projective measurements of a spin degree of freedom, which interacts with the resonator through a Jaynes-Cummings interaction. We show the performance of these cooling schemes, that can be either concatenated—i.e., built by repeating a sequence of dynamical evolutions followed by projective measurements—or single-shot. We characterize the performance of both cooling schemes with numerical simulations and pinpoint the effects of decoherence and noise mechanisms. Due to the ubiquity and experimental relevance of the Jaynes-Cummings model, we argue that our results can be applied in a variety of experimental setups.

Multilayer graphene lattices allow for an additional tunability of the band structure by the strong perpendicular electric field. In particular, the emergence of the new multiple Dirac points in ABA stacked trilayer graphene subject to strong transverse electric fields was proposed theoretically and confirmed experimentally. These new Dirac points dubbed “gullies” emerge from the interplay between strong electric field and trigonal warping. In this work, we first characterize the properties of new emergent Dirac points and show that the electric field can be used to tune the distance between gullies in the momentum space. We demonstrate that the band structure has multiple Lifshitz transitions and higher-order singularity of “monkey saddle” type. Following the characterization of the band structure, we consider the spectrum of Landau levels and structure of their wave functions. In the limit of strong electric fields when gullies are well separated in momentum space, they give rise to triply degenerate Landau levels. In the second part of this work, we investigate how degeneracy between three gully Landau levels is lifted in the presence of interactions. Within the Hartree-Fock approximation we show that the symmetry breaking state interpolates between the fully gully polarized state that breaks $C_3$ symmetry at high displacement field and the gully symmetric state when the electric field is decreased. The discontinuous transition between these two states is driven by enhanced intergully tunneling and exchange. We conclude by outlining specific experimental predictions for the existence of such a symmetry-breaking state.

The effect of proximity-induced interactions such as Rashba spin-orbit coupling (SOC) and exchange interaction on the electronic and optical properties of n-type monolayer (ML) ${\mathrm{MoS}}_2$ is investigated. We predict and demonstrate that the Rashba SOC can induce an in-plane spin splitting with terahertz (THz) energy, while the exchange interaction lifts the energy degeneracy in different valleys. Thus, spin polarization can be achieved in an n-type ML ${\mathrm{MoS}}_2$ and valley Hall or optical Hall effect can be observed using linearly polarized THz radiation. In such a case, the transverse optical conductivity ${\sigma }_{xy}\left(\omega \right)$ results from spin-flip transition within spin-split conduction bands and from the fact that contributions from electrons with different spin orientations in different valleys can no longer be canceled out. Interestingly, we find that for fixed effective Zeeman field (or exchange interaction) the lowest spin-split conduction band in ML ${\mathrm{MoS}}_2$ can be tuned from one in the $K$ valley to another one in the $K^{\prime }$ valley by varying the Rashba parameter ${\lambda }_R$. Therefore, by changing ${\lambda }_R$ we can turn the sign of the spin polarization and $\mathrm{Im}{\sigma }_{xy}\left(\omega \right)$ from positive to negative. Moreover, we find that the dominant contribution of the selection rules to ${\sigma }_{xx}\left(\omega \right)$ is from electrons in the $K$ valley and to ${\sigma }_{xy}\left(\omega \right)$ is from electrons in the $K^{\prime }$ valley. These important and interesting theoretical findings can be helpful to experimental observation of the optical Hall effect in valleytronic systems using linearly polarized THz radiation fields.

${\mathrm{EuTiO}}_3$, a band insulator, and ${\mathrm{LaTiO}}_3$, a Mott insulator, are both antiferromagnetic with transition temperatures ∼5.5 and ∼160 K, respectively. Here, we report the synthesis of ${\mathrm{Eu}}_{1-x}{\mathrm{La}}_x{\mathrm{TiO}}_3$ thin films with $x=0$ to 1 by oxide molecular beam epitaxy. The films in the full range have high crystalline quality and show no phase segregation, allowing us to carry out transport measurements to study their electrical and magnetic properties. From $x=0.03$ to 0.95, ${\mathrm{Eu}}_{1-x}{\mathrm{La}}_x{\mathrm{TiO}}_3$ films show conduction by electrons as charge carriers, with differences in carrier densities and mobilities, contrary to the insulating nature of pure ${\mathrm{EuTiO}}_3$ and ${\mathrm{LaTiO}}_3$. Following a rich phase diagram, the magnetic ground states of the films vary with increasing La-doping level, changing ${\mathrm{Eu}}_{1-x}{\mathrm{La}}_x{\mathrm{TiO}}_3$ from an antiferromagnetic insulator to an antiferromagnetic metal, a ferromagnetic metal, a paramagnetic metal, and back to an antiferromagnetic insulator. These emergent properties reflect the evolutions of the band structure, mainly at the Ti $t_{2\mathrm{g}}$ bands near the Fermi level, when ${\mathrm{Eu}}^{2+}$ are gradually replaced by ${\mathrm{La}}^{3+}$. This work sheds light on this method for designing the electrical and magnetic properties in strongly correlated oxides and completes the phase diagram of the titanate ${\mathrm{Eu}}_{1-x}{\mathrm{La}}_x{\mathrm{TiO}}_3$.

Projective symmetry groups are applied to Raman observations of the Kitaev quantum spin liquids in spherical lattice geometries realized by Platonic and Archimedean polyhedra. Parton single excitations in Kitaev spin polyhedra are characterized by double-valued irreducible representations of their belonging projective symmetry groups, whereas parton geminate excitations relevant to Raman scattering are decomposed into single-valued irreducible representations of the corresponding point symmetry groups. We combine a standard point symmetry group analysis of the Loudon-Fleury vertices and an elaborate projective symmetry group analysis of itinerant spinons against the ground gauge fields to reveal hidden selection rules for Raman scattering in ${\mathbb{Z}}_2$ spin liquids.