We study the optical response of two coupled oriented dipoles with the dimer axis perpendicular to the wave vector of light by analyzing how their scattering matrix can be decomposed. The scattering matrix can be written as a linear combination of three terms with a clear physical meaning: one for each particle and another that is responsible for the coupling and that vanishes for noninteracting or distant particles. We show that the interaction term may generate optical activity for certain scattering directions and that this effect manifests itself mostly in the near field. This simple and intuitive theory based on matrix and vector states of oriented dipoles also describes hybridization processes and Fano resonances. The decomposition method can be also formulated in terms of a hybrid basis that allows us to quantitatively determine the individual contribution of the in-phase and out-of-phase coupling modes to the overall intensity. Our method can help to understand the optical response of more complex nanostructures that can be decomposed into dipole terms. The results are illustrated in gold nanoantenna dimers which exhibit a strong dipolar resonance.

The quasi-two-dimensional nature of ${\mathrm{LaNiO}}_3$ (LNO) thin films on ${\mathrm{SrTiO}}_3$(001) (STO) and ${\mathrm{LaAlO}}_3$(001) (LAO) substrates is examined. Although both samples undergo a metal-insulator transition with decreasing thickness, the spatial distributions of the insulating phase within the films differ from each other. The LNO cell volume of the insulating phase in LNO/STO is larger than that of the metallic phase, and the large cells remain at the interface in thick films with metallic conduction, suggesting a high oxygen-vacancy content at the interface. The cell volume of the insulating phase in LNO/LAO is smaller than that of the metallic phase, and the cell volume of the entire film changes coherently. The cell-volume decrease indicates that the bonding changes from bulklike to covalent.

Topological defects with symmetry-breaking phase transitions have captured much attention. A vortex generated by topological defects exhibits exotic properties, and its flow direction can be switched by altering the spin configurations. Contrary to electromagnetic and acoustic domains, the topological transport of in-plane bulk waves in periodic structures with topological defects is not well explored due to the mode conversion between longitudinal and transverse modes. Here, we propose an elastic topological insulator with spontaneously broken symmetry based on the topological theory of defects and homotopy theory. Multiple topological transitions for in-plane bulk waves are achieved by topologically modifying the ellipse orientation in a triangular lattice of elliptical cylinders. The solid system, independent of the number of molecules in order-parameter space, breaks through the limit of point-group symmetry to emulate elastic pseudospin-orbit coupling. The transport robustness of the edge states is experimentally demonstrated. Our approach provides new possibilities for controlling and transporting in-plane bulk waves.

High-momentum spin waves (magnons) in ultrathin films of cobalt and iron have been explored thoroughly using inelastic scattering of low-energy electrons. The search for magnons in ultrathin nickel films failed, however, although high-energy magnons do exist in bulk nickel. The failure might be due to the weak coupling of nickel magnons to scattering electrons. In order to increase the coupling we deposited layers of cobalt onto Ni films and successfully studied the magnons of such films. The acoustic modes show the same dispersion as pure Co films, which is consistent with the nearly identical stiffness of bulk magnons in nickel and cobalt. Standing magnons in Ni films covered with Co are strongly damped at the Ni/Cu(100) interface when their total wave vector exceeds about $3\mathrm{n}{\mathrm{m}}^{-1}$.

The antiferromagnetic Heisenberg chain is expected to have an extended symmetry, $[\mathrm{SU}\left(2\right)\times \mathrm{SU}\left(2\right)]/{\mathrm{Z}}_2$, in the infrared limit whose physical interpretation is that the spin and dimer order parameters form the components of a common four-dimensional pseudovector. Here we numerically investigate this emergent symmetry using quantum Monte Carlo simulations of a modified Heisenberg chain (the $J\text{−}Q$ model) in which the logarithmic scaling corrections of the conventional Heisenberg chain can be avoided. We show how the two- and three-point spin and dimer correlation functions approach their forms constrained by conformal field theory as the system size increases and numerically confirm the expected effects of the extended symmetry on various correlation functions. We stress that sometimes the leading power laws of three-point (and higher) correlations are not given simply by the scaling dimensions of the lattice operators involved but can be faster decaying because of exact cancellations of contributions from the fields and currents under conformal symmetry.

The magnetic structure of ${\mathrm{TbMn}}_2{\mathrm{O}}_5$ and ${\mathrm{DyMn}}_2{\mathrm{O}}_5$ multiferroics has been studied by high-pressure neutron diffraction in a large pressure range up to 6.6 GPa. In both cases, we observe a pressure-induced commensurate magnetic phase with propagation vector $\left(\frac{1}{2}0\frac{1}{2}\right)$, growing with pressure at the expense of the ambient pressure phases. Being previously observed in ${\mathrm{YMn}}_2{\mathrm{O}}_5$ and ${\mathrm{PrMn}}_2{\mathrm{O}}_5$, this phase is most likely a generic feature of the $R{\mathrm{Mn}}_2{\mathrm{O}}_5$ multiferroic family. A simple model is proposed to explain qualitatively the emergence of this pressure-induced phase. Differences between ${\mathrm{TbMn}}_2{\mathrm{O}}_5$  and ${\mathrm{DyMn}}_2{\mathrm{O}}_5$ behaviors at ambient and low pressures provide clues on the interaction scheme.

Recent demonstrations on manipulating antiferromagnetic (AF) order have triggered a growing interest in antiferromagnetic metal, and potential high-density spintronic applications demand further improvements in the anisotropic magnetoresistance (AMR). The antiferromagnetic semimetals (AFS) are newly discovered materials that possess massless Dirac fermions that are protected by the crystalline symmetries. In this material, a reorientation of the AF order may break the underlying symmetries and induce a finite energy gap. As such, the possible phase transition from the semimetallic to insulating phase gives us a choice for a wide range of resistance, ensuring a large AMR. To further understand the robustness of the phase transition, we study thermal fluctuations of the AF order in AFS at a finite temperature. For macroscopic samples, we find that the thermal fluctuations effectively decrease the magnitude of the AF order by renormalizing the effective Hamiltonian. Our finding suggests that the insulating phase exhibits a gap narrowing at elevated temperatures, which leads to a substantial decrease in AMR. We also examine spatially correlated thermal fluctuations for microscopic samples by solving the microscopic Landau-Lifshitz-Gilbert equation, finding a quantitative difference in the gap narrowing effect from that of the macroscopic sample. For both cases, the semimetallic phase shows a minimal change in its transmission spectrum, illustrating the robustness of the symmetry-protected states in AFS. Our finding may serve as a guideline for estimating and maximizing AMR of the AFS samples at elevated temperatures.

We investigate the vortex lattice and vortex bound states in ${\mathrm{CsFe}}_2{\mathrm{As}}_2$ single crystals by scanning tunneling microscopy/spectroscopy (STM/STS) under various magnetic fields. A possible structural transition or crossover of vortex lattice is observed with the increase of magnetic field, i.e., the vortex lattice changes from a distorted hexagonal lattice to a distorted tetragonal one at the magnetic field near 0.5 T. It is found that a mixture of stripelike hexagonal and square vortex lattices emerges in the crossover region. The vortex bound state is also observed in the vortex center. The tunneling spectra crossing a vortex show that the bound-state peak position holds near zero bias with the STM tip moving away from the vortex core center. The Fermi energy estimated from the vortex bound state energy is very small. Our investigations provide experimental information to both the vortex lattice and the vortex bound states in this iron-based superconductor.

Fractons are gapped pointlike excitations in $d=3$ topological ordered phases whose motion is constrained. They have been discovered in several gapped models but a unifying physical mechanism for generating them is still missing. It has been noticed that in symmetric-tensor $\mathrm{U}\left(1\right)$ gauge theories, charges are fractons and cannot move freely due to, for example, the conservation of not only the charge but also the dipole moment. To connect these theories with fully gapped fracton models, we study Higgs and partial confinement mechanisms in rank-2 symmetric-tensor gauge theories, where charges or magnetic excitations, respectively, are condensed. Specifically, we describe two different routes from the rank-2 $\mathrm{U}\left(1\right)$ scalar charge theory to the X-cube fracton topological order, finding that a combination of Higgs and partial confinement mechanisms is necessary to obtain the fully gapped fracton model. On the other hand, the rank-2 ${\mathbb{Z}}_2$ scalar charge theory, which is obtained from the former theory upon condensing charge-2 matter, is equivalent to four copies of the $d=3$ toric code and does not support fracton excitations. We also explain how the checkerboard fracton model can be viewed as a rank-2 ${\mathbb{Z}}_2$ gauge theory with two different Gauss' law constraints on different lattice sites.

In this work, we introduce a new type of topological order that is protected by subsystem symmetries that act on lower-dimensional subsets of lattice many-body system, e.g., along lines or planes in a three-dimensional system. The symmetry groups for such systems exhibit a macroscopic number of generators in the infinite-volume limit. We construct a set of exactly solvable models in $2D$ and $3D$, which exhibit such subsystem SPT (SSPT) phases with one-dimensional subsystem symmetries. These phases exhibit analogs of phenomena seen in SPTs protected by global symmetries: gapless edge modes, projective realizations of the symmetries at the edge, and nonlocal order parameters. Such SSPT phases are proximate, in theory space, to previously studied phases that break the subsystem symmetries and phases with fracton order, which result upon gauging them.

This paper explores different mechanisms that induce thermal rectification in the nanoscale. The presence of interacting energy channels combined with simple asymmetries is sufficient for promoting the desired behavior. We use simple quantum dot configurations, identifying the basic properties that enhance rectification for each case: the size of a quantum dot state space (which suggests the use of scaled up systems with many interacting channels), tunneling asymmetries due to coherent tunneling in a double quantum dot, or quantum interference in a triangular triple quantum dot. An efficient and tunable thermal diode is proposed using a channel capacitively coupled to a mesoscopic switch.

We study the electronic thermal drag in two different Coulomb-coupled systems, the first one composed of two Coulomb-blockaded metallic islands and the second one consisting of two parallel quantum wires. The two conductors of each system are electrically isolated and placed in the two circuits (the drive and the drag) of a four-electrode setup. The systems are biased, either by a temperature $\mathrm{\Delta }T$ or a voltage $V$ difference, on the drive circuit, while no biases are present on the drag circuit. In the case of a pair of metallic islands we use a master equation approach to determine the general properties of the dragged heat current $I_{\mathrm{drag}}^{\left(\mathrm{h}\right)}$, accounting also for cotunneling contributions and the presence of large biases. Analytic results are obtained in the sequential tunneling regime for small biases, finding, in particular, that $I_{\mathrm{drag}}^{\left(\mathrm{h}\right)}$ is quadratic in $\mathrm{\Delta }T$ or $V$ and nonmonotonic as a function of the interisland coupling. Finally, by replacing one of the electrodes in the drag circuit with a superconductor, we find that heat can be extracted from the other normal electrode. In the case of the two interacting quantum wires, using the Luttinger liquid theory and the bosonization technique, we derive an analytic expression for the thermal transresistivity ${\rho }_{12}^{\left(\mathrm{h}\right)}$, in the weak-coupling limit and at low temperatures. ${\rho }_{12}^{\left(\mathrm{h}\right)}$ turns out to be proportional to the electrical transresistivity, in such a way that their ratio (a kind of Wiedemann-Franz law) is proportional to $T^3$. We find that ${\rho }_{12}^{\left(\mathrm{h}\right)}$ is proportional to $T$ for low temperatures and decreases like $1/T$ for intermediate temperatures or like $1/T^3$ for high temperatures. We complete our analyses by performing numerical simulations that confirm the above results and allow us to access the strong-coupling regime.

The Peierls substitution in the energy functions of a carbon nanotube's sub-bands is carried out for a carbon nanotube of arbitrary chiral indices subjected to a perpendicular uniform magnetic field. The Peierls substitution represents a zeroth-order term of the asymptotically convergent power series in the magnetic field. It provides a very good approximation for the electron energy spectrum of a carbon nanotube subjected to a perpendicular uniform magnetic field, for magnetic fields currently accessible in laboratories.

Quasi-two-dimensional manganese thiophosphate, $\mathrm{MnP}{\mathrm{S}}_3$, is interesting due to its two-dimensional antiferromagnetic structure observed at low temperatures as well as possible technological applications. While the spectroscopic properties of bulk $\mathrm{MnP}{\mathrm{S}}_3$ structures have been extensively studied, the spectroscopic characteristics of freestanding $\mathrm{MnP}{\mathrm{S}}_3$ layers are yet to be explored. We present an experimental study on the spectroscopic properties of a freestanding $\mathrm{MnP}{\mathrm{S}}_3$ single layer obtained through exfoliation from bulk $\mathrm{MnP}{\mathrm{S}}_3$. We find that the position of the main peak in the electron-energy-loss spectrum (EELS) is shifted from approximately 19 eV in bulk $\mathrm{MnP}{\mathrm{S}}_3$ to 15 eV in single $\mathrm{MnP}{\mathrm{S}}_3$ layer. Theoretical calculations show that this peak corresponds to a volume plasmon in bulk $\mathrm{MnP}{\mathrm{S}}_3$ and a damped plasmon peak in single-layer $\mathrm{MnP}{\mathrm{S}}_3$. In addition, the dispersion of this peak was investigated using momentum-resolved electron-energy-loss spectroscopy. The peak dispersion for the single-layer $\mathrm{MnP}{\mathrm{S}}_3$ displays a square-root behavior characteristic of a two-dimensional plasmon. We show that EELS spectra provide both a means to identify single-layer $\mathrm{MnP}{\mathrm{S}}_3$ from bulk structures and also show the effects of low dimensionality on the electronic excitations.

Perovskite nickelate heterostructures consisting of single unit cells of ${\mathrm{EuNiO}}_3$ and ${\mathrm{LaNiO}}_3$ have been grown on a set of single crystalline substrates by pulsed laser interval deposition to investigate the effect of epitaxial strain on electronic and magnetic properties at the extreme interface limit. Despite the variation of substrate in-plane lattice constants and lattice symmetry, the structural response to heterostructuring is primarily controlled by the presence of the ${\mathrm{EuNiO}}_3$ layer. In sharp contrast to bulk ${\mathrm{LaNiO}}_3$ or ${\mathrm{EuNiO}}_3$, the superlattices grown under tensile strains exhibit metal-to-insulator transitions (MIT) below room temperature. The onset of magnetic and electronic transitions associated with the MIT can be further separated by application of large tensile strain. Furthermore, these transitions can be entirely suppressed by very small compressive strain. X-ray resonant absorption spectroscopy measurements reveal that such strain-controlled MIT is directly linked to a strain-induced self-doping effect without any chemical doping.

Atomically thin layers of transition metal dichalcogenides (TMDCs) exhibit exceptionally strong Coulomb interaction between charge carriers due to the two-dimensional carrier confinement in connection with weak dielectric screening. The van der Waals nature of interlayer coupling makes it easy to integrate TMDC layers into heterostructures with different dielectric or metallic substrates. This allows to tailor electronic and optical properties of these materials, as Coulomb interaction inside atomically thin layers is very susceptible to screening by the environment. Here, we theoretically investigate dynamical screening effects in TMDCs due to bulk substrates doped with carriers over a large density range, thereby offering three-dimensional plasmons as a tunable degree of freedom. We report a wide compensation of renormalization effects leading to a spectrally more stable exciton than predicted for static substrate screening, even if plasmons and excitons are in resonance. We also find a nontrivial dependence of the single-particle band gap on substrate doping density due to dynamical screening. Our investigation provides microscopic insight into the mechanisms that allow for manipulations of TMDC excitons by means of arbitrary plasmonic environments on the nanoscale.

We study entanglement dynamics in the nearest-neighbor fermionic chain that is subjected to both dc and ac electric fields. The dynamics gives the well-known Bloch oscillations in the dc field case provided that the system size is larger than the Bloch length, whereas in the ac field case the entropy is bounded and oscillates with the driving frequency at the points of dynamic localization, and has a logarithmic growth at other points. A combined ac+dc field yields super-Bloch oscillations for system sizes larger than the super-Bloch length, which puts a constraint on the device size in a typical nonequilibrium setup to observe super-Bloch oscillations where the device is connected to the leads. Entanglement entropy provides useful signatures for all of these phenomena, and an alternate way to capture the various length scales involved.

The low-temperature regime of charge-qubit decoherence due to its Coulomb interaction with electrons tunneling through Luttinger liquid quantum-point contact (QPC) is investigated. The study is focused on quantum detector properties of Luttinger liquid QPC. Earlier results on related problems were approximate, up to the second order in small electrostatic coupling between charge qubit and QPC. However, here it is shown that in low- (and zero-) temperature limits the respective perturbative decoherence and acquisition of information timescales both tend to diverge, thus shadowing a true picture of low-temperature quantum detection for such quantum systems. Here it is shown that one can successfully circumvent these difficulties in order to restore a complete and exact picture of low-temperature decoherence and quantum detection for charge qubits measured by arbitrary Luttinger liquid QPC. To do this, here I prove two general mathematical statements [summation (S) theorem and (S) lemma] about exact re-exponentiation of Keldysh-contour ordered $T$ exponent for an arbitrary Luttinger liquid tunnel Hamiltonian. The resulting exact formulas are believed to be important in a wide range of those Luttinger liquid problems, where real-time quantum field dynamic is crucial. As the result, decoherence and acquisition of information timescales as well as QPC quantum detector efficiency rate are calculated exactly and are shown to have a dramatic dependence on repulsive interaction between electrons in one-dimensional (1D) leads of QPC. In particular, it is found that at temperatures close to zero there exists a certain well-defined threshold value $g\approx g_{cr}\left(T\right)$ of Luttinger liquid correlation parameter $g$ ($0<g\le 1$) which serves as a sharp boundary between the region of good (or even perfect) quantum detection at $g<g_{cr}$ and the region of quantum detection breakdown for $g>g_{cr}$. Moreover, this abrupt decrease of QPC quantum detector efficiency $Q$ with the increase of $g$ in the close vicinity of value $g_{cr}$ represents evidence of interaction-dependent instability of all the quantum detection procedures for any Luttinger liquid QPC quantum detector at sufficiently low temperatures $T_{cr}\left(g\right)$. The reasons behind these effects are discussed. Also, it is shown that the low-temperature detection instability effect is able to explain the large mismatch between expected and observed decoherence timescales in two recent experiments [Gorman et al., Phys. Rev. Lett. 95, 090502 (2005); Petersson et al., Phys. Rev. Lett. 105, 246804 (2010)] on charge-qubit quantum dynamics.

The lattice dynamics of members of the $M_2{\mathrm{Mo}}_6{\mathrm{Se}}_6$ family of materials with guest ions $M=\mathrm{K}$, Rb, Cs, In, and Tl has been studied using inelastic x-ray scattering and Raman spectroscopy at room temperature, as well as by ab initio calculations. We find a good match between calculations and experiment, both for structure factors (eigenvectors) and for the calculated phonon frequencies. The observed lattice dynamics for ${\mathrm{Tl}}_2{\mathrm{Mo}}_6{\mathrm{Se}}_6$ show no signs of anharmonicity or absence of avoided dispersion crossings, thus ruling out previously hypothesized rattling phonon modes. The reduced mode energies for ${\mathrm{In}}_2{\mathrm{Mo}}_6{\mathrm{Se}}_6$ are identified as only partially responsible for the lower superconducting transition temperature $T_c$ in this material when compared to ${\mathrm{Tl}}_2{\mathrm{Mo}}_6{\mathrm{Se}}_6$.