We report the existence of metallic two-dimensional electron gases (2DEGs) at the (001) and (101) surfaces of bulk-insulating ${\mathrm{TiO}}_2$ anatase due to local chemical doping by oxygen vacancies in the near-surface region. Using angle-resolved photoemission spectroscopy, we find that the electronic structure at both surfaces is composed of two occupied subbands of $d_{xy}$ orbital character. While the Fermi surface observed at the (001) termination is isotropic, the 2DEG at the (101) termination is anisotropic and shows a charge carrier density three times larger than at the (001) surface. Moreover, we demonstrate that intense UV synchrotron radiation can alter the electronic structure and stoichiometry of the surface up to the complete disappearance of the 2DEG. These results open a route for the nanoengineering of confined electronic states, the control of their metallic or insulating nature, and the tailoring of their microscopic symmetry, using UV illumination at different surfaces of anatase.

Superconductivity in aromatic hydrocarbons doped with potassium (K) such as ${\mathrm{K}}_3$ [picene (PCN)] and ${\mathrm{K}}_3$ [phenanthrene (PHN)] is found for only armchair-type polycyclic aromatic hydrocarbon. In this paper the thermodynamics of the reaction processes of PHN or anthracene (AN, zigzag type) with K was studied using differential scanning calorimetry and x-ray diffraction. We show that PHN decomposes during the reaction, triggered by hydrogen abstraction, to give metal hydride KH and unknown amorphous. No stable doped phases exist in ${\mathrm{K}}_{\mathrm{x}}\left(\mathrm{PHN}\right)$ with stoichiometries of $x=1–3$. However, in the case of AN, a stable doped phase forms. We claim that PHN, which has been reported to be energetically more stable in the ground state than AN by first principle calculations, is unstable upon doping. We also suggest that the superconductivity in ${\mathrm{K}}_3\left(\mathrm{PCN}\right)$ is due to the misinterpretation of experimental data, which actually arises from ferromagnetic impurities. We have never detected the superconductivity above 2 K in these compounds. The superconductivity in both ${\mathrm{K}}_x\left(\mathrm{PHN}\right)$ and ${\mathrm{K}}_x\left(\mathrm{PCN}\right)$ is concluded to be highly questionable.

The magnetic excitation spectrum in the bilayer iridate ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ has been investigated using high-resolution resonant inelastic x-ray scattering (RIXS) performed at the iridium $L_3$ edge and theoretical techniques. A study of the systematic dependence of the RIXS spectrum on the orientation of the wave-vector transfer $\mathbf{Q}$, with respect to the iridium-oxide bilayer, has revealed that the magnon dispersion is comprised of two branches well separated in energy and gapped across the entire Brillouin zone. Our results contrast with those of an earlier study which reported the existence of a single dominant branch. While these earlier results were interpreted as two overlapping modes within a spin-wave model of weakly coupled iridium-oxide planes, our results are more reminiscent of those expected for a system of weakly coupled dimers. In this latter approach, the lower- and higher-energy modes find a natural explanation as those corresponding to transverse and longitudinal fluctuations, respectively. We have therefore developed a bond-operator theory which describes the magnetic dispersion in ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$ in terms of quantum dimer excitations. In our model, dimerization is produced by the leading Heisenberg exchange $J_c$, which couples iridium ions in adjacent planes of the bilayer. The Hamiltonian also includes in-plane exchange $J$, as well as further neighbor couplings and relevant anisotropies. The bond-operator theory provides an excellent account of the dispersion of both modes, while the measured $\mathbf{Q}$ dependence of the RIXS intensities is in reasonable qualitative accord with the spin-spin correlation function calculated from the theory. We discuss our results in the context of the quantum criticality of bilayer dimer systems in the presence of anisotropic interactions derived from strong spin-orbit coupling.

Magnetoelectric (ME) materials are of fundamental interest and show broad potential for technological applications. The common dominant contribution to the ME response is the lattice-mediated one, which is proportional to both the Born electric charge $Z^{\mathrm{e}}$ and its analog, the dynamical magnetic charge $Z^{\mathrm{m}}$. Our previous study has shown that exchange striction acting on noncollinear spins induces much larger magnetic charges than those that depend on spin-orbit coupling. The hexagonal manganites $R{\mathrm{MnO}}_3$ and ferrites $R{\mathrm{FeO}}_3$ ($R=\text{Sc}$, Y, In, Ho-Lu) exhibit strong couplings between electric, magnetic, and structural degrees of freedom. The transition-metal ions in the basal plane antiferromagnetically coupled through super-exchange so as to form a ${120}^{\circ }$ noncollinear spin arrangement. In this paper, we present a theoretical study of the magnetic charges, and of the spin-lattice and spin-electronic ME constants, in these hexagonal manganites and ferrites. We clarify the conditions under which exchange striction leads to enhanced $Z^{\mathrm{m}}$ values and anomalously large in-plane spin-lattice ME effects.

We study a pseudo-spin-$\frac{1}{2}$ quantum dot in the cotunneling regime close to the particle-hole symmetric point. For a generic tunneling matrix we find a fixed point with interesting nonequilibrium properties, characterized by effective reservoirs with compensating spin orientation vectors weighted by the polarizations and the tunneling rates. At large bias voltage we study the magnetic field dependence of the dot magnetization and the current. The fixed point can be clearly identified by analyzing the magnetization of the dot. We characterize the universal properties for the case of two reservoirs and discuss deviations from the fixed point model in experimentally realistic situations.

The $\mathrm{WT}{\mathrm{e}}_2$ semimetal, as a typical layered transition-metal dichalcogenide, has recently attracted much attention due to an extremely large, nonsaturating parabolic magnetoresistance in the perpendicular field. Here, we report a systematic study of the angular dependence of the magnetoresistance in a $\mathrm{WT}{\mathrm{e}}_2$ single crystal. The significant anisotropic magnetotransport behavior in different magnetic field directions and violation of the Kohler's rule are observed. Unexpectedly, when the applied field and excitation current are both parallel to the tungsten chains of $\mathrm{WT}{\mathrm{e}}_2$, an exotic large longitudinal linear magnetoresistance as high as $1200\%$ at $15\mathrm{T}$ and $2\mathrm{K}$ is identified. Our results imply that the $\mathrm{WT}{\mathrm{e}}_2$ semimetal, due to its balanced hole and electron populations, seems to be the first material for which a large longitudinal linear magnetoresistance appears when the external magnetic field is parallel to the applied current. Our work may stimulate studies of double-carrier correlated materials and the corresponding quantum physics.

We present an analysis of deep-UV Raman measurements of graphite, graphene, and carbon nanotubes. For excitation energies above the strong optical absorption peak at the $M$ point in the Brillouin zone ($\approx 4.7$ eV), we partially suppress double-resonant scattering processes and observe the two-phonon density of states of carbon nanomaterials. The measured peaks are assigned to contributions from LO, TO, and LA phonon branches, supported by calculations of the phonon dispersion. Moreover, we gain access to the infrared-active $E_{1u}$ mode in graphite. By lowering the excitation energy and thus allowing double-resonant scattering processes, we demonstrate the rise of the $2D$ mode in graphite with ultrashort phonon wave vectors.

We present the ground state extension of the efficient continuous-time quantum Monte Carlo algorithm for lattice fermions of M. Iazzi and M. Troyer, Phys. Rev. B 91, 241118 (2015). Based on continuous-time expansion of an imaginary-time projection operator, the algorithm is free of systematic error and scales linearly with projection time and interaction strength. Compared to the conventional quantum Monte Carlo methods for lattice fermions, this approach has greater flexibility and is easier to combine with powerful machinery such as histogram reweighting and extended ensemble simulation techniques. We discuss the implementation of the continuous-time projection in detail using the spinless $t-V$ model as an example and compare the numerical results with exact diagonalization, density matrix renormalization group, and infinite projected entangled-pair states calculations. Finally we use the method to study the fermionic quantum critical point of spinless fermions on a honeycomb lattice and confirm previous results concerning its critical exponents.

We discover a quantum Monte Carlo (QMC) method to solve the fermion sign problem in interacting fermion models by employing a Majorana representation of complex fermions. We call it the “Majorana QMC” (MQMC). MQMC simulations can be performed efficiently both at finite and zero temperatures. Especially, MQMC is fermion sign free in simulating a class of spinless fermion models on bipartite lattices at half filling and with an arbitrary range of (unfrustrated) interactions. Moreover, we find a class of $SU\left(N\right)$ fermionic models with odd $N$, which are sign free in MQMC but whose sign problem cannot be in solved in other QMC methods, such as continuous-time QMC. To the best of our knowledge, MQMC is the first auxiliary field QMC method to solve the fermion sign problem in spinless (more generally, an odd number of species) fermion models. We conjecture that MQMC could be applied to solve the fermion sign problem in more generic fermionic models.

In the quest for switching of the charge carrier type in conductive materials, we focus on nonsymmorphic crystals, which are expected to have highly anisotropic folded Fermi surfaces due to symmetry requirements. Following a simple tight-binding model simulation, we prepare nonsymmorphic ${\mathrm{IrO}}_2$ single-crystalline films with various growth orientations by molecular beam epitaxy, and systematically quantify their Hall effect for the corresponding field directions. The results clearly demonstrate that the dominant carrier type can be intrinsically controlled by the magnetic field direction, as also evidenced by first-principles calculations revealing nontrivial momentum dependence of the group velocity and mass tensor on the folded Fermi surfaces and its anisotropic nature for the field direction.

The relative stability of dislocation core configurations in body-centered-cubic metals is profoundly modified by the presence of solutes. Considering the Fe(C) system, we demonstrate by using density functional theory that carbon atoms destabilize the usual easy core to the benefit of the hard core configuration of the screw dislocation, which is unstable in pure metals. The carbon atom is at the center of a regular prism in a cementitelike local environment. The same dislocation core reconstruction is also found with other solutes (B, N, O) and in W(C). This unexpected low-energy configuration induces a strong solute-dislocation attraction, leading to dislocation core saturation by solute atoms, even for very low bulk solute concentrations. This core reconstruction will constitute an essential factor to account for in solute-segregation related phenomena, such as strain aging.

We have systematically studied a variety of vanadium dioxide $\left(\mathrm{V}{\mathrm{O}}_2\right)$ crystalline forms, including bulk single crystals and oriented thin films, using infrared (IR) near-field spectroscopic imaging techniques. By measuring the IR spectroscopic responses of electrons and phonons in $\mathrm{V}{\mathrm{O}}_2$ with sub-grain-size spatial resolution $(\sim 20\mathrm{nm})$, we show that epitaxial strain in $\mathrm{V}{\mathrm{O}}_2$ thin films not only triggers spontaneous local phase separations, but also leads to intermediate electronic and lattice states that are intrinsically different from those found in bulk. Generalized rules of strain- and symmetry-dependent mesoscopic phase inhomogeneity are also discussed. These results set the stage for a comprehensive understanding of complex energy landscapes that may not be readily determined by macroscopic approaches.

We provide a fully analytical microscopic theory for the proton correlations in water ice $I_h$. We compute the full diffuse elastic neutron scattering structure factor, which we find to be in excellent quantitative agreement with Monte Carlo simulations. It is also in remarkable qualitative agreement with experiment, in the absence of any fitting parameters. Our theory thus provides a tractable analytical starting point to account for more delicate features of the proton correlations in water ice. In addition, it directly determines an effective field theory of water ice as a topological phase.

The formation of extended electron states in one-dimensional nanostructures is of key importance for the function of molecular electronic devices. Here, we study the effects of strong electron-phonon interaction on the formation of extended electronic states in intentionally created Cl vacancy pairs and chains in a NaCl bilayer on Cu(111). The interaction between the vacancies was tailored by fabricating vacancy pairs and chains of different orientation and separation with atomic precision using vertical manipulation. Small vacancy separations led to the formation of quantum-well-like vacancy states and localized interface states. By using scanning tunneling spectroscopy, we measured their energy splitting and broadening as a function of the intervacancy separation. Remarkably, the energy splitting between the vacancy states is enlarged by level repulsion resulting from the phonon dressing of the electronic states, as evidenced by theory.

More than half a century after first being proposed by Sir Nevill Mott, the deceptively simple question of whether the interaction-driven electronic metal-insulator transition may be continuous remains enigmatic. Recent experiments on two-dimensional materials suggest that when the insulator is a quantum spin liquid, lack of magnetic long-range order on the insulating side may cause the transition to be continuous, or only very weakly first order. Motivated by this, we study a half-filled extended Hubbard model on a triangular lattice strip geometry. We argue, through use of large-scale numerical simulations and analytical bosonization, that this model harbors a continuous (Kosterlitz-Thouless-like) quantum phase transition between a metal and a gapless spin liquid characterized by a spinon Fermi surface, i.e., a “spinon metal.” These results may provide a rare insight into the development of Mott criticality in strongly interacting two-dimensional materials and represent one of the first numerical demonstrations of a Mott insulating quantum spin liquid phase in a genuinely electronic microscopic model.

Using multiple scattering theory, we derived for the first time analytical formulas for electrostrictive tensors for two-dimensional metamaterial systems. The electrostrictive tensor terms are found to depend explicitly on the symmetry of the underlying lattice of the metamaterial, and they also depend explicitly on the direction of a local effective wave vector. These analytical results enable us to calculate light induced body forces inside a composite system (metamaterial) using the Helmholtz stress tensor within the effective medium formalism in the sense that the fields used in the stress tensor are those obtained by solving the macroscopic Maxwell equation with the microstructure of the metamaterial replaced by an effective medium. Our results point to some fundamental questions of using an effective medium theory to determine optical force density. In particular, the fact that Helmholtz tensor carries electrostrictive terms that are explicitly symmetry dependent means that the standard effective medium parameters cannot give sufficient information to determine body force density, even though they can give the correct total force. A more challenging issue is that the electrostrictive terms are related to a local effective wave vector, and it is not always obtainable in systems with boundary reflections within the context of a standard effective medium approach.

We explore the vibrational behavior and stability of ferroelectric ${\text{ZnTiO}}_3$ under high pressure by Raman spectroscopy and second-harmonic-generation (SHG) measurements. Ab initio lattice-dynamics calculations have been employed to solve a controversy concerning the phonon-dispersion relations of ${\text{ZnTiO}}_3$ and to carry out an assignment of the Raman modes. A ferroelectric to paraelectric phase transition has been observed both by Raman spectroscopy and SHG at 20.8 GPa. Contrary to ${\text{LiNbO}}_3$, the ferroelectric soft mode of ${\text{ZnTiO}}_3$ has been found to be the $A_1$(2) and not the $A_1$(1) mode. The calculated eigenvectors show that the $A_1$(2) mode of ferroelectric ${\text{ZnTiO}}_3$ is an antiphase vibration of the Ti atom against the oxygen framework, similar to the soft modes observed in ferroelectric perovskites. The SHG signal of ${\text{ZnTiO}}_3$ has been found to be independent of the grain size below the phase transition, indicating that ${\text{ZnTiO}}_3$ is a phase-matchable compound.

We have studied the structure, magnetic, and transport properties of copper-substituted iron telluride. Our results extend the range of copper substitution to 60% substitution per formula unit, which is far beyond previously stated solubility limits. Substitution of copper into antiferromagnetic iron telluride is found to suppress the signatures of the low-temperature transitions in susceptibility and resistance measurements, giving rise to an insulating, spin-glass state. Upon increasing the copper substitution from 4% to 6%, short-range antiferromagnetic order appears followed by the combined magnetic and structural transition at a lower temperature, although the magnetic order is ultimately not resolution limited with a correlation length of 250 Å in the 6% Cu-substituted sample, in contrast to the magnetic order of the 4% copper-substituted sample, which is resolution limited. Upon warming the 6% Cu-substituted sample in the presence of a 5 T magnetic field oriented along the $b$ axis, magnetic and structural phase transitions are observed at a temperature much lower than those of the magnetic and structural transitions which occur in zero field. Furthermore, these transitions are absent upon cooling in this field. We discuss the field results in the most general terms possible, including possible random field effects.

The dielectric function of silver is a fundamental quantity related to its electronic structure and describes its optical properties. However, results published over the past six decades are in part inconsistent and exhibit significant discrepancies. The measurement is experimentally challenging with the values of dielectric function spanning over five orders of magnitude from the mid-infrared to the visible/ultraviolet spectral range. Using broadband spectroscopic ellipsometry, we determine the complex-valued dielectric function of evaporated and template stripped polycrystalline silver films from 0.05 eV $\left(\lambda =25\mu \text{m}\right)$ to 4.14 eV ($\lambda =300$ nm) with a statistical uncertainty of less than $1\%$. From Drude analysis of the 0.1–3 eV range, values of the plasma frequency $\hslash {\omega }_p=8.9\pm 0.2$ eV, dielectric function at infinite frequency ${\epsilon }_{\infty }=5\pm 2$, and relaxation time $\tau =1/\mathrm{\Gamma }=17\pm 3$ fs are obtained, with the absolute uncertainties estimated from systematic errors and experimental repeatability. Further analysis based on the extended Drude model reveals an increase in $\tau$ with decreasing frequency in agreement with Fermi liquid theory, and extrapolates to $\tau \simeq 22$ fs for zero frequency. A deviation from simple Fermi liquid behavior is suggested at energies below 0.1 eV $\left(\lambda =12\mu \text{m}\right)$ with the onset of a further increase in $\tau$ connecting to the DC value from transport measurements of $\sim 40$ fs. The results are consistent with a wide range of optical and plasmonic experiments throughout the infrared and visible/ultraviolet spectral range. However, due to the polycrystalline nature of our sample, the values measured are not likely reaching the intrinsic limit of silver. The influence of grain boundaries, defect scattering, and surface oxidation is discussed. The results are compared with our previous measurements of the dielectric function of gold [Olmon et al., Phys. Rev. B 86, 235147 (2012)].

Based on first-principles calculations, we have found that a family of two-dimensional transition-metal dichalcogenide haeckelites with square-octagonal lattice $M{X}_2-4-8(M=\mathrm{Mo},W\text{and}X=S,\mathrm{Se}\text{and}\mathrm{Te})$ can host quantum spin Hall effect. The phonon spectra indicate that they are dynamically stable, and the largest band gap is predicted to be around 54 meV. These will pave the way for potential applications of topological insulators. We have also established a simple tight-binding model on a squarelike lattice to achieve a topologically nontrivial quantum state, which extends the study from honeycomb lattice to squarelike lattice and broadens the promising topological material systems greatly.