We show the thickness-dependent transition from metallic conduction to tunneling in three-dimensional (3D) Ag/Si/Ag nanojunctions through layer-by-layer electronic structure and quantum transport calculations. The transmission coefficients are calculated quantum mechanically within the framework of density functional theory in conjunction with nonequilibrium Green's function techniques. Thin junctions show nearly metallic character with no energy gap opening in Si layers due to the metal-induced interface states, and the transmission is independent of the stacking order of Si layers. An energy gap reemerges for Si layers deeply buried within thick junction, and the decay rate of transmission in this insulating region depends on the stacking order. Complex band analysis indicates that the decay of transmission is not determined by a single exponential constant but also depends on the available number of evanescent states. Calculating the electric resistance from the transmission coefficient requires a 3D generalization of the Landauer formula, which is not unique. We examine two approaches, the Landauer-Büttiker formula, with and without subtraction of the Sharvin resistance, and a semiclassical Boltzmann equation with boundary conditions defined by the transmission coefficients at the junction. We identify an empirical upper limit of $\sim 0.05$ per channel in the transmission coefficient, below which the Landauer-Büttiker formula without the Sharvin resistance correction remains a good approximation. In the high transmission limit, the Landauer-Büttiker formula with Sharvin correction and the semiclassical Boltzmann method reach fair agreement.

In this work the influence of film-substrate misorientation on the strain-induced ordering of graphene films on various metallic surfaces is examined using a mesoscopic continuum model and first-principles atomistic calculations. The periodicity and free energy of the Moiré patterns that emerge are studied as a function of film-substrate adhesion strength for misfit strains far from and close to an incommensurate-commensurate phase transition. Interestingly the lowest energy states are found to be at small but finite misorientation even though these states have a higher domain wall density than the zero-misorientation states. First-principles density functional theory calculations are used to connect the results with experimental findings in graphene epitaxy. This combination of mesoscopic and atomistic approaches can be applied to the study of a wide range of strained 2D material systems including the III-nitride monolayer systems.

High-throughput design of new materials with desired electronic properties, based on screening of large collections of crystal structures organized in the from of libraries or databases require fast, widely applicable, consistent and unsupervised methods to calculate the property of interest. In this work we present an approach for the calculation of surface energies of two-dimensional periodic crystal lattices which meets all these requirements. For materials slabs which are terminated with two identical surfaces, the task of calculating the surface energy is trivial. More problematic are the cases where both terminating surfaces are different, as there is no single established method allowing for equal treatment of a wide range of surface morphologies and orientations. Our proposed approach addresses this problem. It relies on appropriately chosen capping atoms, whose bonding energy contributions are used to approximate the total energy of the surface. The choice of the capping atoms is governed by a set of simple guidelines that are applicable for surfaces with different terminations. We present the results for different semiconductor materials and show that our approach leads to surface energies with errors that are below 10%, and that are as low as 2% in many cases. We show that hydrogen is not always the best choice for a capping atom if accurate surface energies are the target of the calculations.

In this article, we report negative thermal expansion and spin frustration in hexagonal ${\mathrm{GdInO}}_3$. Rietveld refinements of the x-ray diffraction patterns reveal that the negative thermal expansion in the temperature range of 50–100 K stems from the triangular lattice of ${\mathrm{Gd}}^{3+}$ ions. The downward deviation of the low-temperature inverse susceptibility $\left({\chi }^{-1}\right)$ versus $T$ plot from the Curie-Weiss law and the large value of the ratio, $|{\theta }_{\text{CW}}|/T_N>28$, where ${\theta }_{\text{CW}}$ and $T_N$ are respectively Curie-Weiss and Neel temperature, indicate a strong spin frustration, which inhibits long-range magnetic ordering down to 1.8 K. Magnetostriction measurements clearly demonstrate a spin-lattice coupling in the system. Low-temperature anomalous phonon softening, as obtained from temperature-dependent Raman measurements, also reveals the same. Our experimental observations are supported by first-principles density functional theory calculations of the electronic and phonon dispersion in ${\mathrm{GdInO}}_3$. The calculations suggest that the ${\mathrm{GdInO}}_3$ lattice is highly frustrated at low temperature. Further, the calculated normal mode frequencies of the Gd-related $\mathrm{\Gamma }$ point phonon modes reveal significant magnetoelastic coupling in this system. The competitive role of magnetic interaction energy and thermal stabilization energy in determining the change in interatomic distances is the possible origin for the negative thermal expansion in ${\mathrm{GdInO}}_3$ over a limited range of temperature.

Oxygen vacancy-vacancy interactions in rutile $\mathrm{Ti}{\mathrm{O}}_2$ are studied in conjunction with polaron formation trends using density functional theory calculations. It is found that polarons strongly enhance the formation of oxygen vacancies in this material and also mediate the interactions between existing vacancies. At distances below 1 nm, two isolated and charge-neutral vacancies exhibit attractive interactions with an equilibrium distance of about 4 Å. The attractive forces between vacancies partly arise from the polaronic transfer of excess electrons to reduce the potential energy. These discoveries provide microscopic explanations to the vacancy clustering phenomena, as well as a practical approach to stabilize the polarons at arbitrary Ti atoms in $\mathrm{Ti}{\mathrm{O}}_2$.

Thin-film multilayered spin-glass CuMn/Cu structures display glassy dynamics. The freezing temperature $T_f$ was measured for 40 layers of CuMn films of thickness $\mathcal{L}=4.5,9.0$, and 20.0 nm, sandwiched between nonmagnetic Cu layers of thickness $\approx 60$ nm. The Kenning effect, $T_f\propto ln\mathcal{L}$, is shown to follow from power-law dynamics where the correlation length grows from nucleation as $\xi (t,T)=c_1{a}_0{(t/{\tau }_0)}^{c_2(T/T_g)}$, leading to $[(T_f/T_g)c_{2}ln(t_{\text{co}}/{\tau }_0)]+ln{c}_1=ln(\mathcal{L}/a_0)$. Here, $T_g$ is the bulk spin-glass temperature, $c_1$ and $c_2$ are constants determined from the spin-glass dynamics, $t_{\text{co}}$ is the time for the correlation length to grow to the film thickness, ${\tau }_0$ is a characteristic exchange time $\approx \hslash /k_B{T}_g$, and $a_0$ is the average Mn-Mn separation. For $t\ge t_{\text{co}}$, the magnetization dynamics are simple activated, with a single activation energy ${\mathrm{\Delta }}_{\text{max}}\left(\mathcal{L}\right)/k_B{T}_g=(1/c_2)[ln(\mathcal{L}/a_0)-ln{c}_1]$ that does not change with time. Values for all these parameters are found for the three values of $\mathcal{L}$ explored in these measurements. We find experimentally ${\mathrm{\Delta }}_{\text{max}}\left(\mathcal{L}\right)/k_B=907$, 1246, and 1650 K, respectively, for the three CuMn thin-film multilayer thicknesses, consistent with power-law dynamics. We perform a similar analysis based on the activated dynamics of the droplet model and find a much larger spread for ${\mathrm{\Delta }}_{\text{max}}\left(\mathcal{L}\right)$ than found experimentally.

The XY pyrochlore antiferromagnet ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ exhibits a rare case of $Z_6$ discrete symmetry breaking in its ${\psi }_2$ magnetic ground state. Despite being well-studied theoretically, systems with high discrete symmetry breakings are uncommon in nature. Thus, ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ provides an experimental playground for the study of broken $Z_n$ symmetry, for $n>2$. A recent theoretical work examined the effect of a magnetic field on a pyrochlore lattice with broken $Z_6$ symmetry and applied it to ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. This study predicted multiple domain transitions depending on the crystallographic orientation of the magnetic field, inducing rich and controllable magnetothermodynamic behavior. In this work, we present neutron scattering measurements on ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ with a magnetic field applied along the [001] and [111] directions and provide experimental observation of these exotic domain transitions. In a [001] field, we observe a ${\psi }_2$ to ${\psi }_3$ transition at a critical field of $0.18\pm 0.05$ T. We are thus able to extend the concept of the spin-flop transition, which has long been observed in Ising systems, to higher discrete $Z_n$ symmetries. In a [111] field, we observe a series of domain-based phase transitions for fields of $0.15\pm 0.03$ T and $0.40\pm 0.03$ T. We show that these field-induced transitions are consistent with the emergence of twofold, threefold, and possibly sixfold Zeeman terms. Considering all the possible ${\psi }_2$ and ${\psi }_3$ domains, these Zeeman terms can be mapped onto an analog clock—exemplifying a literal clock anisotropy. Lastly, our quantitative analysis of the [001] domain transition in ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ is consistent with order-by-disorder as the dominant ground state selection mechanism.

We carry out a realistic simulation of Majorana nanowires in order to understand the latest high-quality experimental data [H. Zhang et al., arXiv:1603.04069 (2016)] and, in the process, develop a comprehensive picture for what physical mechanisms may be operational in realistic nanowires leading to discrepancies between minimal theory and experimental observations (e.g., weakness and broadening of the zero-bias peak and breaking of particle-hole symmetry). Our focus is on understanding specific intriguing features in the data, and our goal is to establish matters of principle controlling the physics of the best possible nanowires available in current experiments. We identify dissipation, finite temperature, multi-sub-band effects, and the finite tunnel barrier as the four most important physical mechanisms controlling the zero-bias conductance peak. Our theoretical results including these realistic effects agree well with the best available experimental data in ballistic nanowires.

A periodically driven rotor is a prototypical model that exhibits a transition to chaos in the classical regime and dynamical localization (related to Anderson localization) in the quantum regime. In a recent work [Phys. Rev. B 94, 085120 (2016)], A. C. Keser et al. considered a many-body generalization of coupled quantum kicked rotors, and showed that in the special integrable linear case, dynamical localization survives interactions. By analogy with many-body localization, the phenomenon was dubbed dynamical many-body localization. In the present work, we study nonintegrable models of single and coupled quantum relativistic kicked rotors (QRKRs) that bridge the gap between the conventional quadratic rotors and the integrable linear models. For a single QRKR, we supplement the recent analysis of the angular-momentum-space dynamics with a study of the spin dynamics. Our analysis of two and three coupled QRKRs along with the proved localization in the many-body linear model indicate that dynamical localization exists in few-body systems. Moreover, the relation between QRKR and linear rotor models implies that dynamical many-body localization can exist in generic, nonintegrable many-body systems. And localization can generally result from a complicated interplay between Anderson mechanism and limiting integrability, since the many-body linear model is a high-angular-momentum limit of many-body QRKRs. We also analyze the dynamics of two coupled QRKRs in the highly unusual superballistic regime and find that the resonance conditions are relaxed due to interactions. Finally, we propose experimental realizations of the QRKR model in cold atoms in optical lattices.

The evolution of the structural and magnetic properties of Fe films during growth on the W(001) surface have been studied with low energy electron diffraction, real-time low energy electron microscopy, and quasi-real-time, spin-polarized low energy electron microscopy in the absence of a magnetic field (virgin state). Depending on the growth temperature, different growth modes are observed: growth of atomically rough and highly strained (10.4% tensile) pseudomorphic films at room temperature, kinetically limited layer-by-layer growth (quasi–Frank-van der Merwe growth mode) of smooth pseudomorphic films up to 4 monolayers at around 500 K and growth of fully relaxed three-dimensional Fe islands on top of a thermodynamically stable 2-monolayer-thick wetting layer (Stranski-Krastanov growth mode) at and above 700 K. Around 500 K, layered growth is terminated by partial (2 monolayers) dewetting of the metastable Fe film and formation of thin, partially relaxed, elongated islands on a thermodynamically stable 2 monolayer film. Ferromagnetic order is first detected during growth at room temperature at 2.35 monolayer Fe film thickness. The magnetization is in-plane with a thickness-dependent direction, rotating from the substrate $\langle 110\rangle$ directions at 3 monolayers toward the $\langle 100\rangle$ directions at 4 monolayers and back again toward the $\langle 110\rangle$ directions at about 8 monolayers. The in-plane spin reorientation that occurs at room temperature is accompanied by significant changes of the magnetic domain structure. In the Frank-van der Merwe growth regime, large magnetic domains are observed in metastable 3 and 4 monolayer films. The isolated three-dimensional Fe islands that form in the Stranski-Krastanov regime have vortex, quasi-single domain ($C$ state), or single magnetic domain structures, depending on their size and shape. The detailed results that are obtained with high thickness, lateral and azimuthal angular resolution with spin-polarized low energy electron microscopy are compared with earlier laterally averaging and laterally resolving magnetic studies, and discrepancies are explained.

We have studied the ground-state properties of para-hydrogen in one dimension and in quasi-one-dimensional configurations using the path-integral ground-state Monte Carlo method. This method produces zero-temperature exact results for a given interaction and geometry. The quasi-one-dimensional setup has been implemented in two forms: the inner channel inside a carbon nanotube coated with ${\mathrm{H}}_2$ and a harmonic confinement of variable strength. Our main result is the dependence of the Luttinger parameter on the density within the stable regime. Going from one dimension to quasi-one dimension, keeping the linear density constant, produces a systematic increase of the Luttinger parameter. This increase is, however, not enough to reach the superfluid regime and the system always remain in the quasicrystal regime, according to Luttinger liquid theory.

The mean-field approximation is at the heart of our understanding of complex systems, despite its fundamental limitation of completely neglecting correlations between the elementary constituents. In a recent work [Phys. Rev. Lett. 117, 130401 (2016)], we have shown that in quantum many-body systems at finite temperature, two-point correlations can be formally separated into a thermal part and a quantum part and that quantum correlations are generically found to decay exponentially at finite temperature, with a characteristic, temperature-dependent quantum coherence length. The existence of these two different forms of correlation in quantum many-body systems suggests the possibility of formulating an approximation, which affects quantum correlations only, without preventing the correct description of classical fluctuations at all length scales. Focusing on lattice boson and quantum Ising models, we make use of the path-integral formulation of quantum statistical mechanics to introduce such an approximation, which we dub quantum mean-field (QMF) approach, and which can be readily generalized to a cluster form (cluster QMF or cQMF). The cQMF approximation reduces to cluster mean-field theory at $T=0$, while at any finite temperature it produces a family of systematically improved, semi-classical approximations to the quantum statistical mechanics of the lattice theory at hand. Contrary to standard MF approximations, the correct nature of thermal critical phenomena is captured by any cluster size. In the two exemplary cases of the two-dimensional quantum Ising model and of two-dimensional quantum rotors, we study systematically the convergence of the cQMF approximation towards the exact result, and show that the convergence is typically linear or sublinear in the boundary-to-bulk ratio of the clusters as $T\to 0$, while it becomes faster than linear as $T$ grows. These results pave the way towards the development of semiclassical numerical approaches based on an approximate, yet systematically improved account of quantum correlations.

Although cloaks are effective at suppressing the observability of static objects, they can be defeated when in motion. Here we discuss a general technique to cloak the motion of objects from static observers, based on compensating the Doppler shift associated with their motion with frequency conversion sustained by a spatiotemporally modulated cover. The concept is theoretically and numerically demonstrated in a system composed of a planar reflector covered by a spatiotemporally modulated slab. It is shown that, for properly selected modulation frequency, the composite system can appear to an external observer as stationary, even though it is actually moving. This concept may pave the way to the minimization of clutter produced by moving objects as well as to new directions in the science of cloaking.

We propose an algorithm to determine maximally localized Wannier functions (MLWFs). This algorithm, based on recent theoretical developments, does not require any physical input such as initial guesses for the Wannier functions, unlike popular schemes based on the projection method. We discuss how the projection method can fail on fine grids when the initial guesses are too far from MLWFs. We demonstrate that our algorithm is able to find localized Wannier functions through tests on two-dimensional systems, simplified models of semiconductors, and realistic DFT systems by interfacing with the wannier90 code. We also test our algorithm on the Haldane and Kane-Mele models to examine how it fails in the presence of topological obstructions.

We revisit the theory of the collective neutral excitation mode in the fractional quantum Hall effect at Landau level filling fractions $\nu =1/3$ and $\nu =7/3$. We include the effect of finite thickness of the two-dimensional electron gas and use extensive exact diagonalizations in the torus geometry. In the lowest Landau level the collective gapped mode, i.e., the magnetoroton always merges in the continuum in the long-wavelength limit. In the second Landau level the mode is well defined only for wave vectors smaller than a critical value and disappears in the continuum beyond this point. Its curvature near zero momentum is opposite to that of the LLL. It is well separated from the continuum even at zero momentum, and the gap of the continuum of higher-lying states is twice the collective mode gap at $k=0$. The shape of the dispersion relation survives a perturbative treatment of Landau level mixing.

Topological materials with both insulating and semimetal phases can be protected by crystalline (e.g., mirror) symmetry. The insulating phase, called a topological crystalline insulator (TCI), has been investigated intensively and observed in three-dimensional materials. However, the predicted two-dimensional (2D) materials with TCI phase are explored much less than 3D TCIs and 2D topological insulators, while the 2D TCIs considered thus far possess almost exclusively a square-lattice structure with the mirror Chern number ${\mathcal{C}}_M=-2$. Here, we predict theoretically that a hexagonal monolayer of Dirac semimetal ${\mathrm{Na}}_3\mathrm{Bi}$ is a 2D TCI with a mirror Chern number ${\mathcal{C}}_M=-1$. The large nontrivial gap of 0.31 eV is tunable and can be made much larger via strain engineering, while the topological phases are robust against strain, indicating a high possibility for room-temperature observation of quantized conductance. In addition, a nonzero spin Chern number ${\mathcal{C}}_S=-1$ is obtained, indicating the coexistence of a 2D topological insulator and a 2D TCI, i.e., the dual topological character. Remarkably, a spin-valley polarization is revealed in the ${\mathrm{Na}}_3\mathrm{Bi}$ monolayer due to the breaking of crystal inversion symmetry. The dual topological character is further explicitly confirmed via the unusual behavior of the edge states under the corresponding symmetry breaking.

Trilayer graphene (TLG) exhibits rich, alternative electronic properties and extraordinary quantum Hall phenomena owing to enhanced electronic interactions and tunable chirality of its quasiparticles. Here, we report direct observation of chirality transition of quasiparticles at stacking solitons of TLG via spatial-resolved Landau level spectroscopy. The one-dimensional stacking solitons with width of the order of 10 nm separate adjacent Bernal-stacked TLG and rhombohedral-stacked TLG. By using high-field tunneling spectra from scanning tunneling microscopy, we measured Landau quantization in both the Bernal-stacked TLG and the rhombohedral-stacked TLG and, importantly, we observed evolution of quasiparticles between the chiral degree $l=1$ and 2 and $l=3$ across the stacking domain-wall solitons. Our experiment indicates that such a chirality transition occurs smoothly, accompanying the transition of the stacking orders of TLG, around the domain-wall solitons. This result demonstrates the important relationship between the crystallographic stacking order and the chirality of quasiparticles in graphene systems.

We report the results of a parquet renormalization group (RG) study of competing instabilities in the full 2D four-pocket, three-orbital low-energy model for iron-based superconductors. We derive and analyze the RG flow of the couplings, which describes all symmetry-allowed interactions between low-energy fermions. Despite that the number of the couplings is large, we argue that there are only two stable fixed trajectories of the RG flow and one weakly unstable fixed trajectory with a single unstable direction. Each fixed trajectory has a finite basin of attraction in the space of initial system parameters. On the stable trajectories, either interactions involving only $d_{xz}$ and $d_{yz}$ or only $d_{xy}$ orbital components on electron pockets dominate, while on the weakly unstable trajectory interactions involving $d_{xz}$ ($d_{yz}$) and $d_{xy}$ orbital states on electron pockets remain comparable. The behavior along the two stable fixed trajectories has been analyzed earlier [Chubukov, Khodas, and Fernandes, Phys. Rev. X 6, 041045 (2016)]. Here we focus on the system behavior along the weakly unstable trajectory and apply the results to FeSe. We find, based on the analysis of susceptibilities along this trajectory, that the leading instability upon lowering the temperature is towards a three-component $d$-wave orbital nematic order. Two components are the differences between fermionic densities on $d_{xz}$ and $d_{yz}$ orbitals on hole pockets and on electron pockets, and the third one is the difference between the densities of $d_{xy}$ orbitals on the two electron pockets. We argue that this order is consistent with the splitting of band degeneracies, observed in recent photoemission data on FeSe by Fedorov et al. [Sci. Rep. 6, 36834 (2016)].

In this paper we explore the meandering instability of vicinal steps with a kinetic Monte Carlo simulations (kMC) model including the attractive next-nearest-neighbor (NNN) interactions. kMC simulations show that increase of the NNN interaction strength leads to considerable reduction of the meandering wavelength and to weaker dependence of the wavelength on the deposition rate F. The dependences of the meandering wavelength on the temperature and the deposition rate obtained with simulations are in good quantitative agreement with the experimental result on the meandering instability of Cu(0 2 24) [T. Maroutian et al., Phys. Rev. B 64, 165401 (2001)]. The effective step stiffness is found to depend not only on the strength of NNN interactions and the Ehrlich-Schwoebel barrier, but also on F. We argue that attractive NNN interactions intensify the incorporation of adatoms at step edges and enhance step roughening. Competition between NNN and nearest-neighbor interactions results in an alternative form of meandering instability which we call “roughening-limited” growth, rather than attachment-detachment-limited growth that governs the Bales-Zangwill instability. The computed effective wavelength and the effective stiffness behave as ${\lambda }_{\mathrm{eff}}\sim F^{-q}$ and ${\tilde{\beta }}_{\mathrm{eff}}\sim F^{-p}$, respectively, with $q\approx p/2$.