We study graphene antidot lattices—superlattices of perforations (antidots) in a graphene sheet—using a model that accounts for the phonon modulation of the $\pi$-electron hopping integrals. We calculate the phonon spectra of selected antidot lattices using two different semiempirical methods. Based on the adopted model, we quantify the nature of charge carriers in the system by computing the quasiparticle weight due to the electron-phonon interaction for an excess electron in the conduction band. We find a very strong phonon-induced renormalization, with the effective electron masses exhibiting nonmonotonic dependence on the superlattice period for a given antidot diameter. Our study provides an indication of polaronic behavior and points to the necessity of taking into account the inelastic degrees of freedom in future studies of transport in graphene antidot lattices.

We study the effect in geometrically frustrated antiferromagnets of weak, random variations in the strength of exchange interactions. Without disorder the simplest classical models for these systems have macroscopically degenerate ground states, and this degeneracy may prevent ordering at any temperature. Weak exchange randomness favors a small subset of these ground states and induces a spin-glass transition at an ordering temperature determined by the amplitude of modulations in interaction strength. We use the replica approach to formulate a theory for this transition, showing that it falls into the same universality class as conventional spin-glass transitions. In addition, we show that a model with a low concentration of defect bonds can be mapped onto a system of randomly located pseudospins that have dipolar effective interactions. We also present detailed results from Monte Carlo simulations of the classical Heisenberg antiferromagnet on the pyrochlore lattice with weak randomness in nearest-neighbor exchange.

Existing force spectroscopy methods in tapping mode atomic force microscopy (AFM) such as higher harmonic inversion [M. Stark, R. W. Stark, W. M. Heckl, and R. Guckenberger, Proc. Natl. Acad. Sci. U.S.A. 99, 8473 (2002)] or scanning probe acceleration microscopy [J. Legleiter, M. Park, B. Cusick, and T. Kowalewski, Proc. Natl. Acad. Sci. U.S.A. 103, 4813 (2006)] or integral relations [M. Lee and W. Jhe, Phys. Rev. Lett. 97, 036104 (2006); S. Hu and A. Raman, Nanotechnology 19, 375704 (2008); H. Hölscher, Appl. Phys. Lett. 89, 123109 (2006); A. J. Katan, Nanotechnology 20, 165703 (2009)] require and assume as an observable the tip dynamics in a single eigenmode of the oscillating microcantilever. We demonstrate that this assumption can distort significantly the extracted tip-sample interaction forces when applied to tapping mode AFM with soft cantilevers in liquid environments. This exception is due to the fact that under these conditions the second eigenmode is momentarily excited and the observed tip dynamics clearly contains contributions from the fundamental and second eigenmodes. To alleviate this problem, a simple experimental method is proposed to screen the second eigenmode contributions in the observed tip deflection signal to allow accurate tip-sample force reconstruction in liquids. The method is implemented experimentally to reconstruct interaction forces on polymer, bacteriorhodopsin membrane, and mica samples in buffer solutions.

We report on angle-resolved photoemission spectroscopic studies on a parent topological insulator, ${\text{Bi}}_2{\text{Se}}_3$. The linewidth of the spectral function (inverse of the quasiparticle lifetime) of the topological metallic states shows an anomalous behavior. This behavior can be reasonably accounted for by assuming the decay of the quasiparticles predominantly into bulk electronic states through electron-electron interaction and defect scattering. Studies on aged surfaces reveal that topological metallic states are very much unaffected by the potentials created by adsorbed atoms or molecules on the surface, indicating that topological states could be indeed protected against weak perturbations.

Commonalities among the order parameters of the ubiquitous antiferromagnetism present in the parent compounds of the iron arsenide high-temperature superconductors are explored. Additionally, comparison is made between the well established two-dimensional Heisenberg-Ising magnet, ${\text{K}}_2{\text{NiF}}_4$, and iron arsenide systems residing at a critical point whose structural and magnetic phase transitions coincide. In particular, analysis is presented regarding two distinct classes of phase-transition behavior reflected in the development of antiferromagnetic and structural order in the three main classes of iron arsenide superconductors. Two distinct universality classes are mirrored in their magnetic phase transitions which empirically are determined by the proximity of the coupled structural and magnetic phase transitions in these materials.

We report on a detailed analysis of the superconducting properties of boron-doped silicon films grown along the 001 direction by gas immersion laser doping. The doping concentration $c_B$ has been varied up to $\sim 10\text{at}\text{.}\%$ by increasing the number of laser shots to 500. No superconductivity could be observed down to 40 mK for doping level below $\sim 2\text{at}\text{.}\%$. The critical temperature $T_c$ then increased steeply to reach $\sim 0.6\text{K}$ for $c_B\sim 8\text{at}\text{.}\%$. No hysteresis was found for the transitions in magnetic field, which is characteristic of a type II superconductor. The corresponding upper critical field ${\mu }_o{H}_{c2}\left(0\right)$ was on the order of 1000 G, much smaller than the value previously reported by Bustarret et al. [E. Bustarret et al., Nature (London) 444, 465 (2006)].

Low-energy electronic structure of (unbiased and undoped) bilayer graphene consists of two Fermi points with quadratic dispersions if trigonal warping is ignored. We show that short-range (or screened Coulomb) interactions are marginally relevant and use renormalization group to study their effects on low-energy properties of the system. We find that the two quadratic Fermi points spontaneously split into four Dirac points. This results in a nematic state that spontaneously breaks the sixfold lattice rotation symmetry (combined with layer permutation) down to a twofold one, with a finite transition temperature. Critical properties of the transition and effects of trigonal warping are also discussed.

In a mean-field-theory treatment the ground state of a graphene bilayer spontaneously breaks inversion symmetry for arbitrarily weak electron-electron interactions when trigonal-warping terms in the band structure are ignored. We report on a perturbative renormalization-group calculation, which assesses the robustness of this instability, comparing with the closely related case of the charge-density-wave instability incorrectly predicted by mean-field theory in a one-dimensional electron gas. We conclude that spontaneous inversion symmetry breaking in graphene is not suppressed by quantum fluctuations but that, because of trigonal warping, it may occur only in high quality suspended bilayers.

The static and dynamic properties of ${\text{V}}^{4+}$ spins $\left(S=1/2\right)$ in the frustrated square-lattice compound ${\text{Pb}}_2\left(\text{VO}\right){\left({\text{PO}}_4\right)}_2$ were investigated by means of magnetic susceptibility $\chi$ and ${}^{31}\text{P}$ nuclear magnetic resonance (NMR) shift $\left(K\right)$ and ${}^{31}\text{P}$ nuclear spin-lattice relaxation rate $1/T_1$ measurements on a single crystal. This compound exhibits long-range antiferromagnetic order below $T_{\text{N}}\simeq 3.65\text{K}$. NMR spectra above $T_{\text{N}}$ show two distinct lines corresponding to two inequivalent P sites present in the crystal structure. The observed asymmetry in hyperfine coupling constant for the in-plane (P1) P site directly points toward a distortion in the square lattice at the microscopic level, consistent with the monoclinic crystal structure. The nearest- and next-nearest-neighbor exchange couplings were estimated by fitting $K$ versus temperature $T$ by a high-temperature series expansion for the spin susceptibility of the frustrated square lattice to be $J_1/k_{\text{B}}=\left(-5.4\pm 0.5\right)\text{K}$ (ferromagnetic) and $J_2/k_{\text{B}}=\left(9.3\pm 0.6\right)\text{K}$ (antiferromagnetic), respectively. $1/\left(T_{1}T\chi \right)$ is almost $T$ independent at high temperatures due to random fluctuation of spin moments. Below 20 K, the compound shows an enhancement of $1/\left(T_{1}T\chi \right)$ which arises from a growth of antiferromagnetic spin correlations above $T_{\text{N}}$. Below $T_{\text{N}}$ and for the field applied along the $c$ axis, the NMR spectrum for the P1 site splits into two satellites and the spacing between them increases monotonically with decreasing $T$ which is a direct evidence of a columnar antiferromagnetic ordering with spins lying in the $ab$ plane. This type of magnetic ordering is consistent with expectation from the $J_2/J_1\simeq -1.72$ ratio. The critical exponent $\beta =0.25\pm 0.02$ estimated from the temperature dependence of the sublattice magnetization as measured by ${}^{31}\text{P}$ NMR at 11.13 MHz is close to the value (0.231) predicted for the two-dimensional XY model.

We analyze the thermoelectric response in layered metals with spontaneously broken rotation symmetry. We identify the anisotropy of the quasiparticle Nernst signal as an extremely sensitive probe of Fermi-surface distortions characteristic of the ordered state. This is due to a subtle interplay of different transport anisotropies, which become additionally enhanced near Van Hove singularities. Applied to recent experiments, our results reinforce the proposal that the underdoped cuprate superconductor ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{6+\delta }$ displays such “electron-nematic” order in the pseudogap regime.

We investigate the spin dynamics of high-mobility two-dimensional electrons in GaAs/AlGaAs quantum wells grown along the [001] and [110] directions by time-resolved Faraday rotation at low temperatures. In measurements on the (001)-grown structures without external magnetic fields, we observe coherent oscillations of the electron-spin polarization about the effective spin-orbit field. In nonquantizing magnetic fields applied normal to the sample plane, the cyclotron motion of the electrons rotates the effective spin-orbit field. This rotation leads to fast oscillations in the spin polarization about a nonzero value and a strong increase in the spin dephasing time in our experiments. These two effects are absent in the (110)-grown structure due to the different symmetry of its effective spin-orbit field. The measurements are in excellent agreement with our theoretical model.

Pristine bilayer graphene is a centrosymmetric material in which parity is a conserved quantity. The high sensitivity of this atomic scale structure to external perturbations that break the inversion symmetry enables significant potentials for device applications. Raman spectroscopy is used here to probe the breakdown of parity conservation in a direct and quantitative manner via a phonon mixing phenomenon. The striking broken-symmetry effects display anticrossing coupling between two opposite parity long-wavelength optical phonons. The spectral intensity transfer between the two observed Raman peaks offers quantitative measurements of the evolution of the phonon wave function and demonstrates a manifestation of broken inversion symmetry in graphene layers.

We study narrow ballistic Josephson weak links in a InAs quantum wells contacted by Nb electrodes and find a dramatic magnetic-field suppression of the Andreev reflection amplitude, which occurs even for in-plane field orientation with essentially no magnetic flux through the junction. Our observations demonstrate the presence of a Doppler shift in the energy of the Andreev levels, which results from diamagnetic screening currents in the hybrid Nb/InAs banks. The data for conductance, excess, and critical currents can be consistently explained in terms of the sample geometry and the McMillan energy, characterizing the transparency of the Nb/InAs interface.

First-principles modeling of ferroelectric capacitors presents several technical challenges due to the coexistence of metallic electrodes, long-range electrostatic forces, and short-range interface chemistry. Here we show how these aspects can be efficiently and accurately rationalized by using a finite-field density-functional theory formalism in which the fundamental electrical variable is the displacement field $\mathbf{D}$. By performing calculations on model $\text{Pt}/{\text{BaTiO}}_3/\text{Pt}$ and $\text{Au}/{\text{BaZrO}}_3/\text{Au}$ capacitors we demonstrate how the interface-specific and bulk-specific properties can be identified and rigorously separated. Then, we show how the electrical properties of capacitors of arbitrary thickness and geometry (symmetric or asymmetric) can be readily reconstructed by using such information. Finally, we show how useful observables such as polarization and dielectric, piezoelectric, and electrostrictive coefficients are easily evaluated as a byproduct of the above procedure. We apply this methodology to elucidate the relationship between chemical bonding, Schottky barriers and ferroelectric polarization at simple-metal/oxide interfaces. We find that ${\text{BO}}_2$-electrode interfaces behave analogously to a layer of linear dielectric put in series with a bulklike perovskite film while a significant nonlinear effect occurs at AO-electrode interfaces.

Transitions in an artificial atom, driven nonadiabatically through an energy-level avoided crossing, can be controlled by carefully engineering the driving protocol. We have driven a superconducting persistent-current qubit with a large-amplitude radio-frequency field. By applying a biharmonic wave form generated by a digital source, we demonstrate a mapping between the amplitude and phase of the harmonics produced at the source and those received by the device. This allows us to image the actual wave form at the device. This information is used to engineer a desired time dependence, as confirmed by the detailed comparison with a simulation.

${\text{LaAlO}}_3/{\text{SrTiO}}_3$ interfaces grown by molecular beam epitaxy show a metal-insulator transition at a critical film thickness of four unit cells of ${\text{LaAlO}}_3$ on ${\text{TiO}}_2$-terminated ${\text{SrTiO}}_3$ substrates. This transition had previously been observed in ${\text{LaAlO}}_3$ films grown by pulsed laser deposition, where defects related to the growth process have been suggested as playing a role in the interface behavior. X-ray photoemission was used to examine the band offsets and look for an electric field across ${\text{LaAlO}}_3$ for a range of film thicknesses on both SrO- and ${\text{TiO}}_2$-terminated ${\text{SrTiO}}_3$ substrates. These results are compared to the predictions of the polar catastrophe model for the emergence of a metallic interface in this system. We do not find the predicted electric field in the ${\text{LaAlO}}_3$ or any dependence on substrate termination. While the observation of metal-insulator transitions in ${\text{LaAlO}}_3/{\text{SrTiO}}_3$ structures grown by different techniques points to an intrinsic effect, the absence of an electric field in the ${\text{LaAlO}}_3$ layer is not consistent with the polar catastrophe model of interface metallicity.

We investigate the charge-density wave (CDW) instability in single and double layers, as well as in the bulk $2{\text{H-NbSe}}_2$. We demonstrate that the density functional theory correctly describes the metallic CDW state in the bulk $2{\text{H-NbSe}}_2$. We predict that both monolayer and bilayer ${\text{NbSe}}_2$ undergo a CDW instability. However, while in the bulk the instability occurs at a momentum ${\mathbf{q}}_{\text{CDW}}\approx \frac{2}{3}\Gamma M$, in freestanding layers it occurs at ${\mathbf{q}}_{\text{CDW}}\approx \frac{1}{2}\Gamma M$. Furthermore, while in the bulk the CDW leads to a metallic state, in a monolayer the ground state becomes semimetallic, in agreement with recent experimental data. We elucidate the key role that an enhancement of the electron-phonon matrix element at $\mathbf{q}\approx {\mathbf{q}}_{\text{CDW}}$ plays in forming the CDW ground state.

We studied the unusual quantum Hall effect (QHE) near the charge neutrality point in high-mobility graphene sample for magnetic fields up to 18 T. We observe breakdown of the delocalized QHE transport and strong increase in resistivities ${\rho }_{xx},\left|{\rho }_{xy}\right|$ with decreasing Landau-level filling for $\nu <2$, where we identify two insulating regimes. First, ${\rho }_{xx,xy}$ increases nearly exponentially within the range of several resistance quanta $R_K$, while the Hall effect gradually disappears and the off-diagonal resistivity ${\rho }_{xy}$ eventually becomes independent of the direction of magnetic field, consistent with the Hall insulator with local transport. Then, at a filling $\nu \approx 1/2$, there is a cusp in ${\rho }_{xx}\left(\nu \right)$ and an onset of even faster growth with the decreasing $\nu$, indicating transition to a collective insulator state. A likely candidate for this state is a pinned Wigner crystal.

The acoustic Faraday rotation in the $4f$ paramagnet ${\text{Tb}}_3{\text{Ga}}_5{\text{O}}_{12}$ has recently been observed by Sytcheva et al. (unpublished). As in earlier examples the rotation angle per unit length of transverse acoustic modes was found to depend linearly on sound frequency. Existing theories for this effect consistently require that it should vary with the square of the frequency. In the present work a solution for this long-standing problem is provided. We propose a model based on magnetoelastic interactions with $4f$ quadrupole moments that includes both acoustic and optical phonons. The symmetry allows a direct and induced coupling between the latter. This leads to an indirect acoustic Faraday rotation via the field-induced splitting of doubly degenerate optical phonon modes. It varies linearly with frequency in accordance with experiment and dominates the rotation angle. It also explains the observed resonance of the rotation angle in the field range between 17–20 T. The mechanism is of general validity for non-Bravais lattices and applies to previous examples of the acoustic Faraday effect.

We have studied a three-unit-cell-thick ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$ (YBCO) superconducting film grown on a slightly vicinal ${\text{SrTiO}}_3$ substrate that was polished from the (001) plane by a small miscut angle (less than $0.5°$). This produced, on its surface, a linear array of single-unit-cell-high steps with a period larger than 40 nm. A characteristic fault, with a displacement corresponding to one third of the $c$-axis length of YBCO, is generated in the film on each substrate step, as observed by atomic force microscopy. Transport measurements with current flows parallel and perpendicular to the fault show that the fault serves as a superconducting weak link, as well as a unique one-dimensional flux-pinning center in an applied magnetic field. In addition, a characteristic fine structure is found in the flux-flow resistivity as a function of magnetic field, the origin of which may be ascribed to a matching effect between the periodically aligned faults and the flux lattice.