Recently, there has been a great deal of interest in the effect of atmospheric gases on the properties of graphene. We investigate the electrical and optical response of graphene-based field effect transistors that have been exposed to high purity oxygen gas using a combination of ultrafast two-pulse correlation (to give high temporal resolution) and low-frequency transport measurements (to monitor the photoinduced changes in the Fermi level). By measuring the Fermi level shifts, we are only sensitive to the oxygen atoms that interact directly with the surface. We compare our results to predictions of the empirical friction model for molecular desorption. We show the time scale of the relaxation associated with oxygen desorption to be $\sim 100$ fs, suggesting the desorption proceeds through hot electron generation in the graphene rather than heating of the lattice through hot phonon generation.

Recently fabricated two-dimensional phosphorene crystal structures have demonstrated great potential in applications of electronics. In this paper, strain effect on the electronic band structure of phosphorene was studied using first-principles methods including density functional theory (DFT) and hybrid functionals. It was found that phosphorene can withstand a tensile stress and strain up to 10 N/m and 30%, respectively. The band gap of phosphorene experiences a direct-indirect-direct transition when axial strain is applied. A moderate −2% compression in the zigzag direction can trigger this gap transition. With sufficient expansion (+11.3%) or compression (−10.2% strains), the gap can be tuned from indirect to direct again. Five strain zones with distinct electronic band structure were identified, and the critical strains for the zone boundaries were determined. Although the DFT method is known to underestimate band gap of semiconductors, it was proven to correctly predict the strain effect on the electronic properties with validation from a hybrid functional method in this work. The origin of the gap transition was revealed, and a general mechanism was developed to explain energy shifts with strain according to the bond nature of near-band-edge electronic orbitals. Effective masses of carriers in the armchair direction are an order of magnitude smaller than that of the zigzag axis, indicating that the armchair direction is favored for carrier transport. In addition, the effective masses can be dramatically tuned by strain, in which its sharp jump/drop occurs at the zone boundaries of the direct-indirect gap transition.

We performed systematic studies of the combined effects of annealing/quenching temperature $\left(T{}_{A/Q}\right)$ and $T=\text{Ni}$, Rh substitution $\left(x\right)$ on the physical properties of $\mathrm{Ca}\left({\mathrm{Fe}}_{1-x}T{}_x\right){}_2{\mathrm{As}}_2$. We constructed two-dimensional, $T{}_{A/Q}$-$x$ phase diagrams for the low-temperature states for both substitutions to map out the relations between ground states and compared them with that of Co substitution. Ni substitution, which brings one more extra electron per substituted atom and suppresses the $c$-lattice parameter at roughly the same rate as Co substitution, leads to a similar parameter range of antiferromagnetic/orthorhombic phase space in the $T{}_{A/Q}$-$x$ space as that found for Co substitution, but the parameter range for superconductivity has been shrunk (roughly by a factor of 2). This result is similar to what is found when Co- and Ni-substituted ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ are compared. On the other hand, Rh substitution, which brings the same amount of extra electrons as does Co substitution, but suppresses the $c$-lattice parameter more rapidly, has a different phase diagram. The collapsed tetragonal phase exists much more pervasively, to the exclusion of the normal, paramagnetic, tetragonal phase. The range of antiferromagnetic/orthorhombic phase space is noticeably reduced, and the superconducting region is substantially suppressed, essentially truncated by the collapsed tetragonal phase. In addition, we found that whereas for Co substitution there was no difference between phase diagrams for samples annealed for 1 or 7 days, for Ni and Rh substitutions a second, reversible effect of annealing was revealed by 7-day anneals.

Twisted graphene layers produce a moiré pattern (MP) structure with a predetermined wavelength for a given twist angle. However, predicting the membrane corrugation amplitude for any angle other than pure AB-stacked or AA-stacked graphene is impossible using first-principles density functional theory (DFT) due to the large supercell. Here, within elasticity theory, we define the MP structure as the minimum-energy configuration, thereby leaving the height amplitude as the only unknown parameter. The latter is determined from DFT calculations for AB- and AA-stacked bilayer graphene in order to eliminate all fitting parameters. Excellent agreement with scanning tunneling microscopy results across multiple substrates is reported as a function of twist angle.

We perform a detailed finite-size scaling analysis of the sheet resistance in Bi films and the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface in the presence and absence of a magnetic field applied perpendicular to the system. Our main aim is to explore the occurrence of Berezinskii-Kosterlitz-Thouless (BKT) and quantum phase transition behavior in the presence of limited size, stemming from the finite extent of the homogeneous domains or the magnetic field. Moreover we explore the implications thereof. Above an extrapolated BKT transition temperature, modulated by the thickness $d$, gate voltage $V_g$, or magnetic field $H$, we identify a temperature range where BKT behavior occurs. Its range is controlled by the relevant limiting lengths, which are set by the extent of the homogeneous domains or the magnetic field. The extrapolated BKT transition lines $T_c(d,V_g,H)$ uncover compatibility with the occurrence of a quantum phase transition where $T_c(d_c,V_{gc},H_c)=0$. However, an essential implication of the respective limiting length is that the extrapolated phase transition lines $T_c(d,V_g,H)$ are unattainable. Consequently, given a finite limiting length, BKT and quantum phase transitions do not occur. Nevertheless, BKT and quantum critical behavior is observable, controlled by the extent of the relevant limiting length. Additional results and implications include: the magnetic-field-induced finite-size effect generates a flattening out of the sheet resistance in the $T\to 0$ limit, while in zero field it exhibits a characteristic temperature dependence and vanishes at $T=0$ only. The former prediction is confirmed in both the Bi films and the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface as well as in previous studies. The latter is consistent with the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface data, while the Bi films exhibit a flattening out.

The spin dimer system ${\mathrm{Ba}}_{3-x}{\mathrm{Sr}}_x{\mathrm{Cr}}_2{\mathrm{O}}_8$ is a solid solution of the triplon Bose-Einstein condensation candidates ${\mathrm{Ba}}_3{\mathrm{Cr}}_2{\mathrm{O}}_8$ and ${\mathrm{Sr}}_3{\mathrm{Cr}}_2{\mathrm{O}}_8$. The magnetic intradimer interaction constant $J_0$ in this spin system can be tuned by varying the Sr content $x$. Very interestingly, this variation of $J_0$ with $x$ is highly nonlinear. In the present study, we show that this peculiar behavior of $J_0$ can be only partly explained by the changes in the average crystal structure alone. We report on neutron powder diffraction experiments to probe the corresponding structural details. Performing extended Hückel tight-binding calculations based on those structural details obtained at liquid-helium temperatures, we found that the change of the magnetic interaction constant can be well reproduced by taking into account the presence of a structural transition due to the Jahn-Teller active Cr${}^{5+}$ ions. This transition, lifting the orbital degeneracy and thereby the magnetic frustration in the system, is heavily influenced by disorder in the system arising from partially exchanging Ba with Sr.

We study the quantum interference correction to the conductivity in HgTe quantum wells using the Bernevig-Hughes-Zhang model. This model consists of two independent species (blocks) of massive Dirac fermions. We describe the crossover between the orthogonal and symplectic classes with increasing the carrier concentration and calculate, respectively, weak localization and antilocalization corrections in the absence of the block mixing and assuming the white-noise disorder within each block. We have calculated the interference-induced magnetoresistance in a wide interval of magnetic fields, in particular, beyond the diffusion regime. Remarkably, each Dirac cone taken separately gives a linear contribution to the low-field magnetoresistance, which turns out to be asymmetric in magnetic field $B$. We present an interpretation of this result in terms of the Berry-phase formalism. The contributions of the two blocks are related to each other by replacing $B$ to $-B$, so that the total magnetoresistance is symmetric and parabolic in the limit $B\to 0$. However, in some range of parameters, field dependence turns out to be strongly nonmonotonous. We also demonstrate that block mixing gives rise to additional singular diffusive modes which do not show up in the absence of mixing.

The modulated structure of the martensite phase of ${\mathrm{Ni}}_2\text{MnGa}$ is revisited using high-resolution synchrotron x-ray powder diffraction measurements, which reveal higher-order satellite reflections up to the third order and phason broadening of the satellite peaks. The structure refinement, using the (3+1) dimensional superspace group approach, shows that the modulated structure of ${\mathrm{Ni}}_2\text{MnGa}$ can be described by orthorhombic superspace group Immm(00γ)$s$00 with lattice parameters $a=\mathrm{4.218\hspace{0.17em}61}\left(2\right)\AA ,b=\mathrm{5.546\hspace{0.17em}96}\left(3\right)\AA$, and $c=\mathrm{4.187\hspace{0.17em}63}\left(2\right) \AA$, and an incommensurate modulation wave vector $\mathbf{q}=0.43160\left(3\right){\mathbf{c}}^{*}=\left(3/7+\delta \right){\mathbf{c}}^{*}$, where $\delta =0.00303\left(3\right)$ is the degree of incommensuration of the modulated structure. Additional satellite peak broadening, which could not be accounted for in terms of the anisotropic strain broadening based on a lattice parameter distribution, has been modeled in terms of phasons using fourth-rank covariant strain-tensor representation for incommensurate structures. The simulation of single-crystal diffraction patterns from the refined structural parameters unambiguously reveals a rational approximant structure with 7M modulation. The inhomogeneous displacement of different atomic sites on account of incommensurate modulation and the presence of phason broadening clearly rule out the adaptive phase model proposed recently by Kaufmann et al. [S. Kaufmann, U. K. Rößler, O. Heczko, M. Wuttig, J. Buschbeck, L. Schultz, and S. Fähler, Phys. Rev. Lett. 104, 145702 (2010)] and suggest that the modulation in ${\mathrm{Ni}}_2\text{MnGa}$ originates from soft-mode phonons.

We present theoretical calculations of the Raman and IR spectra, as well as electronic properties at zero and finite temperature to elucidate the crystal structure of phase III of solid molecular hydrogen. We find that anharmonic finite temperature are particularly important and qualitatively influences the main conclusions. While $P{6}_3/m$ is the most likely candidate for phase III at the nuclear ground state, at finite temperature the $C2/c$ structure appears to be more suitable.

We investigate the one-dimensional long-range random-field Ising magnet with Gaussian distribution of the random fields. In this model, a ferromagnetic bond between two spins is placed with a probability $p\sim r^{-1-\sigma }$, where $r$ is the distance between these spins and $\sigma$ is a parameter to control the effective dimension of the model. Exact ground states at zero temperature are calculated for system sizes up to $L=2^{19}$ via graph theoretical algorithms for four different values of $\sigma \in \{0.25,0.4,0.5,1.0\}$ while varying the strength $h$ of the random fields. For each of these values several independent physical observables are calculated, i.e., magnetization, Binder parameter, susceptibility, and a specific-heat-like quantity. The ferromagnet-paramagnet transitions at critical values $h_c\left(\sigma \right)$ as well as the corresponding critical exponents are obtained. The results agree well with theory, and interestingly we find for $\sigma =1/2$ the data is compatible with a critical random-field strength $h_c>0$.

Here we propose an approach for performing a Taylor series expansion of the first-principles computed energy of a crystal as a function of the nuclear displacements. We enlarge the dimensionality of the existing displacement space and form new variables (i.e., slave modes) which transform like irreducible representations of the point group and satisfy homogeneity of free space. Standard group theoretical techniques can then be applied to deduce the nonzero expansion coefficients a priori. At a given order, the translation group can be used to contract the products and eliminate terms which are not linearly independent, resulting in a final set of slave mode products. While the expansion coefficients can be computed in a variety of ways, we demonstrate that finite difference is effective up to fourth order. We demonstrate the power of the method in the strongly anharmonic system PbTe. All anharmonic terms within an octahedron are computed up to fourth order. A proper unitary transformation demonstrates that the vast majority of the anharmonicity can be attributed to just two terms, indicating that a minimal model of phonon interactions is achievable. The ability to straightforwardly generate polynomial potentials will allow precise simulations at length and time scales which were previously unrealizable.

A continuum of excitations in interacting one-dimensional systems is bounded from below by a spectral edge that marks the lowest possible excitation energy for a given momentum. We analyze short-range interactions between Fermi particles and between Bose particles (with and without spin) using Bethe-ansatz techniques and find that the dispersions of the corresponding spectral edge modes are close to a parabola in all cases. Based on this emergent phenomenon we propose an empirical model of a free, nonrelativistic particle with an effective mass identified at low energies as the bare electron mass renormalized by the dimensionless Luttinger parameter $K$ (or $K_{\sigma }$ for particles with spin). The relevance of the Luttinger parameters beyond the low-energy limit provides a more robust method for extracting them experimentally using a much wider range of data from the bottom of the one-dimensional band to the Fermi energy. The empirical model of the spectral edge mode complements the mobile impurity model to give a description of the excitations in proximity of the edge at arbitrary momenta in terms of only the low-energy parameters and the bare electron mass. Within such a framework, for example, exponents of the spectral function are expressed explicitly in terms of only a few Luttinger parameters.

The Kubo formula for the electrical conductivity is expressed in terms of a weighted sum of Drude-like contributions associated to the exact eigenstates of the interacting system, each characterized by its own frequency-dependent relaxation time. This alternative formulation considerably simplifies the access to the static properties (dc conductivity) and resolves the long-standing difficulty to connect the Boltzmann transport theory and the Kubo formula. In particular, at the lowest order of the perturbation theory, the correct transport scattering lifetime depending on the momentum $k$, which appears in the Boltzmann theory, instead of the single-electron lifetime appearing in the Green function, can be recovered. This alternative formulation is applied to (i) the elastic scattering in metals, and (ii) the inelastic scattering in the Fröhlich polaron model to obtain the exact result of the mobility in the low-temperature weak-coupling limit.

To clarify the universal features of spin glasses, we carefully studied slow dynamics in a geometrically frustrated magnet ${\mathrm{ZnFe}}_2{\mathrm{O}}_4$ with slight disorders, regarded as an “unconventional” Heisenberg spin glass, using time-resolved neutron diffractometry and magnetometry. The results indicate that “aging” can be attributed not to growth of the short-range order detected by a diffuse scattering but to aging of a hidden aperiodic correlation, as expected from theories for spin glasses. Concerning aging, peculiar behavior was found; the decay of thermoremanent magnetization is extremely accelerated if the sample is heated/cooled briefly midway through the isothermal slow relaxation. Conversely, magnetization surprisingly increases despite the absence of a magnetic field when the temperature returns after the brief heating/cooling. The behavior can be explained as a destabilization of the aged spin configuration due to the thermal perturbations and subsequent spontaneous restoration of the original spin configuration after the destabilization. Whereas such destabilization and restoration do not occur during freezing into numerous metastable states in a fixed energy landscape, these are possible in an energy landscape with a temperature-sensitive funnel-like structure. These features, consistent with the ghost domain scenario of the droplet picture, are the same as for conventional Heisenberg spin glasses such as dilute magnetic alloys and dilute magnetic semiconductors. In other words, they are universal features in Heisenberg spin glasses including unconventional ones.

We investigate the spin-1/2 Heisenberg model on the delta chain (sawtooth chain) with ferromagnetic nearest-neighbor and antiferromagnetic next-neighbor interactions. For a special ratio between these interactions there is a class of exact ground states formed by localized magnons and the ground state is macroscopically degenerate. The degree of this degeneration is found analytically. It is shown that the residual entropy per spin is $s_0=\frac{1}{2}ln2$. An important feature of this model is a sharp decrease of the gaps for excited states with an increase of the number of magnons. These excitations give an essential contribution to the low-temperature thermodynamics. The corresponding thermodynamic quantities are calculated using full exact diagonalization of finite chains up to $N=22$. The behavior of the considered model is compared with that of the delta chain with both antiferromagnetic interactions.

We numerically investigate the spin-orbit (SO) coupling effect on the magnetic flux evolution of energy and supercurrent in mesoscopic $d$-wave superconducting loops by solving the spin-generalized Bogoliubov–de Gennes equations self-consistently. It is found that the energy spectrum splits when the SO interaction is involved and the Majorana zero mode can be realized in the [100] edges of square systems for an appropriate SO coupling strength. Superconducting phase transitions appear when the energy gap closes, accompanied by energy jumps between different energy parabolas in the ground state, which provides a possible mechanism to support fractional flux periodicity of supercurrent. Moreover, in the case of rectangular loops with SO coupling, the jumps of the ground-state energy gradually disappear by increasing the ratio of length to height of the sample, and a paramagnetic response with opposite direction of the screening current around zero flux value can occur in such systems.

Thermotropic phase boundaries (TPBs), as thermal analogs of morphotropic phase boundaries (MPBs), are associated with the thermal inter-ferroelectric phase transitions. Similar to an MPB, a TPB exhibits a characteristically flattened energy profile which favors polarization rotation, thus giving rise to a structurally bridging low-symmetry phase. We report on the kinetic process of thermal inter-ferroelectric phase transitions in ${\mathrm{BaTiO}}_3$ and ${\mathrm{KNbO}}_3$ using the phase-field method. The domain structures are found to play key roles in stabilizing the monoclinic phase. In simple domain structures, the monoclinic phase is a transient phase and cannot be stabilized into its neighboring phase regimes. However, by introducing structural inhomogeneity (orthogonal in-plane domain twins), we found that the monoclinic phase can be stabilized over a range of over 100 K across the transition. As a result, the piezoelectric properties are enhanced due to the stabilized monoclinic phase. In addition to the emergence of new piezoelectric components with monoclinic symmetry, most of the original components present in the tetragonal symmetry also show substantial enhancement with the rotation of polarization.