We present measurements of weak localization on hexagonal boron nitride encapsulated bilayer ${\mathrm{MoS}}_2$. From the analysis we obtain information regarding the phase coherence and the spin diffusion of the electrons. We find that the encapsulation with boron nitride provides higher mobilities in the samples, and the phase coherence shows improvement, while the spin relaxation does not exhibit any significant enhancement compared to nonencapsulated ${\mathrm{MoS}}_2$. The spin relaxation time is in the order of a few picoseconds, indicating a fast intravalley spin-flip rate. Lastly, the spin-flip rate is found to be independent from electron density in the current range, which can be explained through counteracting spin-flip scattering processes based on electron-electron Coulomb scattering and extrinsic Bychkov-Rashba spin-orbit coupling.

To assess the energetics of Cu intercalation on defective graphite, the chemical potentials and binding energies for Cu at graphite step edges are calculated for three main configurations: an isolated atom, a chain, and an atom attached to a chain. As expected, for Cu interacting directly with a graphite step edge, the strength of interaction depends on the stability of the step, with Cu binding more strongly at a less-stable step. However, the relationship is reversed when considering binding of a Cu atom attached to a chain. Taken together, these trends mean that if the graphite step is less stable, as for the zigzag step, then decorating the step with a Cu chain facilitates intercalation by additional Cu atoms (which are less strongly bound to the decorated step). For more stable steps, intercalation is optimal without decoration. We also calculate the diffusion barrier for atomic Cu on top of the graphite terrace and, in the uppermost gallery, find values of 0.008 and 0.021 eV, respectively. These values are very small, indicating that the minimum barrier for a Cu atom to detach from a step and move to a terrace or gallery is dominated by the difference in binding energies. For intercalation, this minimum barrier is 1.4 to 3.1 eV and depends strongly on step configuration.

We investigate transient spin-dependent thermoelectric signatures in a single-molecule magnet under the effect of a time-dependent voltage pulse. We model the system using nonequilibrium Green's functions and a generalized spin equation of motion incorporating the dynamic electronic structure of the molecule. We show that the generated heat current in the system is due to both charge and spin contributions, related to the Peltier and the spin-dependent Peltier effect. There is also a clear signature in the heat current due to the spin dynamics of the single molecule and a possibility to control the spin-dependent heat currents by bias, tunneling coupling, and exchange interaction. A reversal of the net heat transfer in the molecule is found for increasing bias voltage due to the local Zeeman split and we can correlate the net heat transfer with the local anisotropies and dynamic exchange fields in the system.

A monolayer of ${\mathrm{WTe}}_2$ has been shown to display quantum spin Hall (QSH) edge modes persisting up to 100 K in transport experiments. Based on density-functional theory calculations and symmetry-based model building including the role of correlations and substrate support, we develop an effective electronic model for ${\mathrm{WTe}}_2$ that fundamentally differs from other prototypical QSH settings: we find a remarkably strong transverse localization of QSH edge modes in ${\mathrm{WTe}}_2$ related to the glide symmetry due to which the topological gap opens away from high-symmetry points in momentum space. While the indirect bulk gap is much smaller, a large direct gap of up to 1 eV in the Brillouin zone region of the dispersing edge modes determines their properties.

Monolayer ${\mathrm{MoS}}_2$ is a promising two-dimensional material for electronic and optoelectronic devices. As-grown ${\mathrm{MoS}}_2$ is an $\mathit{n}$-type semiconductor, however, the origin of this unintentional doping is still not clear. Here, using hybrid density functional theory, we carried out an extensive study of the often observed native point defects, i.e., $V_{\text{S}}$, $V_{\text{Mo}}$, $V_{\text{S2}}$, $V_{\text{MoS3}}$, $V_{\text{MoS6}}$, ${\mathrm{Mo}}_{\text{S2}}$, and $\mathrm{S}{2}_{\text{Mo}}$, and found that none of them cause $\mathit{n}$-type doping. Specifically, the S vacancy ($V_{\text{S}}$), which has been widely attributed to $\mathit{n}$-type conductivity, turns out to be an electron compensating center. We report that hydrogen, which is almost always present in the growth environments, is most stable in its interstitial (${\mathrm{H}}_{\text{i}}$) and H-S adatom forms in ${\mathrm{MoS}}_2$ and acts as a shallow donor, provided the sample is grown under S-rich condition. Furthermore, they have high migration barriers (in excess of 1 eV), which would ensure their stability even at higher temperatures, and hence lead to $\mathit{n}$-type conductivity.

The adsorption geometry, the electronic properties, and the adsorption energy of the prototype organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on a monolayer of hexagonal boron nitride (hBN) grown on the Cu(111) surface were determined experimentally. The perylene core is at a large height of 3.37 $\AA$ and only a minute downward displacement of the functional anhydride groups (0.07 Å) occurs, yielding adsorption heights that agree with the sum of the involved van der Waals radii. Thus, already a single hBN layer leads to a decoupled (physisorbed) molecule, contrary to the situation on the bare Cu(111) surface.

We report on the clear evidence of massless Dirac fermions in two-dimensional system based on III-V semiconductors. Using a gated Hall bar made on a three-layer InAs/GaSb/InAs quantum well, we restore the Landau level fan chart by magnetotransport and unequivocally demonstrate a gapless state in our sample. Measurements of cyclotron resonance at different electron concentrations directly indicate a linear band crossing at the $\mathrm{\Gamma }$ point of the Brillouin zone. Analysis of experimental data within an analytical Dirac-like Hamiltonian allows us not only to determine the velocity ($v_F=1.8\times {10}^5$ m/s) of massless Dirac fermions, but also to demonstrate a significant nonlinear dispersion at high energies.

Spin-orbit interaction (SOI) in superconductors can protect Cooper pairs from external magnetic fields and thus enhance the upper critical field. This effect is most significant in ultrathin superconductor films when the field is in plane. Recently, it was found that this protection is especially efficient in so-called Ising superconductors whose special form of SOI pins the electron spins in the out-of-plane direction and the in-plane critical field is enhanced up to tens of Tesla. We report that a strong SOI can be induced in metallic thin films by proximity to monolayer tungsten diselenide ($\mathrm{WS}{\mathrm{e}}_2$), a semiconductor with intrinsic strong Ising SOI. We demonstrate that the upper critical field of the thin-film superconductor is enhanced by the induced SOI, even though the zero-field critical temperature is unchanged or reduced.

Magnetic refrigeration at room-temperature is a technology that could potentially be more environmentally friendly, efficient, and affordable than traditional refrigeration. The search for suitable materials for magnetocaloric refrigeration led to the study of double-perovskites ${\mathrm{La}}_2{\mathrm{MnNiO}}_6,{\mathrm{La}}_2{\mathrm{MnCoO}}_6$, and ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$. While ${\mathrm{La}}_2{\mathrm{MnNiO}}_6$ and ${\mathrm{La}}_2{\mathrm{MnCoO}}_6$ are ferromagnets with near room-temperature $T_C\mathrm{s}$, a previous theoretical study of double-perovskite ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$ revealed that this material is a ferrimagnet due to strong electronic interactions in $\mathrm{Fe}\text{−}d$ orbitals. Here we investigate the double-perovskites ${\mathrm{La}}_2{\mathrm{MnRuO}}_6$ and $\mathrm{La}A{\mathrm{MnFeO}}_6(A$ = Ba, Ca, and Sr) with density functional theory (DFT) as materials that can counteract the effects the strong repulsion present in the in $\mathrm{Fe}\text{−}d$ shells of ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$ and lead to a ferromagnetic state. Our study reveals that while ${\mathrm{La}}_2{\mathrm{MnRuO}}_6$ is also a ferrimagnet, but with a higher net magnetic moment per formula than ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$, doubly ordered $\mathrm{La}A{\mathrm{MnFeO}}_6$ are ferromagnets. By mapping the total energy of the $\mathrm{La}A{\mathrm{MnFeO}}_6$ compounds obtained from DFT calculations to the Ising model, we also calculate their magnetic exchange couplings. This allows us to estimate the trend in $T_C$ of the three doped ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$ materials with classical Monte Carlo calculations and predict that doubly ordered ${\mathrm{LaBaMnFeO}}_6$ and ${\mathrm{LaSrMnFeO}}_6$ could be suitable materials for room-temperature magnetic refrigeration.

We study a simple model system for the conjugated $\pi$ bonds in benzene, the Pariser-Parr-Pople (PPP) model, within the parquet approximation (PA), exemplifying the prospects of the PA for molecules. Advantages of the PA are its polynomial scaling with the number of orbitals, and the natural calculation of one- and two-particle spectral functions as well as of response and correlation functions. We find large differences in the electronic correlations in the PPP model compared to a Hubbard model with only local interactions. The quasiparticle renormalization (or mass enhancement) is much weaker in the PPP than in the Hubbard model, but the static part of the self-energy enhances the band gap of the former. Furthermore, the vertex corrections to the optical conductivity are much more important in the PPP model. Because nonlocal interactions strongly alter the self-energy, we conclude that the PA is more suitable for calculating conjugated $\pi$ bonds in molecules than single site dynamical mean-field theory.

We present an investigation of the electronic structure of $1T\text{−}{\mathrm{TiTe}}_2$ single crystals using high-resolution angle-resolved photoemission spectroscopy utilizing tunable photon energy excitation. The typical semimetal-like electronic structure is observed and examined, where multiple hole pockets related to $\mathrm{Te}5p$ bands and one-electron pockets related to Ti $3d$ bands are populated by carriers. The obtained results reveal (i) a pronounced three-dimensional (3D) electronic band structure of $1T\text{−}{\mathrm{TiTe}}_2$ with typical semi-metallic features for both the Ti $3d$ and the $\mathrm{Te}5p$ states; (ii) multiple Fermi surface sheets and complex band structure; and (iii) an obvious kink in the dispersion at an energy of about 18 meV below the Fermi energy, which is an experimental observation of the kink structure in $1T\text{−}{\mathrm{TiTe}}_2$ single crystal. We find that this kink is induced by electron-phonon coupling in the material. These important and significant findings can help us to gain an in-depth understanding of the 3D electronic structure of semimetallic $1T\text{−}{\mathrm{TiTe}}_2$.

Bilayer graphene provides a unique platform to explore the rich physics in quantum Hall effect. The unusual combination of spin, valley, and orbital degeneracy leads to interesting symmetry-broken states with electric and magnetic field. Conventional transport measurements, like resistance measurements, have been performed to probe the different ordered states in bilayer graphene. However, not much work has been done to directly map the energetics of those states in bilayer graphene. Here, we have carried out the magnetocapacitance measurements with electric and magnetic field in a hexagonal boron nitride encapsulated dual-gated bilayer graphene device. At zero magnetic field, using the quantum capacitance technique we measure the gap around the charge neutrality point as a function of perpendicular electric field and the obtained value of the gap matches well with the theory. In the presence of a perpendicular magnetic field, we observe Landau-level crossing in our magnetocapacitance measurements with electric field. The gap closing and reopening of the lowest Landau level with electric and magnetic field shows the transition from one ordered state to another. Furthermore, we observe the collapsing of the Landau levels near the band edge at higher electric field ($\overline{D}>0.5$ V/nm), which was predicted theoretically. The complete energetics of the Landau levels of bilayer graphene with electric and magnetic field in our experiment paves the way to unravel the nature of ground states of the system.

The impact of electron-hole pairing on the spectrum of plasma excitations in double layer systems is investigated. The theory is developed with reference to a double monolayer graphene. Taking into account the coupling of scalar potential oscillations with oscillations of the order parameter $\mathrm{\Delta }$, we show that the spectrum of antisymmetric (acoustic) plasma excitations contains two modes: a weakly damped mode below the gap $2\mathrm{\Delta }$ and a strongly damped mode above the gap. The lower mode can be interpreted as an analog of the Carlson-Goldman mode. This mode has an acoustic dispersion relation at small wave vectors and it saturates at the level $2\mathrm{\Delta }$ at large wave vectors. Its velocity is larger than the velocity of the Anderson-Bogoliubov mode $v_{AB}=v_F/\sqrt{2}$, and it can be smaller than the Fermi velocity $v_F$. The damping rate of this mode strongly increases under increase of temperature. Out-of-phase oscillations of two order parameters in two spin subsystems are also considered. This part of the spectrum contains two more modes. One of them is interpreted as an analog of the Anderson-Bogoliubov (phase) mode and the other as an analog of the Schmid (amplitude) mode. With minor modifications the theory can be extended to describe collective modes in a double bilayer graphene as well.

Quantum information protocols, such as quantum error correction and quantum phase estimation, have been widely used to enhance the performance of quantum sensors. While these protocols have relied on single-shot detection, in most practical applications only an averaged readout is available, as in the case of room-temperature sensing with the electron spin associated with a nitrogen-vacancy center in diamond. Here, we theoretically investigate the application of the quantum phase estimation algorithm for high dynamic-range magnetometry, when single-shot readout is not available. We show that, even in this case, Bayesian estimation provides a natural way to efficiently use the available information. We apply Bayesian analysis to achieve an optimized sensing protocol for estimating a time-independent magnetic field with a single electron spin associated to a nitrogen-vacancy center at room temperature and show that this protocol improves the sensitivity over previous protocols by more than a factor of 3. Moreover, we show that an extra enhancement can be achieved by considering the timing information in the detector clicks.

Quantum anomalous Hall (QAH) multilayers provide a platform for topological materials with high Chern numbers. We investigate the localization routes of bilayer QAH systems with Chern number $C=2$ during the process of increasing disorder, by numerical simulations on their quantum transport properties and the Chern-Simons axion coupling. The localization trajectories present richer behaviors than those in the monolayer with $C=2$. For example, there exists a stable intermediate state with $C=1$ before localization, which was always unstable in a monolayer. In some cases, this $C=1$ state is “weak” in the sense that its Hall plateau is hardly visible in a mesoscopic sample, but is still stable in the sense of renormalization group. The underlying physics is discussed. During the localization process, the Chern-Simons axion coupling shows a surprising peak which is even more remarkable in the large size limit. The physical origin of this peak is understood by a real-space analysis of the electronic states. As a result, the disordered QAH multilayers can be good candidates for this nontrivial magnetoelectric coupling mediated by orbital motions.

We propose a self-consistent site-dependent Hubbard $U$ approach for density functional theory (DFT)+$U$ calculations of defects in complex transition metal oxides, using Hubbard parameters computed via linear response theory. The formation of a defect locally perturbs the chemical environment of Hubbard sites in its vicinity, resulting in different Hubbard $U$ parameters for different sites. Using oxygen vacancies in ${\mathrm{SrMnO}}_3$ as a model system, we investigate the dependence of $U$ on the chemical environment and study its influence on the structural, electronic, and magnetic properties of defective bulk and strained thin-film structures. Our results show that a self-consistent $U$ improves the description of stoichiometric bulk ${\mathrm{SrMnO}}_3$ with respect to generalized gradient approximation (GGA) or GGA+$U$ calculations using an empirical $U$. For defective systems, $U$ changes as a function of the distance of the Hubbard site from the defect, its oxidation state, and the magnetic phase of the bulk structure. Taking into account this dependence, in turn, affects the computed defect formation energies and the predicted strain- and/or defect-induced magnetic phase transitions, especially when occupied localized states appear in the band gap of the material upon defect creation.

Motivated by the growing interest in self-correcting quantum memories, we study the feasibility of self-correction in classical lattice systems composed of bounded degrees of freedom with local interactions. We argue that self-correction, including a requirement of stability against external perturbation, cannot be realized in system with broken global symmetries such as the two-dimensional Ising model, but that systems with local, i.e., gauge symmetries, have the required properties. Previous work identified a three-dimensional (3D) quantum system which realizes a self-correcting classical memory [E. Dennis et al., J. Math. Phys. 43, 4452 (2002)]. Here, we show that a purely classical 3D system, Wegner's Ising lattice gauge model [F. J. Wegner, J. Math. Phys. 12, 2259 (1971)], can also realize this self-correction despite having an extensive ground-state degeneracy. We give a detailed numerical study to support the existence of a self-correcting phase in this system, even when the gauge symmetry is explicitly broken. More generally, our results obtained by studying the memory lifetime of the system are in quantitative agreement with the phase diagram obtained from conventional analysis of the system's specific heat [I. S. Tupitsyn et al., Phys. Rev. B 82, 085114 (2010)], except that self-correction extends beyond the topological phase, past the lower critical temperature.

We report on a detailed study of the optical response and $T_c\text{−}\rho$ phase diagram ($T_c$ being the superconducting critical temperature and $\rho$ the normal state resistivity of the film) of granular aluminum, combining transport measurements and a high resolution optical spectroscopy technique. The $T_c\text{−}\rho$ phase diagram is discussed as resulting from an interplay between the phase stiffness, the Coulomb repulsion, and the superconducting gap $\mathrm{\Delta }$. We provide direct evidence for two different types of well resolved subgap absorptions, at ${\omega }_1\simeq \mathrm{\Delta }$ and at $\mathrm{\Delta }\lesssim {\omega }_2\lesssim 2\mathrm{\Delta }$ (decreasing with increasing resistivity).