The electronic band structure of graphene is strongly dependent on the amount of strain and/or doping present. We performed a comprehensive study of temperature-dependent strain and doping in isotopically labeled graphene mono- and bilayers on a ${\mathrm{SiO}}_2/\mathrm{Si}$ substrate by Raman spectral mapping at well-defined temperatures between 300 and 10 K. The principal Raman active modes of the graphene (G, 2D) were subjected to correlation analysis, which enabled reliable separation of the strain and doping contributions. The influence of strain on the monolayer and top and bottom layers of the bilayer graphene is large and shows a pronounced temperature-dependent variation. A clear difference is observed in local strain fluctuations on length scales much smaller than the laser spot. In both the monolayer and the bottom layer of the bilayer, which are in contact with the substrate, a significant amount of local strain is induced when the temperature is varied. In contrast, the influence of local strain on the top layer of the bilayer is much smaller. Temperature dependence of the doping is clearly present in both layers, suggesting equalization of the captured charge in the bilayer down to low temperatures.

We perform large-scale ab initio density functional theory calculations to study the lattice strain and the vibrational properties of colloidal semiconductor core-shell nanoclusters with up to one thousand atoms (radii up to 15.6 Å). For all the group IV, III-V, and II-VI semiconductors studied, we find that the atom positions of the shell atoms seem unaffected by the core material. In particular, for group-IV core-shell clusters the shell material remains unstrained, while the core adapts to the large lattice mismatch (compressive or tensile strain). For InAs-InP and CdSe-CdS, both the cores and the shells are compressively strained corresponding to pressures up to 20 GPa. We show that this compression, which contributes a large blueshift of the vibrational frequencies, is counterbalanced, to some degree, by the undercoordination effect of the near-surface shell, which contributes a redshift to the vibrational modes. These findings lead to a different interpretation of the frequency shifts of recent Raman experiments, while they confirm the speculated interface nature of the low-frequency shoulder of the high-frequency Raman peak.

We have measured the thermal properties of suspended membranes from 10 to 300 K for two amplitudes of internal stress (about 0.1 and 1 GPa) and for two different thicknesses (50 and 100 nm). The use of the original $3\omega$-Volklein method has allowed the extraction of both the specific heat and the thermal conductivity of each SiN membrane over a wide temperature range. The mechanical properties of the same substrates have been measured at helium temperatures using nanomechanical techniques. Our measurements show that the thermal transport in freestanding SiN membranes is not affected by the presence of internal stress. Consistently, mechanical dissipation is also unaffected even though $Q$'s increase with increasing tensile stress. We thus demonstrate that the theory developed by Wu and Yu [J. Wu and C. C. Yu, Phys. Rev. B 84, 174109 (2011)] does not apply to this amorphous material in this stress range. On the other hand, our results can be viewed as a natural consequence of the “dissipation dilution” argument [Y. L. Huang and P. R. Saulson, Rev. Sci. Instrum. 69, 544 (1998)], which has been introduced in the context of mechanical damping.

Insulating hexagonal boron nitride monolayers $\left(h\mathrm{BN}\right)$ are best known for being resistant to chemical functionalization. This property makes $h\mathrm{BN}$ an excellent substrate for graphene heterostructures, but limits its application as an active element in nanoelectronics where tunable electronic properties are needed. Moreover, the two-dimensional–materials' community wishes to learn more about the adsorption and intercalation characteristics of alkali metals on $h\mathrm{BN}$, which have direct relevance to several electrochemistry experiments that are envisioned with layered materials. Here we provide results on ionic functionalization of $h\mathrm{BN}/\text{metal}$ interfaces with K and Li dopants. By combining angle-resolved photoemission spectroscopy (ARPES), x-ray photoelectron spectroscopy, and density functional theory calculations, we show that the metallic substrate readily ionizes the alkali dopants and exposes $h\mathrm{BN}$ to large electric fields and band-energy shifts. In particular, if $h\mathrm{BN}$ is in between the negatively charged substrate and the positive alkali ion, this allows us to directly study, using ARPES, the effects of large electric fields on the electron energy bands of $h\mathrm{BN}$.

We investigate the scattering of a wave packet describing low-energy electrons in graphene by a time-dependent finite-step potential barrier. Our results demonstrate that, after Klein tunneling through the barrier, the electron acquires an extra energy which depends on the rate of change of the barrier height with time. If this rate is negative, the electron loses energy and ends up as a valence band state after leaving the barrier, which effectively behaves as a positively charged quasiparticle.

The behavior of an atom in a molecule, liquid, or solid is governed by the force it experiences. If the dependence of this vectorial force on the atomic chemical environment can be learned efficiently with high fidelity from benchmark reference results—using “big-data” techniques, i.e., without resorting to actual functional forms—then this capability can be harnessed to enormously speed up in silico materials simulations. The present contribution provides several examples of how such a force field for Al can be used to go far beyond the length-scale and time-scale regimes presently accessible using quantum-mechanical methods. It is argued that pathways are available to systematically and continuously improve the predictive capability of such a learned force field in an adaptive manner, and that this concept can be generalized to include multiple elements.

We calculate the Raman response for the Kitaev spin model on the $\mathcal{H}-0, \mathcal{H}-1$, and $\mathcal{H}-\infty$ harmonic honeycomb lattices. We identify several quantitative features in the Raman spectrum that are characteristic of the spin liquid phase. Unlike the dynamical structure factor, which probes both the Majorana spinons and flux excitations that emerge from spin fractionalization, the Raman spectrum in the Kitaev models directly probes a density of states of pairs of fractional, dispersing Majorana spinons. As a consequence, the Raman spectrum in all these models is gapless for sufficiently isotropic couplings, with a low-energy power law that results from the Fermi lines (or points) of the dispersing Majorana spinons. We show that the polarization dependence of the Raman spectrum contains crucial information about the symmetry of the ground state. We also discuss to what extent the features of the Raman response that we find reflect generic properties of the spin liquid phase, and comment on their possible relevance to $\alpha$-, $\beta$-, and $\gamma -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ compounds.

We use a generalized Schwinger boson approach to investigate the nature of the intermediate phase (separating the antiferromagnetic and dimer phases) in the Shastry-Sutherland model with exchange anisotropy. Our results confirm the plaquette nature of the intermediate phase and demonstrate that exchange anisotropy (both Ising-like and $XY$-like) weakens the plaquette order and ultimately suppresses it altogether for sufficiently strong anisotropy. Interestingly, we find that the low-energy excitation spectrum of the plaquette state changes dramatically near the boundary with the dimer phase—the location of the minimum in the (gapped) lowest excitation mode changes from $\mathbf{k}=(0,0)$ to four degenerate points.

Many-body localization in an XY model with a long-range interaction is investigated. We show that in the regime of a high strength of disordering compared to the interaction an off-resonant flip-flop spin-spin interaction (hopping) generates the effective Ising interactions of spins in the third order of perturbation theory in a hopping. The combination of hopping and induced Ising interactions for the power-law distance dependent hopping $V\left(R\right)\propto R^{-\alpha }$ always leads to the localization breakdown in a thermodynamic limit of an infinite system at $\alpha <3d/2$ where $d$ is a system dimension. The delocalization takes place due to the induced Ising interactions $U\left(R\right)\propto R^{-2\alpha }$ of “extended” resonant pairs. This prediction is consistent with the numerical finite size scaling in one-dimensional systems. Many-body localization in an XY model is more stable with respect to the long-range interaction compared to a many-body problem with similar Ising and Heisenberg interactions requiring $\alpha \ge 2d$ which makes the practical implementations of this model more attractive for quantum information applications. The full summary of dimension constraints and localization threshold size dependencies for many-body localization in the case of combined Ising and hopping interactions is obtained using this and previous work and it is the subject for the future experimental verification using cold atomic systems.

Compounds based on the $\mathrm{F}{\mathrm{e}}_2\mathrm{P}$ structure have continued to attract interest because of the interplay between itinerant and localized magnetism in a noncentrosymmetric crystal structure, and because of the recent developments of these materials for magnetocaloric applications. Here we report the growth and characterization of millimeter-sized single crystals of $\mathrm{FeMn}{\mathrm{P}}_{0.8}\mathrm{S}{\mathrm{i}}_{0.2}$ with the $\mathrm{F}{\mathrm{e}}_2\mathrm{P}$ structure. Single-crystal x-ray diffraction, magnetization, resistivity, and Hall and heat capacity data are reported. The crystals exhibit itinerant antiferromagnetic order below 158 K with no hint of ferromagnetic behavior in the magnetization curves and with the spins ordered primarily in the $ab$ plane. The room-temperature resistivity is close to the Ioffe-Regel limit for a metal. Single-crystal x-ray diffraction indicates a strong preference for Mn to occupy the larger pyramidal $3g$ site. The cation site preference in the as-grown crystals and the antiferromagnetism are not changed after high-temperature anneals and a rapid quench to room temperature.

In the face of mounting numerical evidence, Metlitski and Grover (arXiv:1112.5166) have given compelling analytical arguments that systems with spontaneous broken continuous symmetry contain a subleading contribution to the entanglement entropy that diverges logarithmically with system size. They predict that the coefficient of this log is a universal quantity that depends on the number of Goldstone modes. In this paper, we confirm the presence of this log term through quantum Monte Carlo calculations of the second Rényi entropy on the spin-1/2 $XY$ model. Devising an algorithm to facilitate convergence of entropy data at extremely low temperatures, we demonstrate that the single Goldstone mode in the ground state can be identified through the coefficient of the log term. Furthermore, our simulation accuracy allows us to obtain an additional geometric constant additive to the Rényi entropy, that matches a predicted fully universal form obtained from a free bosonic field theory with no adjustable parameters.

We study the spin-orbit coupling in metallic carbon nanotubes (CNTs) within the many-body Tomonaga-Luttinger liquid framework. For a well-defined subclass of metallic CNTs, that contains both achiral zigzag as well as a subset of chiral tubes, an effective low-energy field theory description is derived. We aim to describe systems at finite dopings, but close to the charge neutrality point (commensurability). A new regime is identified where the spin-orbit coupling leads to an inverted hierarchy of minigaps of bosonic modes. We then add a proximity coupling to a superconducting (SC) substrate and show that the only order parameter that is supported within the spin-orbit induced phase is a topologically trivial s-SC.

In graphene spintronics, interaction of localized magnetic moments with the electron spins paves a new way to explore the underlying spin-relaxation mechanism. A self-assembled layer of organic cobalt porphyrin (CoPP) molecules on graphene provides a desired platform for such studies via the magnetic moments of porphyrin-bound cobalt atoms. In this work a study of spin-transport properties of graphene spin-valve devices functionalized with such CoPP molecules as a function of temperature via nonlocal spin-valve and Hanle spin-precession measurements is reported. For the functionalized (molecular) devices, we observe a decrease in the spin-relaxation time ${\tau }_s$ even up to 50%, which could be an indication of enhanced spin-flip scattering of the electron spins in graphene in the presence of the molecular magnetic moments. The effect of the molecular layer is masked for low-quality samples (low mobility), possibly due to dominance of Elliot-Yafet-type spin relaxation mechanisms.

Ultrafast pump-probe spectroscopy of two-dimensional tungsten disulfide monolayers $\left(2\mathrm{D}\mathrm{W}{\mathrm{S}}_2\right)$ grown on sapphire substrates revealed two transient absorption spectral peaks that are attributed to distinct negative trions at $\sim 2.02\mathrm{eV} \left(T_1\right)$ and $\sim 1.98\mathrm{eV} \left(T_2\right)$. The dynamics measurements indicate that trion formation by the probe is enabled by photodoped 2D ${\mathrm{WS}}_2$ crystals with electrons remaining after trapping of holes from excitons or free electron-hole pairs at defect sites in the crystal or on the substrate. Dynamics of the characteristic absorption bands of excitons $X_A$ and $X_B$ at $\sim 2.03$ and $\sim 2.40\mathrm{eV}$, respectively, were separately monitored and compared to the photoinduced absorption features. Selective excitation of the lowest exciton level $X_A$ using ${\lambda }_{\mathrm{pump}}<2.4\mathrm{eV}$ forms only trion $T_1$, implying that the electron remaining from dissociation of exciton $X_A$ is involved in the creation of this trion with a binding energy $\sim 10\mathrm{meV}$ with respect to $X_A$. The absorption peak corresponding to trion $T_2$ appears when ${\lambda }_{\mathrm{pump}}<2.4\mathrm{eV}$, which is just sufficient to excite exciton $X_B$. The dynamics of trion $T_2$ formation are found to correlate with the disappearance of the bleach of the $X_B$ exciton, indicating the involvement of holes participating in the bleach dynamics of exciton $X_B$. Static electrical-doping photoabsorption measurements confirm the presence of an induced absorption peak similar to that of $T_2$. Since the proposed trion formation process here involves exciton dissociation through hole trapping by defects in the 2D crystal or substrate, this discovery highlights the strong role of defects in defining optical and electrical properties of 2D metal chalcogenides, which is relevant to a broad spectrum of basic science and technological applications.

We demonstrate an efficient control of ${}^{29}\mathrm{Si}$ nuclear spins for specific lattice sites near ${}^{31}\mathrm{P}$ donors in silicon at temperatures below 1 K and in a high magnetic field of 4.6 T. Excitation of the forbidden electron-nuclear transitions leads to a pattern of well-resolved holes and peaks in the electron spin resonance (ESR) lines of ${}^{31}\mathrm{P}$. The pattern originates from dynamic polarization (DNP) of the ${}^{29}\mathrm{Si}$ nuclear spins near the donors via the solid effect. DNP of ${}^{29}\mathrm{Si}$ is demonstrated also with the Overhauser effect where the allowed ESR transitions are excited. In this case mostly the remote ${}^{29}\mathrm{Si}$ nuclei having weak interaction with the donors are polarized, which results in a single hole and a sharp peak pair in the ESR spectrum. Our work shows that the solid effect can be used for initialization of ${}^{29}\mathrm{Si}$ nuclear spin qubits near the donors.

We develop a theoretical approach to study the transient dynamics and the time-dependent statistics for the Anderson-Holstein model in the regime of strong electron-phonon coupling. For this purpose we adapt a recently introduced diagrammatic approach to the time domain. The generating function for the time-dependent charge transfer probabilities is evaluated numerically by discretizing the Keldysh contour. The method allows us to analyze the system evolution to the steady state after a sudden connection of the dot to the leads, starting from different initial conditions. Simple analytical results are obtained in the regime of very short times. We study in particular the apparent bistable behavior occurring for strong electron-phonon coupling, small bias voltages, and a detuned dot level. The results obtained are in remarkably good agreement with numerically exact results obtained by quantum Monte Carlo methods. We analyze the waiting time distribution and charge transfer probabilities, showing that only a single electron transfer is responsible for the rich structure found in the short-time regime. A universal scaling (independent of the model parameters) is found for the relative amplitude of the higher order current cumulants in the short-time regime, starting from an initially empty dot. We finally analyze the convergence to the steady state of the differential conductance and of the differential Fano factor at the inelastic threshold, which exhibits a peculiar oscillatory behavior.

Diminution of the thermal conductivity is a crucial aspect in thermoelectric research. We report a systematic and significant reduction of the cross-plane thermal conductivity in a model system consisting of dc sputtered TiNiSn and HfNiSn half-Heusler superlattices. The reduction of $\kappa$ is measured by the $3\omega$ method and originates from phonon scattering at the internal interfaces. Heat transport in the superlattices is calculated based on Boltzmann transport theory, including a diffusive mismatch model for the phonons at the internal interfaces. Down to a superlattice periodicity of 3 nm the phonon spectrum mismatch between the superlattice components quantitatively explains the reduction of $\kappa$. For very thin individual layers the interface model breaks down and the artificial crystal shows an enhanced $\kappa$.