Our first-principles calculations show that an energy gap around 0.12–0.25 eV can be engineered in epitaxial graphene on SiC(0001) through the noncovalent intercalation of transition or alkali metals but originated from the distinct mechanisms. The former is attributed to the combined effects of a metal-induced perpendicular electric field and interaction, while the latter is solely attributed to the built-in electric field. A great advantage of this scheme is that the gap size is almost independent of the gate voltage up to 1 V/nm, thus reserving the electric means to tune the Fermi level of graphene when configured as field-effect transistors. Given the recent progress in experimental techniques for intercalated graphene, our findings provide a practical way to incorporate graphene in the current semiconductor industry.

We study the ground-state magnetism of the half-filled Hubbard model on the anisotropic triangular lattice, where two out of three bonds have hopping $t$ and the third one has $t^{\prime }$ in a unit triangle. Working in a spin-rotating frame and using the density matrix renormalization group method as an impurity solver, we provide a proper description of incommensurate magnetizations at zero temperature in the framework of the dynamical mean-field theory (DMFT). It is shown that the incommensurate spiral magnetic order for $t^{\prime }/t\gtrsim 0.7$ survives the dynamical fluctuations of itinerant electrons in the Hubbard interaction range from the strong-coupling (localized-spin) limit down to the insulator-to-metal transition. We also find that when the anisotropy parameter $t^{\prime }/t$ increases from the Néel-to-spiral transition, the magnitude of the magnetic moment exhibits a maximum at the isotropic triangular lattice point $t^{\prime }/t=1$ and then rapidly decreases in the range of larger $t^{\prime }/t$. This work gives a solid foundation for further extension of the study including nonlocal correlation effects neglected at the standard DMFT level.

We examine the microwave frequency $\left(f\right)$ variation of the angular phase shift, ${\theta }_0$, observed in the polarization-angle dependence of microwave-induced magnetoresistance oscillations in a high-mobility GaAs/AlGaAs two-dimensional electron system. By fitting the diagonal resistance $R_{xx}$ versus $\theta$ plots to an empirical cosine square law, we extract ${\theta }_0$ and trace its quasicontinuous variation with $f$. The results suggest that the overall average of ${\theta }_0$ extracted from Hall bar device sections with length-to-width ratios of $L/W=1$ and 2 is the same. We compare the observations with expectations arising from the “ponderomotive force” theory for microwave radiation-induced transport phenomena.

Nanowires made of superconducting Mo-Ge alloys undergo a superconductor-insulator transition when their cross-sectional area is reduced. On the insulating side of the transition, the differential resistance of the nanowires drops with voltage and displays a positive zero-bias anomaly (ZBA). To reveal the origin of this ZBA, we fabricated and studied a series of nanowires made of amorphous alloys with composition $\mathrm{M}{\mathrm{o}}_{50}\mathrm{G}{\mathrm{e}}_{50}$. The length of wires was in the range 150 nm–11 μm and width was in the range 10–20 nm. We also fabricated and measured several more complex nanowire-based structures: (i) a nanowire gated by a nearby film electrode, (ii) a nanowire connected to film electrodes with an “adiabatically reduced” width, (iii) a nanowire with a multielectrode configuration which allowed comparison of different sections of the same nanowire, and (iv) a nanowire with different sizes of film electrodes. We found that for $\mathrm{M}{\mathrm{o}}_{50}\mathrm{G}{\mathrm{e}}_{50}$ nanowires all experimental parameters of the ZBA and their dependence on nanowire length can be explained by electron heating. Several physical processes thought to be responsible for the ZBA have been analyzed and rejected.

Inspired by pulsed laser-assisted thermal relaxation experiments, we simulated the response of a nanometer-size monolayer graphene sample to thermal impulses by means of nonequilibrium molecular dynamics. By following the time evolution of the temperature profile until equilibrium is eventually reached, we obtain a direct estimation of the thermal conductivity of the excited graphene sample. It is shown that the predicted thermal conductivity depends on which vibrational degrees of freedom are in fact excited by the thermal impulse, suggesting that its determination depends, among many other issues, on the actual phonon population generated in the sample. The situation here explored is typically found in experiments or simulations addressing a transient regime of approach to equilibrium.

The electrical resistance of pristine and acceptor-doped single-wall carbon nanotubes assembled into transparent films was measured in the temperature range of 5 to 300 K. The doping was accomplished by filling the nanotubes with iodine or CuCl from the gas phase. After doping the films resistance appeared to drop down by one order of magnitude, to change the nonmonotonic temperature behavior, and to reduce the crossover temperature. The experimental data have been perfectly fitted in frames of the known heterogeneous model with two contributions: from the nanotube bundles (with quasi-one-dimensional conductivity) and from the interbundle electron tunneling. The doping was observed to decrease the magnitudes of both contributions. In this paper we have revealed the main reason of changes in the nanotube part. It is considered to be connected with the involvement of low-energy phonons, which start to participate in the intravalley scattering due to the shift of the Fermi level after doping. The values of the Fermi level shift into the valence band are estimated to be equal to $-0.6$ eV in the case of iodine doping and $-0.9$ eV in the case of CuCl doping. These values are in qualitative agreement with the optical absorption data.

Utilizing thermal changes in solid-state materials strategically offers caloric-based alternatives to replace current vapor-compression technology. To make full use of multiple forms of the entropy and achieve higher efficiency for designs of cooling devices, the multicaloric effect appears as a cutting-edge concept encouraging researchers to search for multicaloric materials with outstanding caloric properties. Here we report the multicaloric effect in $\mathrm{BaTi}{\mathrm{O}}_3$ single crystals driven simultaneously by mechanical and electric fields and described via a thermodynamic phenomenological model. It is found that the multicaloric behavior is mainly dominated by the mechanical field rather than the electric field, since the paraelectric-to-ferroelectric transition is more sensitive to mechanical field than to electric field. The use of uniaxial stress competes favorably with pressure due to its much higher caloric strength and negligible elastic thermal change. It is revealed that multicaloric response can be significantly larger than just the sum of mechanocaloric and electrocaloric effects in temperature regions far above the Curie temperature but cannot exceed this limit near the Curie temperature. Our results also show the advantage of the multicaloric effect over the mechanically mediated electrocaloric effect or electrically mediated mechanocaloric effect. Our findings therefore highlight the importance of ferroelectric materials to develop multicaloric cooling.

Nonequilibrium transport properties of quantum systems have recently become experimentally accessible in a number of platforms in so-called full-counting experiments that measure transient and steady-state nonequilibrium transport dynamics. We show that the effect of the measurement back-action can be exploited to establish general relationships between transport coefficients in the transient regime which take the form of fluctuation-dissipation theorems in the steady state. This result becomes most conspicuous in the transient dynamics of open quantum systems under strong-coupling to non-Markovian environments in nonequilibrium settings. In order to explore this regime, a new simulation method based in a hierarchy of equations of motion has been developed. We instantiate our proposal with the study of energetic conductance between two baths connected via a few level system.

CePdAl with Ce $4f$ moments forming a distorted kagome network is one of the scarce materials exhibiting Kondo physics and magnetic frustration simultaneously. As a result, antiferromagnetic (AF) order setting in at $T_{\mathrm{N}}=2.7$ K encompasses only two-thirds of the Ce moments. We report measurements of the specific heat, $C$, and the magnetic Grüneisen parameter, ${\mathrm{\Gamma }}_{\mathrm{mag}}$, on single crystals of ${\mathrm{CePd}}_{1-x}{\mathrm{Ni}}_x\mathrm{Al}$ with $x\le 0.16$ at temperatures down to 0.05 K and magnetic fields $B$ up to $8\mathrm{T}$. Field-induced quantum criticality for various concentrations is observed with the critical field decreasing to zero at $x_c\approx 0.15$. Remarkably, two-dimensional AF quantum criticality of Hertz-Millis-Moriya type arises for $x=0.05$ and $x=0.1$ at the suppression of three-dimensional magnetic order. Furthermore, ${\mathrm{\Gamma }}_{\mathrm{mag}}\left(B\right)$ shows an additional contribution near $2.5\mathrm{T}$ for all concentrations, which is ascribed to correlations of the frustrated one-third of Ce moments.

In a number of recent publications hidden magnetic properties at high temperatures have been reported for $\mathrm{EuTi}{\mathrm{O}}_3$ (ETO), which orders antiferromagnetically below $T_N=5.7\mathrm{K}$. In addition, structural phase transitions have been discovered which correlate with the magnetic responses and can be tuned by a magnetic field. In order to identify the magnetic properties of ETO at temperatures well above $T_N$, low-energy muon-spin rotation (μSR) experiments have been performed on thin films of ETO which exhibit all properties observed in bulk materials and are thus well suited to conclude about the magnetic order of the bulk. The μSR data reveal anomalies at 282 and 200 K related to the structural phase transitions in accordance with birefringence results. In addition, a transition to some kind of magnetic order below 100 K was observed as previously indirectly deduced from conductivity and dielectric constant measurements.

Free electrons can screen out long-range Coulomb interaction and destroy the polar distortion in some ferroelectric materials, whereas the coexistence of polar distortion and metallicity were found in several non-central-symmetric metals (NCSMs). Therefore, the mechanisms and designing of NCSMs have attracted great interest. In this work, by first-principles calculation, we found the polar distortion in the lone-pair driven ferroelectric material ${\mathrm{PbTiO}}_3$ cannot only persist but also increase with electron doping. We further analyzed the mechanisms of the persisting of the polar distortion. We found that the Ti site polar instability is suppressed, but the Pb site polar instability is intact with the electron doping. The Pb-site instability is due to the lone-pair mechanism which can be viewed as a pseudo-Jahn-Teller effect, a mix of the ground state and the excited state by ion displacement from the central symmetric position. The lone-pair mechanism is not strongly affected by the electron doping because neither the ground state nor the excited state involved is at the Fermi energy. The enhancement of the polar distortion is related to the increasing of the Ti ion size by doping. These results show that the long-pair stereoactive ions can be used for designing NCSMs.

We examined the formation mechanisms of magnetic bubbles in an $M$-type hexaferrite via Lorentz microscopy. When magnetic fields were perpendicularly applied to a thin sample of $\mathrm{BaF}{{\mathrm{e}}_{12}}_{-x-0.05}\mathrm{S}{\mathrm{c}}_x\mathrm{M}{\mathrm{g}}_{0.05}{\mathrm{O}}_{19}(x=1.6)$, Bloch lines, which were identified as reversals of domain-wall chirality, appeared, and magnetic bubbles were formed when the magnetic stripes were pinched off at these Bloch lines. The number of Bloch lines increased with the amount of Sc in $\mathrm{BaF}{\mathrm{e}}_{12-x-0.05}\mathrm{S}{\mathrm{c}}_x\mathrm{M}{\mathrm{g}}_{0.05}{\mathrm{O}}_{19}$, probably because of the reduction in magnetic anisotropy. A Lorentz microscopic observation revealed that Bloch lines with high magnetostatic energy may play an important role in the formation of magnetic bubbles.

In the ionic Hubbard model, the on-site repulsion $U$, which drives a Mott insulator, and the ionic potential $V$, which drives a band insulator, compete with each other to open up a window of charge fluctuations when $U\sim V$. We study this model on square and cubic lattices in the limit of large $U$ and $V$, with $V\sim U$. Using an effective Hamiltonian and a slave-boson approach with both doublons and holons, we find that the system undergoes a phase transition as a function of $V$ from an antiferromagnetic Mott insulator to a paramagnetic insulator with strong singlet correlations, which is driven by a condensate of “neutral” doublon-holon pairs. On further increasing $V$, the system undergoes another phase transition to a superconducting phase driven by condensate of “charged” doublons and holons. The superfluid phase, characterized by the presence of a coherent (but gapped) fermionic quasiparticle and $hc/e$ flux quantization, has a high $T_c\sim t$, which shows a dome-shaped behavior as a function of $V$. The paramagnetic insulator phase has a deconfined U(1) gauge field and associated gapless photon excitations. We also discuss how these phases can be detected in the ultracold-atom context.

Since the discovery of iron-based superconductors, a number of theories have been put forward to explain the qualitative origin of pairing, but there have been few attempts to make quantitative, material-specific comparisons to experimental results. The spin-fluctuation theory of electronic pairing, based on first-principles electronic structure calculations, makes predictions for the superconducting gap. Within the same framework, the surface wave functions may also be calculated, allowing, e.g., for detailed comparisons between theoretical results and measured scanning tunneling topographs and spectra. Here we present such a comparison between theory and experiment on the Fe-based superconductor LiFeAs. Results for the homogeneous surface as well as impurity states are presented as a benchmark test of the theory. For the homogeneous system, we argue that the maxima of topographic image intensity may be located at positions above either the As or Li atoms, depending on tip height and the setpoint current of the measurement. We further report the experimental observation of transitions between As- and Li-registered lattices as functions of both tip height and setpoint bias, in agreement with this prediction. Next, we give a detailed comparison between the simulated scanning tunneling microscopy images of transition-metal defects with experiment. Finally, we discuss possible extensions of the current framework to obtain a theory with true predictive power for scanning tunneling microscopy in Fe-based systems.

A comprehensive study of heat and electric transport, magnetic, and electronic structure (experiment and calculations) properties is reported for a skutterudite-related ${\mathrm{Ce}}_3{\mathrm{Ru}}_4{\mathrm{Sn}}_{13}$ heavy fermion system with the respective substitution of Co and Sb into Ru and Sn sites. ${\mathrm{Ce}}_3{\mathrm{Ru}}_4{\mathrm{Sn}}_{13}$ is obtained as a heavy fermion system with high electronic contribution to the specific heat $C\left(T\right)/T$ of $\sim 3\text{J}/{\mathrm{K}}^2{\text{mol}}_{\text{Ce}}$, and a significant Schottky anomaly below about 10 K. The complex study gives a consistent interpretation of the impact of doping on the crystal electric-field effect and Kondo temperature. For ${\mathrm{Ce}}_3{\mathrm{Ru}}_4{\mathrm{Sn}}_{13}$ we report a field-induced phase transition between the magnetically correlated heavy fermion phase and the single-ion Kondo impurity state, which does not depend on the type of dopant. We also demonstrate that doping does not improve the poor thermoelectric properties of ${\mathrm{Ce}}_3{\mathrm{Ru}}_4{\mathrm{Sn}}_{13}$.

A calculation of the high-harmonic generation (HHG) in $\alpha$-quartz using the time-dependent density functional theory is reported. The interband process is attributed to the dominant in HHG above the band gap. The photon energy is set to 1.55 eV, and the cutoff energy of the plateau region is found to be limited at the 19th harmonic (30 eV). The dependence of the HHG efficiency at the cutoff energy region on laser intensity is consistent with that of the hole density in the lowest-lying valence band. Numerical results indicate that electron-hole recombination plays a crucial role in HHG in $\alpha$-quartz. It is found that a 200 attosecond pulse train is produced using HHG around the plateau cutoff energy.

Considerable discussions have occurred about the critical role played by free electrons in the transport of heat in pure metals. In principle, any environment that can influence the dynamical behaviors of electrons would have impact on electronic thermal conductivity $\left({\kappa }_{\mathrm{el}}\right)$ of metals. Over the past decades, significant progress and comprehensive understanding have been gained from theoretical, as well as experimental, investigations by taking into account the effects of various conditions, typically temperature, impurities, strain, dimensionality, interface, etc. However, the effect of external magnetic field has received less attention. In this paper, the magnetic-field dependence of electron-phonon scattering, the electron's lifetime, and ${\kappa }_{\mathrm{el}}$ of representative metals (Al, Ni, and Nb) are investigated within the framework of all-electron spin-density functional theory. For Al and Ni, the induced magnetization vector field and difference in electron density under external magnetic-field aggregate toward the center of unit cell, leading to the enhanced electron-phonon scattering, the damped electron's lifetime, and thus the reduced ${\kappa }_{\mathrm{el}}$. On the contrary, for Nb with strong intrinsic electron-phonon interaction, the electron's lifetime and ${\kappa }_{\mathrm{el}}$ slightly increase as external magnetic field is enhanced. This is mainly attributed to the separately distributed magnetization vector field and difference in electron density along the corner of unit cell. This paper sheds light on the origin of influence of external magnetic field on ${\kappa }_{\mathrm{el}}$ for pure metals and offers a new route for robust manipulation of electronic thermal transport via applying external magnetic field.

We performed the angle dependent magnetoresistance (MR), Hall effect measurements, the temperature dependent magneto-thermoelectric power (TEP) $S$($T$) measurements, and the first-principles calculations to study the electronic properties of orthorhombic phase $\mathrm{Mo}{\mathrm{Te}}_2(T_d-\mathrm{Mo}{\mathrm{Te}}_2)$, which was proposed to be electronically two dimensional (2D). There are some interesting findings about $T_d-\mathrm{Mo}{\mathrm{Te}}_2$. (1) A scaling approach ${\varepsilon }_{\theta }={(\mathrm{si}{\mathrm{n}}^2\theta +{\gamma }^{-2}\mathrm{co}{\mathrm{s}}^2\theta )}^{1/2}$ is applied, where $\theta$ is the magnetic field angle with respect to the $c$ axis of the crystal and $\gamma$ is the mass anisotropy. Unexpectedly, the electronically three-dimensional (3D) character with $\gamma$ as low as 1.9 is observed in $T_d-\mathrm{Mo}{\mathrm{Te}}_2$. (2) The possible Lifshitz transition and the following electronic structure change can be verified around $T\sim 150\mathrm{K}$ and $T\sim 60\mathrm{K}$, which is supported by the evidence of the slope changing of the temperature dependence of TEP, the carrier density extracted from Hall resistivity, and the onset temperature of $\gamma$ obtained from the MR measurements. The extremely large MR effect in $T_d-\mathrm{Mo}{\mathrm{Te}}_2$ could originate from the combination of the electron-hole compensation and a particular orbital texture on the electron pocket, which is supported by the calculations of electronic structure. Our results may provide a general scaling relation for the anisotropic MR and help to recognize the origins of the MR effect in other systems, such as the Weyl semimetals and the Dirac ones.

We study the bond-alternating Heisenberg model using the finite-size density-matrix renormalization-group (DMRG) technique and analytical arguments based on the matrix product state, where we pay particular attention to the boundary-condition dependence on the entanglement spectrum of the system. We show that, in the antiperiodic boundary condition (APBC), the parity quantum numbers are equivalent to the topological invariants characterizing the topological phases protected by the bond-centered inversion and $\pi$ rotation about the $z$ axis. We also show that the odd parity in the APBC, which characterizes topologically nontrivial phases, can be extracted as a twofold degeneracy in the entanglement spectrum even with finite system size. We then determine the phase diagram of the model with the uniaxial single-ion anisotropy using the level spectroscopy method in the DMRG technique. These results not only suggest the detectability of the symmetry protected topological (SPT) phases via general twisted boundary conditions but also provide a useful and precise numerical tool for discussing the SPT phases in the exact diagonalization and DMRG techniques.