Successful thermal management in nanostructured devices relies on control of interfacial thermal transport. Recent measurements have revealed poor thermal transport across interfaces between two dissimilar materials, e.g., organic semiconductors and metals. In such systems, the interfacial thermal conductance $G_b$ is dominated by the strength of interfacial bonding, but existing analytical models still fail to accurately predict $G_b$ especially for organic-metal interfaces. Growing interest in this research area calls for comprehensive understanding of interfacial thermal transport across hybrid material interfaces. Here we demonstrate that spatial nonuniformity has to be assessed in the calculation of $G_b$ for interfaces with partial coverage or incommensurate growth that is characteristic of these interfaces. The interface between copper phthalocyanine and fcc metals (Ag, Al, and Au) exhibits a sixfold difference between the metal's (∼4-Å) and the organic molecule's (∼25-Å) lattice constant. Molecular dynamics simulations reveal the spatial variation in $G_b$, and a model is developed that considers the spatial variations in phonon transmission, successfully predicting $G_b$ for many organic-metal interfaces.

The classical criterion for classification of superconductors as type I or type II based on the isotropic Ginzburg-Landau theory is generalized to arbitrary temperatures for materials with anisotropic Fermi surfaces and order parameters. We argue that the relevant quantity for this classification is the ratio of the upper and thermodynamic critical fields $H_{c2}/H_c$, rather than the traditional ratio of the penetration depth and the coherence length $\lambda /\xi$. Even in the isotropic case, $H_{c2}/H_c$ coincides with $\sqrt{2}\lambda /\xi$ only at the critical temperature $T_c$ and they differ as $T$ decreases, the long-known fact. Anisotropies of Fermi surfaces and order parameters may amplify this difference and render false the criterion based on the value of $\kappa =\lambda /\xi$.

Pairing in the Weyl semimetal appearing on the surface of a topological insulator is considered. It is shown that due to an “ultrarelativistic” dispersion relation there is a quantum critical point governing the zero-temperature transition to a superconducting state. Starting from the microscopic Hamiltonian with local attraction, we calculated using the Gor'kov equations, the phase diagram of the superconducting transition at arbitrary chemical potential, and its magnetic properties and critical exponents close to the quantum critical point. The Ginzburg-Landau (GL) effective theory is derived for small chemical potential, allowing us to consider effects of spatial dependence of order parameters in a magnetic field. The GL equations are very different from the conventional ones reflecting the chiral universality class of the quantum phase transition. The order-parameter distribution of a single vortex is found to be different as well. The magnetization near the upper critical field is found to be quadratic, not linear as usual. We discuss the application of these results to recent experiments in which surface superconductivity was found for some three-dimensional topological insulators, and we estimate feasibility of the phonon pairing.

The quasiparticle interference (QPI) patterns in ${\mathrm{BiS}}_2$-based superconductors are theoretically investigated by taking into account the spin-orbital coupling and assuming the recently proposed $d_{x^2-y^2}^{*}$-wave pairing symmetry. We found two distinct scattering wave vectors whose evolution can be explained based on the evolution of the constant-energy contours. In addition, we also studied the QPI spectra in the conventional $s$-wave, $d$-wave, and extended $s$-wave cases and propose to further measure the Knight shift to distinguish the $d_{x^2-y^2}^{*}$-wave pairing from the $d$-wave one. The QPI spectra as well as the Knight shift presented in this paper can thus be compared with future scanning tunneling microscopy and NMR experiments to test whether the pairing symmetry is $d_{x^2-y^2}^{*}$ wave in ${\mathrm{BiS}}_2$-based superconductors.

${\mathrm{BiCu}}_2{\mathrm{PO}}_6$, a spin-gapped material in a frustrated two-leg ladder lattice, exhibits a complex $(H,T)$ phase diagram in external magnetic fields. We have characterized the field-induced phases through specific heat, magnetocaloric effect, Faraday rotation, and magnetization measurements in fields up to 120 T. In addition to observing a magnetic phase above 60 T which we attribute to the crystallization of triplet excitations, our measurements reveal the emergence and the subsequent disappearance of a high-entropy (short-range-ordered) phase at first and second critical magnetic fields, respectively. This is in good agreement with the theoretical prediction of a quantum soliton lattice at intermediate fields.

It is well established that long-range magnetic order is suppressed in magnetic systems whose interactions are low dimensional. The prototypical example is the $S-\frac{1}{2}$ Heisenberg antiferromagnetic chain ($S-\frac{1}{2}$ HAFC) whose ground state is quantum critical. In real $S-\frac{1}{2}$ HAFC compounds interchain coupling induces long-range magnetic order although with a suppressed ordered moment and reduced Néel temperature compared to the Curie-Weiss temperature. Recently, it was suggested that order can also be suppressed if the interchain interactions are frustrated, as for the Nersesyan-Tsvelik model. Here, we study the new $S-\frac{1}{2}$ HAFC, $\left(\mathrm{NO}\right)\left[\mathrm{Cu}\right({\mathrm{NO}}_3{)}_3]$. This material shows extreme suppression of order which furthermore is incommensurate revealing the presence of frustration consistent with the Nersesyan-Tsvelik model.

Using first-principles calculations, we investigated the impact of chromium (Cr) and vanadium (V) impurities on the magnetic anisotropy and spin polarization in Fe/MgO magnetic tunnel junctions. It is demonstrated, using layer resolved anisotropy calculation technique, that while the impurity near the interface has a drastic effect in decreasing the perpendicular magnetic anisotropy (PMA), its position within the bulk allows maintaining high interfacial PMA while reducing the bulk magnetization and correlatively the easy-plane demagnetizing energy. As a result, the effective magnetic anisotropy tends to increase as a function of the Cr or V concentration resulting in an increase in the critical magnetic thickness at which the crossover from out-of-plane to in-plane anisotropy takes place. At the same time, the interfacial spin polarization is not affected by the magnetic layer bulk doping by Cr or V impurities and even enhanced in most situations thus favoring an increase of tunnel magnetoresistance (TMR) amplitude.

We investigated the growth and magnetic properties of Tm atoms and monolayers deposited on a W(110) surface using scanning tunneling microscopy and x-ray magnetic circular and linear dichroism. The equilibrium structure of Tm monolayer films is found to be a strongly distorted hexagonal lattice with a Moiré pattern due to the overlap with the rectangular W(110) substrate. Monolayer as well as isolated Tm adatoms on W present a trivalent ground-state electronic configuration, contrary to divalent gas phase Tm and weakly coordinated atoms in quench-condensed Tm films. Ligand field multiplet simulations of the x-ray absorption spectra further show that Tm has a $|J=6,J_z=\pm 5\rangle$ electronic ground state separated by a few meV from the next lowest substates $|J=6,J_z=\pm 4\rangle$ and $|J=6,J_z=\pm 6\rangle$. Accordingly, both the Tm atoms and monolayer films exhibit large spin and orbital moments with out-of-plane uniaxial magnetic anisotropy. X-ray magnetic dichroism measurements as a function of temperature show that the Tm monolayers develop antiferromagnetic correlations at about 50 K. The triangular structure of the Tm lattice suggests the presence of significant magnetic frustration in this system, which may lead to either a noncollinear staggered spin structure or intrinsic disorder.

We consider a bilayer quantum spin model with anisotropic intralayer exchange couplings. By varying the anisotropy, the quantum critical phenomena changes from XY to Heisenberg to Ising universality class, with two modes, three modes, and one mode, respectively, becoming gapless simultaneously. We use series-expansion methods to calculate the second and third Renyi entanglement entropies when the system is bipartitioned into two parts. Leading area-law terms and subleading entropies associated with corners are separately calculated. We find clear evidence that the logarithmic singularity associated with the corners is universal in each class. Its coefficient along the Ising critical line is in excellent agreement with those obtained previously for the transverse-field Ising model. Our results provide strong evidence for the idea that the universal terms in the entanglement entropy provide an approximate measure of the low energy degrees of freedom in the system.

Projected entangled pair states (PEPS) are a promising ansatz for the study of strongly correlated quantum many-body systems in two dimensions. However, due to their high computational cost, developing and improving PEPS algorithms is necessary to make the ansatz widely usable in practice. Here we analyze several algorithmic aspects of the method. On the one hand, we quantify the connection between the correlation length of the PEPS and the accuracy of its approximate contraction and discuss how purifications can be used in the latter. On the other hand, we present algorithmic improvements for the update of the tensor that introduce drastic gains in the numerical conditioning and the efficiency of the algorithms. Finally, the state-of-the-art general PEPS code is benchmarked with the Heisenberg and quantum Ising models on lattices of up to $21\times 21$ sites.

We develop a theory for the Higgs (gap-amplitude) mode excitation in electric-field-induced (EFI) superconductors. The Higgs mode can be excited directly by an oscillating electric field in EFI superconductors since the gap value depends sensitively on the external electric field that induces superconducting carriers. The mass of the Higgs mode in EFI superconductors does not coincide with the threshold energy of pair-breaking two-particle excitations.

Neutron diffraction and muon spin relaxation measurements are used to obtain a detailed phase diagram of ${\mathrm{PrFe}}_{1-x}{\mathrm{Ru}}_x\mathrm{AsO}$. The isoelectronic substitution of Ru for Fe acts effectively as spin dilution, suppressing both the structural and magnetic phase transitions. The temperature $T_S$ of the tetragonal-orthorhombic structural phase transition decreases gradually as a function of $x$. Slightly below $T_S$, coherent precessions of the muon spin are observed corresponding to static magnetism, possibly reflecting a significant magnetoelastic coupling in the FeAs layers. Short-range order in both the Fe and Pr moments persists for higher levels of $x$. The static magnetic moments disappear at a concentration coincident with that expected for percolation of the $J{}_1\text{−}J{}_2$ square-lattice model.

We employ molecular beam epitaxy to stabilize ${\mathrm{Ba}}_2{\mathrm{IrO}}_4$ thin films and utilize in situ angle-resolved photoemission spectroscopy to investigate the evolution of its electronic structure through the Néel temperature $T_{\mathrm{N}}$. Our measurements indicate that dispersions of the relativistic $J_{\mathrm{eff}}=1/2$ and 3/2 bands exhibit an unusual dichotomy in their behavior through the Néel transition. Although the charge gap survives into the paramagnetic state, only the $J_{\mathrm{eff}}=1/2$ state exhibits a strong temperature dependence and its gap softens with increasing temperature approaching $T_{\mathrm{N}}$, while the nearly fully occupied $J_{\mathrm{eff}}=3/2$ state which remains nearby in energy exhibits negligible changes with temperature.

The energy positions of frontier orbitals in organic electronic materials are often studied experimentally by (inverse) photoemission spectroscopy and theoretically within density functional theory. However, standard exchange-correlation functionals often result in too small fundamental gaps, may lead to wrong orbital energy ordering, and do not capture polarization-induced gap renormalization. Here we examine these issues and a strategy for overcoming them by studying the gas phase and bulk electronic structure of the organic molecule quinacridone (5Q), a promising material with many interesting properties for organic devices. Experimentally we perform angle-resolved photoemission spectroscopy (ARUPS) on thin films of the crystalline $\beta$ phase of 5Q. Theoretically we employ an optimally tuned range-separated hybrid functional (OT-RSH) within density functional theory. For the gas phase molecule, our OT-RSH result for the ionization potential (IP) represents a substantial improvement over the semilocal PBE and the PBE0 hybrid functional results, producing an IP in quantitative agreement with experiment. For the bulk crystal we take into account the correct screening in the bulk, using the recently developed optimally tuned screened range-separated hybrid (OT-SRSH) approach, while retaining the optimally tuned parameters for the range separation and the short-range Fock exchange. This leads to a band gap narrowing due to polarization effects and results in a valence band spectrum in excellent agreement with experimental ARUPS data, with respect to both peak positions and heights. Finally, full-frequency $G_0{W}_0$ results based on a hybrid functional starting point are shown to agree with the OT-SRSH approach, improving substantially on the PBE-starting point.

A magnetic field applied perpendicularly to the chiral two-dimensional electron gas (C2DEG) in a Bernal-stacked bilayer graphene quantizes the kinetic energy into a discrete set of Landau levels $N=0,\pm 1,\pm 2,....$ While Landau level $N=0$ is eightfold degenerate, higher Landau levels $\left(\left|N\right|\ge 1\right)$ are fourfold degenerate when spin and valley degrees of freedom are counted. In this work, the Hartree-Fock approximation is used to study the phase diagram of the C2DEG at integer fillings $\tilde{\nu }=1,2,3$ of these higher Landau levels. At these filling factors, the C2DEG is a valley or spin Ising quantum Hall ferromagnet. At odd fillings, the C2DEG is spin polarized and has all its electrons in one valley or the other. There is no intervalley coherence, in contrast with most of the ground states in Landau level $N=0.$ At even filling $\tilde{\nu }=2,$ the C2DEG is either fully spin polarized with electrons occupying both valleys or spin unpolarized with electrons occupying one of the two valleys. A finite electric field (or bias) applied perpendicularly to the plane of the C2DEG induces a series of first-order phase transitions between these different ground states. The transport gap or its slope is discontinuous at the bias where a transition occurs. Such discontinuity may result in a change in the transport properties of the C2DEG at that bias.

Recent experiments reveal that a scanning tunneling microscopy (STM) probe tip can generate a highly localized strain field in a graphene drumhead, which in turn leads to pseudomagnetic fields in the graphene that can spatially confine graphene charge carriers in a way similar to a lithographically defined quantum dot (QD). While these experimental findings are intriguing, their further implementation in nanoelectronic devices hinges upon the knowledge of key underpinning parameters, which still remain elusive. In this paper we first summarize the experimental measurements of the deformation of graphene membranes due to interactions with the STM probe tip and a back-gate electrode. We then carry out systematic coarse-grained (CG) simulations to offer a mechanistic interpretation of STM tip-induced straining of the graphene drumhead. Our findings reveal the effect of (i) the position of the STM probe tip relative to the graphene drumhead center, (ii) the sizes of both the STM probe tip and graphene drumhead, as well as (iii) the applied back-gate voltage, on the induced strain field and corresponding pseudomagnetic field. These results can offer quantitative guidance for future design and implementation of reversible and on-demand formation of graphene QDs in nanoelectronics.

In the presence of axial magnetic fields that can be realized in deliberately buckled monolayer graphene, quasirelativistic Dirac fermions may find themselves in a variety of broken-symmetry phases even for weak repulsive interactions. Through a detailed Hartree self-consistent numerical calculation in finite strained graphene with cylindrical and open boundaries, we establish the possibility of realizing a charge-density wave order for the spinless fermions in the presence of weak nearest-neighbor repulsion. Such an instability gives rise to a staggered pattern of average fermionic density between bulk and boundary of the system as well as among two sublattices of honeycomb lattice, due to the spatial separation of the zero-energy states localized on opposite sublattices. Although with fermions spin restored, an unconventional magnetic order driven by the onsite repulsion possibly leads to the dominant instability at the Dirac point, the proposed charge-density wave order can nevertheless be realized at finite doping, which is always accompanied by a finite ferromagnetic moment. Additionally, the charge-density wave phase supports a quantized charge or spin Hall conductivity when its formation away from the Dirac point is further preceded by the appearance of topological anomalous or spin Hall insulator, respectively. The topological orders in strained graphene can be supported by weak second-neighbor repulsion, for example. Therefore, depending on the relative strength of various short-range components of the Coulomb interaction, several broken-symmetry phases can be realized within the zero-energy manifold in strained graphene.

The interaction between a graphene layer and a hexagonal boron nitride (hBN) substrate induces lateral displacements and strains in the graphene layer. The displacements lead to the appearance of commensurate regions and the existence of an average gap in the electronic spectrum of graphene. We present a simple, but realistic, model, with which the displacements, strains, and spectral gap can be derived analytically from the adhesion forces between hBN and graphene. When the lattice axes of graphene and the substrate are aligned, strains reach a value of the order of 2%, leading to effective magnetic fields above 100 T. The combination of strains and induced scalar potential gives a sizable contribution to the electronic gap. Commensuration effects are negligible due to the large stiffness of graphene.

The excitonic behavior of anisotropic two-dimensional crystals is investigated using numerical methods. We employ a screened potential arising due to the system polarizability to solve the central-potential problem using the Numerov approach. The dependence of the exciton energies on the interaction strength and mass anisotropy is demonstrated. We use our results to obtain the exciton binding energy in phosphorene as a function of the substrate dielectric constant.

We theoretically investigate photothermoelectric effects, i.e., the photoenabled versions of the Seebeck and Nernst effects in monolayer transition metal dichalcogenides (TMDCs) using semiclassical transport theory. We find that monolayer TMDCs subjected to circular polarized light show anomalous thermoelectric properties as a result of strong spin-orbit interaction and broken inversion symmetry. The Seebeck coefficient for photogenerated carriers is relatively large and changes its sign at a critical carrier density. In addition, a nontrivial photo-Nernst effect emerges in the absence of an external magnetic field or magnetic moments.