In designing and optimizing new-generation nanomaterials and related quantum devices, dissipation versus decoherence phenomena are often accounted for via local scattering models, such as relaxation-time and Boltzmann-like schemes. Here we show that the use of such local scattering approaches within the Wigner-function formalism may lead to unphysical results, namely anomalous suppression of intersubband relaxation, incorrect thermalization dynamics, and violation of probability-density positivity. Furthermore, we propose a quantum-mechanical generalization of relaxation-time and Boltzmann-like models, resulting in nonlocal scattering superoperators that enable one to overcome such limitations.

We investigate the many-body properties of graphene on top of a piezoelectric substrate, focusing on the interaction between graphene electrons and piezoelectric acoustic phonons. We calculate the electron and phonon self-energies as well as the electron mobility limited by the substrate phonons. We emphasize the importance of proper screening of the electron-phonon vertex, and we discuss the various limiting behaviors as a function of electron energy, temperature, and doping level. The effect of piezoelectric acoustic phonons on graphene electrons is compared with that of intrinsic deformation acoustic phonons. Substrate phonons tend to dominate over intrinsic ones for low doping levels at high and low temperatures.

We demonstrate that the degree of charge delocalization has a strong impact on polarization energy and thereby on the position of the transport band edge in organic semiconductors. This gives rise to long-range potential fluctuations, which govern the electronic transport through delocalized states in organic crystalline layers. This concept is employed to formulate an analytic model that explains a negative field dependence coupled with a positive temperature dependence of the charge mobility observed by a lateral time-of-flight technique in a high-mobility crystalline organic layer. This has important implications for the further understanding of the charge transport via delocalized states in organic semiconductors.

We theoretically analyze the Andreev bound states and their coupling to external radiation in superconductor-nanowire-superconductor Josephson junctions. We provide an effective Hamiltonian for the junction projected onto the Andreev level subspace and incorporating the effects of nanowire multichannel structure, Rashba spin-orbit coupling, and Zeeman field. Based on this effective model, we investigate the dependence of the Andreev levels and the matrix elements of the current operator on system parameters such as chemical potential, nanowire dimensions, and normal transmission. We show that the combined effect of the multichannel structure and the spin-orbit coupling gives rise to finite current matrix elements between odd-parity states having different spin polarizations. Moreover, our analytical results allow to determine the appropriate parameters range for the detection of transitions between even as well as odd states in circuit-QED-like experiments, which may provide a way for the Andreev spin-qubit manipulation.

We calculate the dielectric function and plasmonic response of armchair (aGNRs) and zigzag (zGNRs) graphene nanoribbons using the self-consistent-field approach within the Markovian master equation formalism (SCF-MMEF). We accurately account for electron scattering with phonons, ionized impurities, and line-edge roughness and show that electron scattering with surface optical phonons is much more prominent in GNRs than in graphene. We calculate the loss function, plasmon dispersion, and the plasmon propagation length in supported GNRs. Midinfrared plasmons in supported (3N+2)-aGNRs can propagate as far as several microns at room temperature, with 4–5-nm-wide ribbons having the longest propagation length. In other types of aGNRs and in zGNRs, the plasmon propagation length seldom exceeds 100 nm. Plasmon propagation lengths are much longer on nonpolar (e.g., diamondlike carbon) than on polar substrates (e.g., ${\mathrm{SiO}}_2$ or hBN), where electrons scatter strongly with surface optical phonons. We also show that the aGNR plasmon density is nearly uniform across the ribbon, while in zGNRs, because of the highly localized edge states, plasmons of different spin polarization are accumulated near the opposite edges.

We study a two coupled Su-Schrieffer-Heeger (SSH) chains system, which is shown to contain rich quantum phases associated with topological invariants protected by symmetries. In the weak coupling region, the system supports two nontrivial topological insulating phases, characterized by winding number $\mathcal{N}=\pm 1$, and two types of edge states. The boundary between the two topological phases arises from two band closing points, which exhibit topological characteristics in one-dimensional $k$ space. By mapping Bloch states on a vector field in $k$ space, the band degenerate points correspond to a pair of kinks of the field, with opposite topological charges. Two topological nodal points move and merge as the interchain coupling strength varies. This topological invariant is protected by the translational and inversion symmetries, rather than the antiunitary operation. Furthermore, we find that when a pair of nodal points is created, a second order quantum phase transition (QPT) occurs, associating with a gap closing and spontaneously symmetry breaking. This simple model demonstrates several central concepts in the field of quantum materials and provides a theoretical connection between them.

Topological phononic states, which facilitate unique acoustic transport around defects and disorders, have significantly revolutionized our scientific cognition of acoustic systems. Here, by introducing a zone folding mechanism, we realize the topological phase transition in a double Dirac cone of the rotatable triangular phononic crystal with $C_{3v}$ symmetry. We then investigate the distinct topological edge states on two types of interfaces of our phononic insulators. The first one is a zigzag interface which simultaneously possesses a symmetric mode and an antisymmetric mode. Hybridization of the two modes leads to a robust pseudospin-dependent one-way propagation. The second one is a linear interface with a symmetric mode or an antisymmetric mode. The type of mode is dependent on the topological phase transition of the phononic insulators. Based on the rotatability of triangular phononic crystals, we consider several complicated contours defined by the topological zigzag interfaces. Along these contours, the acoustic waves can unimpededly transmit without backscattering. Our research develops a route for the exploration of the topological phenomena in experiments and provides an excellent framework for freely steering the acoustic backscattering-immune propagation within topological phononic structures.

The complex structure and magnetism of ${\mathrm{Pr}}_{2-x}{\mathrm{Bi}}_x{\mathrm{Ru}}_2{\mathrm{O}}_7$ was investigated by neutron scattering and extended x-ray absorption fine structure. Pr has an approximate doublet ground state and the first excited state is a singlet. While the $B$-site (Ru) is well ordered throughout, this is not the case for the $A$-site (Pr/Bi). A broadened distribution for the Pr-O2 bond length at low temperature indicates the Pr environment varies from site to site even for $x=0$. The environment about the Bi site is highly disordered ostensibly due to the $6s$ lone pairs on ${\mathrm{Bi}}^{3+}$. Correspondingly, we find that the non-Kramers doublet ground-state degeneracy, otherwise anticipated for Pr in the pyrochlore structure, is lifted so as to produce a quadrupolar singlet ground state with a spatially varying energy gap. For $x=0$, below $T_N$, the Ru sublattice orders antiferromagnetically, with propagation vector $\mathbf{k}=(0,0,0)$ as for ${\mathrm{Y}}_2{\mathrm{Ru}}_2{\mathrm{O}}_7$. No ordering associated with the Pr sublattice is observed down to 100 mK. The low-energy magnetic response of ${\mathrm{Pr}}_{2-x}{\mathrm{Bi}}_x{\mathrm{Ru}}_2{\mathrm{O}}_7$ features a broad spectrum of magnetic excitations associated with inhomogeneous splitting of the Pr quasidoublet ground state. For $x=0$ ($x=0.97$), the spectrum is temperature dependent (independent). It appears disorder associated with Bi alloying enhances the inhomogeneous Pr crystal-field level splitting so that intersite interactions become irrelevant for $x=0.97$. The structural complexity for the $A$-site may be reflected in the hysteretic uniform magnetization of $B$-site ruthenium in the Néel phase.

The quantum nature of a microscopic system can only be revealed when it is sufficiently decoupled from surroundings. Interactions with the environment induce relaxation and decoherence that turn the quantum state into a classical mixture. Here, we study the timescales of these processes for a qubit encoded in the collective state of a set of magnetic atoms deposited on a metallic surface. For that, we provide a generalization of the commonly used definitions of $T_1$ and $T_2$ characterizing relaxation and decoherence rates. We calculate these quantities for several atomic structures, including a collective spin, a setup implementing a decoherence-free subspace, and two examples of spin chains. Our work contributes to the comprehensive understanding of the relaxation and decoherence processes and shows the advantages of the implementation of a decoherence free subspace in these setups.

We study nanostructures based on two ultrathin superconducting nanowires connected in parallel to form a superconducting quantum interference device (SQUID). The measured function of the critical current versus magnetic field, $I_C\left(B\right)$, is multivalued, asymmetric, and its maxima and minima are shifted from the usual integer and half integer flux quantum points. We also propose a low-temperature-limit model which generates accurate fits to the $I_C\left(B\right)$ functions and provides verifiable predictions. The key assumption of our model is that each wire is characterized by a sample-specific critical phase ${\varphi }_C$ defined as the phase difference at which the supercurrent in the wire is the maximum. For our nanowires ${\varphi }_C$ is much greater than the usual $\pi /2$, which makes a qualitative difference in the behavior of the SQUID. The nanowire current-phase relation is assumed linear, since the wires are much longer than the coherence length. The model explains single-valuedness regions where only one vorticity value $n_v$ is stable. Also, it predicts regions where multiple vorticity values are stable because the Little-Parks (LP) diamonds, which describe the region of stability for each winding number $n_v$ in the current-field diagram, can overlap. We also observe and explain regions in which the standard deviation of the switching current is independent of the magnetic field. We develop a technique that allows a reliable detection of hidden phase slips and use it to determine the boundaries of the LP diamonds even at low currents where $I_C\left(B\right)$ is not directly measurable.

We measured the Josephson radiation emitted by an InSb semiconductor nanowire junction utilizing photon-assisted quasiparticle tunneling in an ac-coupled superconducting tunnel junction. We quantify the action of the local microwave environment by evaluating the frequency dependence of the inelastic Cooper-pair tunneling of the nanowire junction and find the zero-frequency impedance $Z\left(0\right)=492\mathrm{\Omega }$ with a cutoff frequency of $f_0=33.1\mathrm{GHz}$. We extract a circuit coupling efficiency of $\eta \approx 0.1$ and a detector quantum efficiency approaching unity in the high-frequency limit. In addition to the Josephson radiation, we identify a shot noise contribution with a Fano factor $F\approx 1$, consistently with the presence of single electron states in the nanowire channel.

We studied the temperature and magnetic field dependence of vortex dissipation and critical current in the mixed state of unconventional superconducting alloys $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$($0.044\le x\le 0.100$) through current-voltage measurements. Our results reveal that all the electric field $E$ vs current density $j$ curves in the Ohmic regime merge to one point ($j_0,E_0$) and that there is a simple relationship between the critical current density $j_c$ and flux-flow resistivity ${\rho }_{\mathrm{ff}}$: ${\rho }_{\mathrm{ff}}/{\rho }_{\mathrm{n}}={(1-j_c/j_0)}^{-1}$, where ${\rho }_{\mathrm{n}}=E_0/j_0$ is the normal-state resistivity just above the superconducting transition. In addition, $E_0$ is positive for all five dopings, reflecting the abnormal behavior of the flux-flow resistivity ${\rho }_{\mathrm{ff}}$: it increases with decreasing magnetic field. In contrast, $E_0$ is negative for the conventional superconductor Nb since, as expected, ${\rho }_{\mathrm{ff}}$ decreases with decreasing magnetic field. Furthermore, in the underdoped and overdoped single crystals of $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$, the parameter $E_0$ remains temperature independent, while it decreases with increasing temperature for the single crystals around optimal doping ($0.060\le x\le 0.072$). This result points to the coexistence of superconductivity with some other phase around optimal doping.

Starting from first principles, we show the formation and evolution of superconducting gaps in ${\mathrm{MgB}}_2$ at its ultrathin limit. Atomically thin ${\mathrm{MgB}}_2$ is distinctly different from bulk ${\mathrm{MgB}}_2$ in that surface states become comparable in electronic density to the bulklike $\sigma$ and $\pi$ bands. Combining the ab initio electron-phonon coupling with the anisotropic Eliashberg equations, we show that monolayer ${\mathrm{MgB}}_2$ develops three distinct superconducting gaps, on completely separate parts of the Fermi surface due to the emergent surface contribution. These gaps hybridize nontrivially with every extra monolayer added to the film owing to the opening of additional coupling channels. Furthermore, we reveal that the three-gap superconductivity in monolayer ${\mathrm{MgB}}_2$ is robust over the entire temperature range that stretches up to a considerably high critical temperature of 20 K. The latter can be boosted to $>50\mathrm{K}$ under biaxial tensile strain of $\sim 4\%$, which is an enhancement that is stronger than in any other graphene-related superconductor known to date.

Understanding the lattice thermal conductivity at high temperatures is important for many applications. We characterize phonon quasiparticles numerically through a hybrid approach that combines first-principles molecular dynamics and lattice dynamics. We find no lower-bound limits on phonon mean free paths in the ${\mathrm{MgSiO}}_3$ perovskite. This contradicts the widely used minimal mean free path idea. The clear identification of phonon quasiparticles validates the use of a phonon gas model when the phonon mean free paths are shorter than the lattice constants of solids. Using the phonon quasiparticle properties, we have calculated the lattice thermal conductivity of the ${\mathrm{MgSiO}}_3$ perovskite. The results are reasonable compared to recent experimental measurements.