We theoretically investigate the backaction of a sensor quantum dot with strong local Coulomb repulsion on the transient dynamics of a qubit that is probed capacitively. We show that the measurement backaction induced by the noise of electron cotunneling through the sensor is surprisingly mitigated by the recently identified coherent backaction [M. Hell, M. R. Wegewijs, and D. P. DiVincenzo, Phys. Rev. B 89, 195405 (2014)] arising from quantum fluctuations. This indicates that a sensor with quantized states may be switched off better than naively expected. This renormalization effect is missing in semiclassical stochastic fluctuator models and typically also in Born-Markov approaches, which try to avoid the calculation of the nonstationary, nonequilibrium state of the qubit plus sensor. Technically, we integrate out the current-carrying electrodes to obtain kinetic equations for the joint, nonequilibrium detector-qubit dynamics. We show that the sensor current response, level renormalization, cotunneling broadening, and leading non-Markovian corrections always appear together and cannot be turned off individually in an experiment or ignored theoretically. We analyze the backaction on the reduced qubit state—capturing the full non-Markovian effects imposed by the sensor quantum dot on the qubit—by applying a Liouville-space decomposition into quasistationary and rapidly decaying modes. Importantly, the sensor cannot be eliminated completely even in the simplest high-temperature, weak-measurement limit since the qubit state experiences an initial slip depending on the initial preparation of qubit plus sensor quantum dot. The slip persists over many qubit cycles, i.e., also on the time scale of the qubit decoherence induced by the backaction. A quantum-dot sensor can thus not be modeled as usual as a “black box” without accounting for its dynamical variables; it is part of the quantum circuit. We furthermore find that the Bloch vector relaxes (rate $1/T_1$) along an axis that is not orthogonal to the plane in which the Bloch vector dephases (rate $1/T_2$), blurring the notions of relaxation and dephasing times. Moreover, the precessional motion of the Bloch vector is distorted into an ellipse in the tilted dephasing plane.

A theoretical study of surface phonon polaritons (SPhPs) on periodically poled lithium niobate and periodically poled lithium tantalate surfaces is presented. We calculate the dielectric response for six different superlattice orientations and the associated SPhP dispersion relations. Our study of SPhPs accounts for the anisotropic nature of the dielectric response of the semi-infinite piezoelectric superlattices. We find that two different types of SPhPs can be supported. The first type consists of real surface dipole oscillations coupled to photons. The second type consists of virtual surface dipole oscillations driven by the incident photons. The dependence of the SPhPs on temperature and superlattice geometry is addressed. The use of these metamaterial excitations is discussed in the context of hybrid quantum systems.

Oxygen octahedra tilting is a common structural phenomenon in perovskites and has been subject of intensive studies, particularly in rhombohedral $\mathrm{Pb}\left(\mathrm{Zr},\mathrm{Ti}\right){\mathrm{O}}_3$ (PZT). Early reports suggest that the tilted octahedra may strongly affect the domain switching behavior as well as the dielectric and piezoelectric properties of PZT ceramics. In a way similar to that proposed for tilts, however, charged defects, associated with oxygen vacancies, may also inhibit the motion of the domain walls, resulting macroscopically in pinched hysteresis loops and reduced piezoelectric response. Here, we revisit the early studies on rhombohedral PZT ceramics with tilted octahedra by considering a possible role of both tilts and charged defects on domain-wall motion. We show that the observed pinched hysteresis loops are likely associated with the presence of defect complexes containing charged oxygen vacancies, and not tilts, as suggested in some cases. Regardless of the presence or absence of long-range ordered tilts in rhombohedral PZT, the effect of charged defects is also prominent in weak-field permittivity and piezoelectric properties, particularly at sub-Hertz driving conditions where the conductivity, related to the motion of oxygen vacancies, gives rise to strong frequency dispersion.

The changes to the topological and chemical ordering in the network-forming isostatic glass ${\mathrm{GeSe}}_4$ are investigated at pressures up to $\sim 14.4$ GPa by using a combination of neutron diffraction and first-principles molecular dynamics. The results show a network built from corner- and edge-sharing Ge(${\mathrm{Se}}_{1/2}$)${}_4$ tetrahedra, where linkages by ${\mathrm{Se}}_2$ dimers or longer ${\mathrm{Se}}_n$ chains are prevalent. These linkages confer the network with a local flexibility that helps to retain the network connectivity at pressures up to $\sim 8$ GPa, corresponding to a density increase of $\sim 37\%$. The network reorganization at constant topology maintains a mean coordination number $\overline{n}\simeq 2.4$, the value expected from mean-field constraint-counting theory for a rigid stress-free network. Isostatic networks may therefore remain optimally constrained to avoid stress and retain their favorable glass-forming ability over a large density range. As the pressure is increased to around 13 GPa, corresponding to a density increase of $\sim 49\%$, Ge(${\mathrm{Se}}_{1/2}$)${}_4$ tetrahedra remain as the predominant structural motifs, but there is an appearance of 5-fold coordinated Ge atoms and homopolar Ge-Ge bonds that accompany an increase in the fraction of 3-fold coordinated Se atoms. The band gap energy decreases with increasing pressure, and midgap states appear at pressures beyond $\sim 6.7$ GPa. The latter originate from undercoordinated Se atoms that terminate broken ${\mathrm{Se}}_n$ chains.

We report the design procedure for an acoustic add-drop filter (ADF) composed of two line-defect waveguides coupled through a ring resonator cavity (RRC) all based on a phononic crystal (PnC) platform. Using finite difference time domain and plane wave expansion methods, we study the propagation of acoustic waves through the PnC based ADF structures. Numerical results show that the quality factor for the ADF with a quasisquare ring resonator with a frequency band of 95 Hz centered about 75.21 kHz is $Q$ ∼ 800. We show that the addition of an appropriate scatterer at each RRC corner can reduce the scattering loss, enhancing the quality factor and the transmission efficiency. Moreover, it is also shown that by increasing the coupling gaps between the RRC and waveguides the quality factor can be increased by ∼25 times, at the expense of a significant reduction in the transmission efficiency this is attributed to the enhanced selectivity in expense of weakened coupling. Finally, by varying the effective path length of the acoustic wave in the RRC, via selectively varying the inclusions physical and geometrical properties, we show how one can ultra-fine and fine-tune the resonant frequency of the ADF.

We study a system coupled to external degrees of freedom, called bath, where we assume that the total system consisting of system and bath is in equilibrium. An expansion in the coupling between system and bath leads to a general form of the reduced density matrix of the system as a function of the bath self energy. The coupling to the bath results in a renormalization of the energies of the system and in a change of the eigenbasis. This theory is applicable to quantum emulators in thermal equilibrium. Undesired external degrees of freedom can affect their reliability. We study the influence of bosonic degrees of freedom on the state of a six qubit system.

A spin-orbital quantum liquid state is theoretically proposed in the honeycomb lattice of ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$, enabled by dynamic short-range correlations between the spin and orbital degrees of freedom. Using neutron diffraction, the local atomic structure of ${\mathrm{Ba}}_3{\mathrm{Cu}}_{1+x}{\mathrm{Sb}}_{2-x}{\mathrm{O}}_9$ at $x=0$ and 0.1 is obtained via the pair density function analysis. The results indicate that both compositions exhibit local Jahn-Teller (JT) distortions with the elongated ${\mathrm{CuO}}_3$ octahedral configuration. In ${\mathrm{Ba}}_3{\mathrm{Cu}}_{1.1}{\mathrm{Sb}}_{1.9}{\mathrm{O}}_9$, JT ordering of the distorted ${\mathrm{CuO}}_3$ octahedra gives rise to the orthorhombic symmetry with ferro-orbital order. On the other hand, in ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$, even though the ${\mathrm{CuO}}_3$ octahedra are JT distorted, there is no long-range ordering hence the symmetry is hexagonal. Furthermore, the local singlet excitation at 5.8 meV observed in ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ below 50 K is absent in ${\mathrm{Ba}}_3{\mathrm{Cu}}_{1.1}{\mathrm{Sb}}_{1.9}{\mathrm{O}}_9$. Instead, an excitation at 2.5 meV is observed in the latter, which is likely associated with short-range spin order.

Using dynamical mean-field theory (DMFT) we study a simplified model for heterostructures involving superconductors. The system is driven out of equilibrium by a voltage bias, imposed as an imbalance of chemical potential at the interface. We solve the self-consistent DMFT equations using iterative second-order perturbation theory in the Nambu-Keldysh formalism. We show that the superconducting state is destabilized by voltage biases of the order of the energy gap. We demonstrate that the transition to the normal state occurs through an intermediate bad-superconducting state, which is characterized by a smaller value of the order parameter and incoherent excitations. We discuss the energetic balance behind the stabilization of such exotic superconducting state.

As titanium is a highly utilized metal for structural lightweighting, its phases, transformation pathways (transition states), and structures have scientific and industrial importance. Using a proper solid-state nudged elastic band method employing two climbing images combined with density functional theory DFT + U methods for accurate energetics, we detail the pressure-induced $\alpha$ (ductile) to $\omega$ (brittle) transformation at the coexistence pressure. We find two transition states along the minimal-enthalpy path and discover a metastable body-centered orthorhombic structure, with stable phonons, a lower density than the end-point phases, and decreasing stability with increasing pressure.

Topological superconductors possess a nodeless superconducting gap in the bulk and gapless zero energy modes, known as “Majorana zero modes,” at the boundary of a finite system. In this work, we introduce a new class of topological superconductors, which are protected by nonsymmorphic crystalline symmetry and thus dubbed “topological nonsymmorphic crystalline superconductors.” We construct an explicit Bogoliubov–de Gennes type of model for this superconducting phase in the D class and show how Majorana zero modes in this model are protected by glide plane symmetry. Furthermore, we generalize the classification of topological nonsymmorphic crystalline superconductors to the classes with time reversal symmetry, including the DIII and BDI classes, in two dimensions. Our theory provides guidance to search for new topological superconducting materials with nonsymmorphic crystal structures.

The response of small-grained metals to mechanical stress is investigated by a theoretical study of the elementary mechanisms occurring during the shear-coupled migration of grain boundaries (GB). Investigating a model $\mathrm{\Sigma }17\left(410\right)$ GB in a copper bicrystal, both $\langle 110\rangle$ and $\langle 100\rangle$ GB migration modes are studied focusing on both the structural and energetic characteristics. The minimum energy paths of these shear-coupled GB migrations are computed using the nudge elastic band method. For both modes, the GB migration occurs through the nucleation and motion of disconnections. However, the atomic mechanisms of both modes qualitatively differ: While the $\langle 110\rangle$ mode presents no metastable state, the $\langle 100\rangle$ mode shows multiple metastable states, some of them evidencing some kinks along the disconnection lines. Disconnection kinks nucleation and motion activation energies are evaluated. Besides, the activation energies of the $\langle 100\rangle$ mode are smaller than those of the $\langle 110\rangle$ one except for very high stresses. These results significantly improve our knowledge of the GB migration mechanisms and the conditions under which they occur.

Compresed BiTeBr has been studied from a joint experimental and theoretical perspective. Room-temperature x-ray diffraction, Raman scattering, and transport measurements at high pressures have been performed in this layered semiconductor and interpreted with the help of ab initio calculations. A reversible first-order phase transition has been observed above 6–7 GPa, but changes in structural, vibrational, and electrical properties have also been noted near 2 GPa. Structural and vibrational changes are likely due to the hardening of interlayer forces rather than to a second-order isostructural phase transition while electrical changes are mainly attributed to changes in the electron mobility. The possibility of a pressure-induced electronic topological transition and of a pressure-induced quantum topological phase transition in BiTeBr and other bismuth tellurohalides, like BiTeI, is also discussed.

${\mathrm{NiS}}_{2-2x}{\mathrm{Se}}_x$ is revisited to determine the magnetic structure using neutron diffraction and magnetic representational analysis. Upon cooling, the insulating parent compound, ${\mathrm{NiS}}_2$, becomes antiferromagnetic with two successive magnetic transitions. The first transition $\left(M1\right)$ occurs at $T_N\sim 39\text{K}$ with ${\mathrm{\Gamma }}_1{\psi }_1$ symmetry and a magnetic propagation vector of $k=\left(000\right)$. The second transition $\left(M2\right)$ occurs at $T_N\sim 30\text{K}$ with $k=(0.5,0.5,0.5)$ and a ${\mathrm{\Gamma }}_1{\psi }_2$ symmetry with face-centered translations, giving rise to four possible magnetic domains. With doping, the system becomes metallic. The transition to the $M2$ state is suppressed prior to $x=0.4$ while the $M1$ state persists. The $M1$ magnetic structure gradually vanishes by $x\sim 0.8$ at a lower concentration than previously reported. The details of the magnetic structures are provided.

A multiple time scale relaxation dynamic is revealed by alternating current (ac) susceptibility studies of a single crystal of $\mathrm{E}{\mathrm{r}}_2\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_7$ performed at high temperatures ($k_{B}T\gg J/k_B$) in a wide range of static magnetic fields. An analysis of the frequency dependence of the ac susceptibility revealed the existence of two relaxation mechanisms, identified as an Orbach process with the pronounced effect of a phonon bottleneck and cross-tunneling. The origin of the phonon bottleneck is attributed to resonant phonon trapping. The relevance of the obtained results for relaxation phenomena found in other rare-earth pyrochlores, studied under similar conditions, is discussed.

We studied the structural, electronic, and magnetic properties of a cubic perovskite ${\mathrm{BaFeO}}_{3-\delta }$ ($0\le \delta \le 0.5$) within the density-functional theory using a generalized gradient approximation (GGA) and a GGA + $U$ method. According to our calculations, ${\mathrm{BaFeO}}_3$ in its stoichiometric cubic structure should be half-metallic and strongly ferromagnetic, with an extremely high Curie temperature ($T_C$) of 700–900 K. However, such an estimate of $T_C$ disagrees with all available experiments, which report that $T_C$ of the ${\mathrm{BaFeO}}_3$ and undoped ${\mathrm{BaFeO}}_{3-\delta }$ films varies between 111 and 235 K, or, alternatively, that no ferromagnetic order was detected there. Fitting the calculated x-ray magnetic circular dichroism spectra to the experimental features seen for ${\mathrm{BaFeO}}_3$, we concluded that good agreement can be obtained when oxygen vacancies are included in our model. Thus, the relatively low $T_C$ measured in ${\mathrm{BaFeO}}_3$ can be explained by oxygen vacancies intrinsically present in the material. Since iron species near the O vacancy change their oxidation state from $4+$ to $3+$, the interaction between ${\mathrm{Fe}}^{4+}$ and ${\mathrm{Fe}}^{3+}$, which is antiferromagnetic, weakens the effective magnetic interaction in the system, which is predominantly ferromagnetic. With increasing $\delta$ in ${\mathrm{BaFeO}}_{3-\delta }$, its $T_C$ decreases down to the critical value when the magnetic order becomes antiferromagnetic. Our calculations of the electronic structure of ${\mathrm{BaFeO}}_{3-\delta }$ illustrate how the ferromagnetism originates and also how one can keep this cubic perovskite robustly ferromagnetic far above room temperature.

The emergence of the electron-pocket-only iron-based superconductor $A_x\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2 \left(A=\mathrm{alkali}\mathrm{metal}\right)$ challenges the Fermi-surface nesting picture established in iron pnictides. It was widely believed that magnetism is correlated with the superconductivity in $A_x\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2$. Unfortunately, the highly anisotropic exchange parameters and the disagreement between theoretical calculations and experimental results triggered a fierce debate on the nature of magnetism in $A_x\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2$. Here we find that the strong magnetic anisotropy is from the anisotropic biquadratic interaction. In order to accurately obtain the magnetic interaction parameters, we propose a universal method, which does not need to include other high-energy configurations as in the conventional energy-mapping method. We show that our model successfully captures the magnetic interactions in $A_x\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2$ and correctly predicts the spin-wave spectrum, in quantitative agreement with the experimental observation. These results suggest that the local moment picture, including the biquadratic term, can describe accurately the magnetic properties and spin excitations in $A_x\mathrm{F}{\mathrm{e}}_{2-y}\mathrm{S}{\mathrm{e}}_2$, which sheds new light on the future study of the high-$T_{\mathrm{c}}$ iron-based superconductors.

Modeling the adsorption of atoms and molecules on surfaces requires efficient electronic-structure methods that are able to capture both covalent and noncovalent interactions in a reliable manner. In order to tackle this problem, we have developed a method within density-functional theory (DFT) to model screened van der Waals interactions (vdW) for atoms and molecules on surfaces (the so-called DFT+${\text{vdW}}^{\mathrm{surf}}$ method). The relatively high accuracy of the DFT+${\text{vdW}}^{\mathrm{surf}}$ method in the calculation of both adsorption distances and energies, as well as the high degree of its reliability across a wide range of adsorbates, indicates the importance of the collective electronic effects within the extended substrate for the calculation of the vdW energy tail. We examine in detail the theoretical background of the method and assess its performance for adsorption phenomena including the physisorption of Xe on selected close-packed transition metal surfaces and 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on Au(111). We also address the performance of DFT+${\text{vdW}}^{\mathrm{surf}}$ in the case of non-close-packed surfaces by studying the adsorption of Xe on Cu(110) and the interfaces formed by the adsorption of a PTCDA monolayer on the Ag(111), Ag(100), and Ag(110) surfaces. We conclude by discussing outstanding challenges in the modeling of vdW interactions for studying atomic and molecular adsorbates on inorganic substrates.

The Jarzynski equality relates nonequilibrium thermodynamics and equilibrium states. We realize a coupled quantum dot-electron reservoir system in which the time resolved observation of the tunneling dynamics is used to explicitly measure the exerted work and dissipated heat per single charge. We determine accurate values of the equilibrium free energy change over a large range of final state energies by driving the system far from equilibrium.

In this paper, we present a theoretical investigation of the energy transfer efficiency between a pair of quantum emitters placed in proximity to a conducting graphene nanodisk. The energy transfer efficiency quantifies the contribution of the energy transfer process to the relaxation of the donor quantum system, as compared to the spontaneous emission rate of the donor in the absence of the acceptor. We use in our calculations the Green's tensor formalism in the electrostatic limit. This approximation works very well for the nanodisks considered here, for which the radius is much smaller than the emission wavelength of the donor. The approximate analytical solutions obtained are used to investigate the decay rate of a single quantum emitter and the energy transfer rate between quantum emitters in the vicinity of the graphene nanodisk. We find that these rates are enhanced several orders of magnitude compared with their free-space values. We determine the resonance frequencies of the spontaneous emission rate of a single quantum emitter to a graphene nanodisk, and the energy transfer rate between a pair of quantum emitters in proximity to a graphene nanodisk. We identify the surface modes which give the largest contributions to the energy transfer function. We connect the resonance frequency values and their surface plasmon wave numbers, which depend on the radius of the graphene nanodisk, with the dispersion relation of an infinite graphene monolayer at the same chemical potential. Analyzing the distance dependence of these rates, we are able to fit the full numerical results with a simple analytical expression which depends only on the geometrical characteristics of the graphene nanodisk, i.e., its radius. We show that the interaction distance depends on the transition dipole moment orientation and the different order resonance frequencies. The interaction distance between a pair of quantum emitters increases from a free-space value of $20\text{nm}$ to reach values of 120 nm in the presence of a graphene nanodisk.

We determine the bands and gaps in graphene subjected to the magnetic field of an Abrikosov lattice of vortices in the underlying superconducting film. The spectrum features one nondispersive magnetic miniband at zero energy, separated by the largest gaps in the miniband spectrum from a pair of minibands resembling a slightly broadened first Landau level in graphene, suggesting the persistence of $\nu =\pm 2$ and $\pm 6$ quantum Hall effect states. Also, we identify an occasional merging point of magnetic minibands with a Dirac-type dispersion at the miniband edges.