Co-doped $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ has been previously shown to have an unusually significant improvement of $T_c$ (up to 2 K, or almost 10%) with annealing 1–2 weeks at 700 °C or 800 °C, where such annealing conditions are insufficient to allow significant atomic diffusion. While confirming similar behavior in optimally Co-doped $\mathrm{SrF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ samples, the influence on $T_c$ of strain induced by grinding to $\sim 50\mu \mathrm{m}$ sized particles, followed by pressing the powder into a pellet using 10 kbar pressure, was found to increase the annealed transition width of 1.5 K by approximately a factor of ten. Also, the bulk discontinuity in the specific heat at $T_c$, ΔC, on the same pellet sample was completely suppressed by grinding. This evidence for a strong sensitivity of superconductivity to strain was used to optimize single-crystal growth of Co-doped $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$. This strong dependence (both positive via annealing and negative via grinding) of superconductivity on strain in these two iron based 122 structure superconductors is compared to the unconventional heavy Fermion superconductor $\mathrm{UP}{\mathrm{t}}_3$, where grinding is known to completely suppress superconductivity, and to recent reports of strong sensitivity of $T_c$ to damage induced by electron-irradiation-induced point defects in other 122 structure iron-based superconductors, $\mathrm{Ba}{\left(\mathrm{F}{\mathrm{e}}_{0.76}\mathrm{R}{\mathrm{u}}_{0.24}\right)}_2\mathrm{A}{\mathrm{s}}_2$ and $\mathrm{B}{\mathrm{a}}_{1-x}{\mathrm{K}}_x\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$. Both the electron irradiation and the introduction of strain by grinding are believed to only introduce nonmagnetic defects, and argue for unconventional superconducting pairing.

The nature of a material's Fermi surface is crucial to understanding its electronic, magnetic, optical, and thermal characteristics. Traditional measurements such as angle-resolved photoemission spectroscopy and de Haas–van Alphen quantum oscillations can be difficult to perform in the vicinity of a pressure-driven quantum phase transition, although the evolution of the Fermi surface may be tied to the emergence of exotic phenomena. We demonstrate here that magnetic x-ray diffraction in combination with Hall effect measurements in a diamond anvil cell can provide valuable insight into the Fermi surface evolution in spin- and charge-density-wave systems near quantum phase transitions. In particular, we track the gradual evolution of the Fermi surface in elemental chromium and delineate the critical pressure and absence of Fermi surface reconstruction at the spin-flip transition.

Some well-established examples of itinerant-electron ferromagnetism in one dimension occur in a Mott-insulating phase. We examine the consequences of doping a ferromagnetic insulator and coupling magnons to gapless charge fluctuations. Using a bosonization scheme for strongly interacting electrons, we derive an effective field theory for the magnon-holon interaction. When the magnon momentum matches the Fermi momentum of the holons, the backscattering of the magnon at low energies gives rise to a Kondo effect of a pseudospin defined from the chirality degree of freedom (right- or left-moving particles). The crossover between weak-coupling and strong-coupling fixed points of the effective mobile-impurity model is then investigated using a numerical renormalization group approach.

We present a theoretical factorization scheme for interactions between many-body systems with localized discrete and delocalized quasicontinuous states. The projection operator technique combines a conventional correlation expansion with an exact diagonalization scheme. Our approach is capable of treating the continuum dynamically and to apply non-Markovian treatments beyond the Markovian approximation typically used in Lindblad formalism. Therefore it can be an important tool to describe a variety of systems as well as interaction mechanisms.

We study the dynamics of entanglement of two electron spins in two quantum dots, in which each electron is interacting with its nuclear spin environment. Focusing on the case of uncoupled dots, and starting from either Bell or Werner states of two qubits, we calculate the decay of entanglement due to the hyperfine interaction with the nuclei. We mostly focus on the regime of magnetic fields in which the bath-induced electron spin flips play a role, for example, their presence leads to the appearance of entanglement sudden death at finite time for two qubits initialized in a Bell state. For these fields, the intrabath dipolar interactions and spatial inhomogeneity of hyperfine couplings are irrelevant on the time scale of coherence (and entanglement) decay, and most of the presented calculations are performed using the uniform-coupling approximation to the exact hyperfine Hamiltonian. We provide a comprehensive overview of entanglement decay in this regime, considering both free evolution of the qubits, and an echo protocol with simultaneous application of $\pi$ pulses to the two spins. All the currently relevant for experiments bath states are considered: the thermal state, narrowed states (characterized by diminished uncertainty of one of the components of the Overhauser field) of two uncorrelated baths, and a correlated narrowed state with a well-defined value of the $z$ component of the Overhauser field interdot gradient. While we mostly use concurrence to quantify the amount of entanglement in a mixed state of the two electron spins, we also show that their entanglement dynamics can be reconstructed from measurements of the currently relevant for experiments entanglement witnesses and the fidelity of quantum teleportation, performed using a partially disentangled state as a resource.

We derive the dispersion relations of the guided modes in wire media slabs using a transmission and reflection matrix approach, providing a physically insightful perspective into wave propagation in wire media slabs. We find new approximations to the dispersion relations, and assess their accuracy and the regimes for which they hold. We do so both analytically, and against exact calculations of the electromagnetic fields without reference to effective media. We then analyze the effects of the background refractive index on the guided modes, proposing explanations for the observed behavior.

The effect of electron doping on the van Hove singularities (vHs) which develop in twisted graphene bilayers (tBLs) is studied for a broad range of rotation angles $\theta (1.5^{\circ }<\theta <{15}^{\circ })$ by means of scanning tunneling microscopy and spectroscopy. Bilayer and trilayer graphene islands were grown on the 6H-SiC(000-1) $(3\times 3)$ surface, which results in tBLs doped in the ${10}^{12}{\mathrm{cm}}^{-2}$ range by charge transfer from the substrate. For large angles, doping manifests in a strong asymmetry of the positions of the upper (in empty states) and lower (in occupied states) vHs with respect to the Fermi level. The splitting of these vHs energies is found essentially independent of doping for the whole range of $\theta$ values, but the center of theses vHs shifts towards negative energies with increasing electron doping. Consequently, the upper vHs crosses the Fermi level for smaller angles (around $3^{\circ }$). The analysis of the data performed using tight-binding calculations and simple electrostatic considerations shows that the interlayer bias remains small $(<100\mathrm{mV})$ for the doping level resulting from the interfacial charge transfer $(\simeq 5\times {10}^{12}{\mathrm{cm}}^{-2})$.

Based on ${}^{29}\mathrm{Si}$ nuclear spin-lattice relaxation time $\left(T_1\right)$ data on a single crystal of ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$, the coexisting Fermi liquid and non-Fermi liquid states around the quantum critical phase transition have been found to be sensitive to the direction of the applied magnetic field. Augmented scaling analysis shows that the anisotropic characteristic temperature and field are roughly ten times larger for $H\parallel c$ than for $H\parallel a$, consistent with a previously obtained anisotropic phase diagram.

The interplay of electronic nematic modulations, magnetic order, superconductivity, and structural distortions in strongly correlated electron materials calls for methods which allow characterizing them simultaneously, to allow establishing directly the relationship between these different phenomena. Spin-polarized STM enables studying both electronic excitations as well as magnetic structure in the same measurement at the atomic scale. Here we demonstrate preparation of magnetic tips, both ferromagnetic and antiferromagnetic, on single crystals of FeTe. This opens up preparation of spin-polarized tips without the need for sophisticated ultrahigh-vacuum preparation.

We consider theoretically spin correlations in a one-dimensional quantum wire with Rashba-Dresselhaus spin-orbit interaction (RDI). The correlations of noninteracting electrons display electron spin resonance at a frequency proportional to the RDI coupling. Interacting electrons, upon varying the direction of the external magnetic field, transit from the state of Luttinger liquid (LL) to the spin-density wave (SDW) state. We show that the two-time total-spin correlations of these states are significantly different. In the LL, the projection of total spin to the direction of the RDI-induced field is conserved and the corresponding correlator is equal to zero. The correlators of two components perpendicular to the RDI field display a sharp electron-spin resonance driven by the RDI-induced intrinsic field. In contrast, in the SDW state, the longitudinal projection of spin dominates, whereas the transverse components are suppressed. This prediction indicates a simple way for an experimental diagnostic of the SDW in a quantum wire. We point out that the Luttinger model does not respect the spin conservation since it assumes the infinite Fermi sea. We propose a proper cutoff to correct this failure.

Low temperature scanning tunneling microscopy is used to image the doped topological insulator ${\text{Bi}}_{2-x}{\text{Fe}}_x{\text{Se}}_3$. Interstitial Fe defects allow the detection of quasiparticle interference, and the reconstruction of the empty state band structure. Quantitative comparison between measured data and density functional theory calculations reveals the unexpected coexistence of quantum well states (QWSs) with topological surface states (TSSs) on the atomically clean surface of ${\text{Bi}}_{2-x}{\text{Fe}}_x{\text{Se}}_3$. Spectroscopic measurements quantify the breakdown of linear dispersion due to hexagonal warping. Nonetheless, both QWSs and TSSs remain spin polarized and protected from backscattering to almost 1 eV above the Dirac point, suggesting their utility for spin-based applications.

Using high-resolution angle-resolved photoemission spectroscopy (ARPES), surface states that were proposed to be useful for constructing the topological superconducting state in sodium cobaltates are experimentally explored. Two energy bands are observed to cross the Fermi level in the large $x$ region. Through in situ surface electron doping, temperature-dependent ARPES measurements, and spin-resolved ARPES measurements, we demonstrate the existence of surface states due to less Na on the surface than in the bulk, as predicted in theory.

We use the multiscale entanglement renormalization ansatz (MERA) to numerically investigate three critical quantum spin chains with $Z_2\times Z_2$ on-site symmetry: a staggered XXZ model, a transverse field cluster model, and the quantum Ashkin-Teller model. All three models possess a continuous one-parameter family of critical points. Along this critical line, the thermodynamic limit of these models is expected to be described by classes of $c=1$ conformal field theories (CFTs) of two possible types: the $S^1$ free boson and its $Z_2$ orbifold. Our numerics using MERA with explicitly enforced $Z_2\times Z_2$ symmetry allow us to extract conformal data for each model, with strong evidence supporting the identification of the staggered XXZ model and critical transverse field cluster model with the $S^1$ boson CFT, and the Ashkin-Teller model with the $Z_2$-orbifold boson CFT. Our first two models describe the phase transitions between symmetry-protected topologically ordered phases and trivial phases, which lie outside the usual Landau-Ginsburg-Wilson paradigm of symmetry breaking. Our results show that a range of critical theories can arise at the boundary of a single symmetry protected phase.

To explore the origin of the insulating property of layered system $R_2{\mathrm{O}}_2\mathrm{Sb}$ ($R=\mathrm{rare}\text{−}\mathrm{earth}$ element) and to find their distorted superstructures, we have investigated the band structures and phonon dispersions of ${\mathrm{La}}_2{\mathrm{O}}_2\mathrm{Sb}$ and ${\mathrm{Er}}_2{\mathrm{O}}_2\mathrm{Sb}$ based on the ab initio density functional theory. We have found three dominant Fermi surface nesting vectors of ${\mathbf{q}}_1=0.25{\mathbf{a}}^{*},{\mathbf{q}}_2=0.5{\mathbf{a}}^{*}+0.25{\mathbf{b}}^{*}$, and ${\mathbf{q}}_3=0.25{\mathbf{a}}^{*}+0.25{\mathbf{b}}^{*}$. Phonon softening instabilities occur at the same $\mathbf{q}$ vectors, which suggests that the lattice instability in Sb layers is driven by the charge density wave instability through the electron-phonon coupling. The linearly superposed mode $\left({{\eta }_{\mathbf{q}}}_3^{*}\right)$ of two degenerate normal modes at ${\mathbf{q}}_3$ produces the most stable state with a herringbone-type arrangement of Sb dimers. In the ground state, the distorted Sb square sheet shows the insulating behavior in agreement with existing experiments.

Optical measurements and first-principles calculations of the band structure and exciton states in direct-gap bulk and few-layer $\mathrm{Pb}{\mathrm{I}}_2$ indicate that the $n=1$ exciton is Frenkel-like in nature in that its energy exhibits a weak dependence on thickness down to atomic-length scales. Results reveal large increases in the gap and exciton binding energy with a decreasing number of layers and a transition of the fundamental gap, which becomes indirect for one and two monolayers. Calculated values are in reasonable agreement with a particle-in-a-box model relying on the Wannier-Mott theory of exciton formation. General arguments and existing data suggest that the Frenkel-like character of the lowest exciton is a universal feature of wide-gap layered semiconductors whose effective masses and dielectric constants give bulk Bohr radii that are on the order of the layer spacing.

An optimal band gap that allows for a high solar-to-fuel energy conversion efficiency is one of the key factors to achieve sustainability. We investigate computationally the band gaps and optical spectra of functional perovskites composed of layers of the two cubic perovskite semiconductors ${\mathrm{BaSnO}}_3$ and ${\mathrm{BaTaO}}_2\mathrm{N}$. Starting from an indirect gap of around $3.3\mathrm{eV}$ for ${\mathrm{BaSnO}}_3$ and a direct gap of $1.8\mathrm{eV}$ for ${\mathrm{BaTaO}}_2\mathrm{N}$, different layerings can be used to design a direct gap of the functional perovskite between $2.3$ and $1.2\mathrm{eV}$. The variations of the band gap can be understood in terms of quantum confinement and tunneling. We also calculate the light absorption of the different heterostructures and demonstrate a large sensitivity to the detailed layering.

Zigzag graphene nanoribbons have spin-polarized edges, antiferromagnetically coupled in the ground state with total spin zero. Customarily, these ribbons are made ferromagnetic by producing an imbalance between the two sublattices. Here we show that zigzag ribbons can be ferromagnetic due to the presence of reconstructed divacancies near one edge. This effect takes place even though the divacancies are produced by removing two atoms from opposite sublattices, which were balanced before reconstruction to 5-8-5 defects. We demonstrate that there is a strong interaction between the defect-localized and edge bands which mix and split away from the Fermi level. This splitting is asymmetric, yielding a net edge spin polarization. Therefore, the formation of reconstructed divacancies close to the edges of the nanoribbons can be a practical way to make them partially ferromagnetic.

A major focus within the field of magnonics involves the manipulation and control of spin-wave modes. This is usually done by patterning continuous soft magnetic films. Here, we report on work in which we use topographic modifications of a continuous magnetic thin film, rather than lithographic patterning techniques, to modify the ferromagnetic resonance spectrum. To demonstrate this technique we have performed in-plane, broadband, ferromagnetic resonance studies on a 100-nm-thick permalloy film sputtered onto a colloidal crystal with individual sphere diameters of 200 nm. Effects resulting from the, ideally, sixfold-symmetric underlying colloidal crystal were studied as a function of the in-plane field angle through experiment and micromagnetic modeling. Experimentally, we find two primary modes; the ratio of the intensities of these two modes exhibits a sixfold dependence. Detailed micromagnetic modeling shows that both modes are quasiuniform and nodeless in the unit cell but that they reside in different demagnetized regions of the unit cell. Our results demonstrate that topographic modification of magnetic thin films opens additional directions for manipulating ferromagnetic resonant excitations.

Large Chern-number topological phase is always an important topic in modern physics. Here we investigate the topological superfluids in a coupled-layer system, in which transitions between different topological superfluids can be realized by controlling the binding energy, interlayer tunneling, and layer asymmetry, etc. These topological transitions are characterized by energy gap closing and reopening at the critical points at zero momentum, where the Chern number and sign of Pfaffian undergo a discontinuous change. Topological protected edge modes at the boundaries are ensured by the bulk-edge correspondence. In a trapped potential the edge modes are spatially localized at the interfaces between distinct topological superfluids, where the number of edge modes is equal to the Chern-number difference between the left and right superfluids. These topological transitions can be detected by spin texture at or near zero momentum, which changes discretely across the critical points due to band inversion. The model can be generalized to a multilayer system in which the Chern number can be equal to any positive integer. These large Chern-number topological superfluids provide fertile grounds for exploring exotic quantum matters in the context of ultracold atoms.

Electromagnetic pulse propagation in a quantum metamaterial, an artificial, globally quantum coherent optical medium, is numerically simulated. We show that a one-dimensional quantum metamaterial based on superconducting quantum bits, initialized in an easily reachable factorized excited state, demonstrates lasing in the microwave range, accompanied by the chaotization of qubit states and generation of higher harmonics. These effects may provide a tool for characterization and optimization of quantum metamaterial prototypes.