The static and the dynamic spin correlations in the low-temperature collapsed tetragonal and the high-temperature tetragonal phase in $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$ have been investigated by ${}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements. Through the temperature ($T$) dependence of the nuclear spin lattice relaxation rates (1$/T_1$) and the Knight shifts, although stripe-type antiferromagnetic (AFM) spin correlations are realized in the high-temperature tetragonal phase, no trace of the AFM spin correlations can be found in the nonsuperconducting, low-temperature, collapsed tetragonal (c$\mathcal{T}$) phase. Given that there is no magnetic broadening in ${}^{75}\mathrm{As}$ NMR spectra, together with the $T$-independent behavior of magnetic susceptibility $\chi$ and the $T$ dependence of 1$/T_{1}T\chi$, we conclude that Fe spin correlations are completely quenched statically and dynamically in the nonsuperconducting c$\mathcal{T}$ phase in $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{As}}_2$.

A set of broken-symmetry two-dimensional ground states is predicted in (111)-oriented ${\left(\mathrm{La}\mathrm{Ni}{\mathrm{O}}_3\right)}_N$/${\left(\mathrm{La}\mathrm{Al}{\mathrm{O}}_3\right)}_M$ ($N$/$M$) superlattices, based on density functional theory calculations including a Hubbard $U$ term. An unanticipated Jahn-Teller distortion with $d_{z^2}$ orbital polarization and a ferromagnetic Mott-insulating (and multiferroic) phase emerges in the double perovskite (1/1), that shows strong susceptibility to strain-controlled orbital engineering. The $\mathrm{La}Ni{\mathrm{O}}_3$ bilayer with graphene topology has a switchable multiferroic (ferromagnetic and ferroelectric) insulating ground state with inequivalent Ni sites. Beyond $N=3$ the confined $\mathrm{La}Ni{\mathrm{O}}_3$ slab undergoes a metal-to-insulator transition through a half-semimetallic phase with conduction originating from the interfaces. Antiferromagnetic arrangements allow combining motifs of the bilayer and single trigonal layer band structures in designed artificial mixed phases.

We present a methodology to determine the periodic potential of a surface from angle-resolved photoemission spectroscopy. By mapping the band structure and measuring the energy gap widths at high symmetry points of the surface Brillouin zone, the first Fourier components of the potential can be estimated. This method has been applied in the case of the reconstructed Ag/Cu(111) surface where controlled potassium atom doping has been used to precisely tune the band gap energies. This potential determination allows one to calculate the local density of states which is found to be in very good agreement with scanning tunneling spectroscopy data.

We report on a contribution to the microwave response of a two-dimensional electron system in a magnetic field which originates from excitation of virtual Bernstein modes. These collective modes emerge as a result of interaction between the usual magnetoplasmon mode and cyclotron resonance harmonics. The electrons are found to experience a strongly enhanced radiation field when its frequency falls in a gap of the Bernstein modes spectrum. This field can give rise to nonlinear effects, one of which, the parametric cyclotron resonance, is discussed. We argue that this resonance leads to a plasma instability in the ultraclean system. The instability-induced heating is responsible for the giant photoresistivity spike recently observed in the vicinity of the second cyclotron resonance harmonic.

We perform first-principles calculations to study the stability and electronic structure of the (111) surface of SnTe, a representative topological crystalline insulator (TCI). We find three stable surface phases, which support two qualitatively different types of topological surface states: type I with four Dirac points at $\overline{\Gamma }$ and three $\overline{\mathrm{M}}$ points and type II with two Dirac points nearby but not at $\overline{\Gamma }$. Their appearance can be controlled by varying growth conditions. Under an Sn-poor condition, the Te-terminated surface without reconstruction is stable, resulting in the type-I surface states. While under an Sn-rich condition, the (2$\times$1)-reconstructed Sn-terminated surface becomes more stable. The reconstruction folds the surface Brillouin zone and effectively induces interactions between the Dirac points at the $\overline{\Gamma }$ and $\overline{\mathrm{M}}$ points. Surface states thus change from type I to type II accompanied by a Lifshitz transition. Under intermediate growth conditions, the ($\sqrt{3}\times \sqrt{3}$)-reconstructed Sn-terminated surface gets stabilized, which recovers the type-I surface states. Our work suggests a promising alternative way to control the topological surface states of TCIs besides selecting different surface orientations.

We present an analytical investigation of the optical absorption spectrum of monolayer molybdenum-disulfide. Based on the density matrix formalism, our approach gives insights into the microscopic origin of excitonic transitions, their relative oscillator strength, and binding energy. We show analytical expressions for the carrier-light coupling element, which contains the optical selection rules and describes well the valley-selective polarization in ${\mathrm{MoS}}_2$. In agreement with experimental results, we find the formation of strongly bound electron-hole pairs due to the efficient Coulomb interaction. The absorption spectrum of MoS${}_2$ features two pronounced peaks corresponding to the A and B exciton. For MoS${}_2$ on a SiO${}_2$ substrate, these are characterized by binding energies of 455 meV and 465 meV, respectively. Our calculations reveal their relative oscillator strength and predict the appearance of further low-intensity excitonic transitions at higher energies. The presented approach is applicable to other transition metal dichalcogenides and can be extended to investigations of trion and biexcitonic effects.

We present theoretical foundations and numerical demonstration of an efficient method for performing time-dependent many-electron simulations for electronic transport. The method employs the concept of stroboscopic wave-packet basis for the description of electrons' dynamics in the semi-infinite leads. The rest of the system can be treated using common propagation schemes for finite electronic systems. We use the implementation of our method to study the time-dependent current response in armchair graphene nanoribbons (AGNRs) with sizes up to 800 atoms described within tight-binding approximation. The character of the time-dependent current is studied for different magnitudes of the bias voltage, variable width and length of AGNRs, different positions of the current measurement, and for full and reduced coupling of the AGNRs to the electrodes.

We examine the stability of classical states with a generic incommensurate spiral order against quantum fluctuations. Specifically, we focus on the frustrated spin-1/2 $XY$ and Heisenberg models on the honeycomb lattice with nearest-neighbor $J_1$ and next-nearest-neighbor $J_2$ antiferromagnetic couplings. Our variational approach is based on the Jastrow wave functions, which include quantum correlations on top of classical spin waves. We perform a systematic optimization of wave vectors and Jastrow pseudopotentials within this class of variational states and find that quantum fluctuations favor collinear states over generic coplanar spirals. The Néel state with $\mathbf{Q}=(0,0)$ extends its stability well beyond the classical value $J_2/J_1=1/6$. Most importantly, the collinear states with $\mathbf{Q}=(0,2\pi /\sqrt{3})$ (and the two symmetry-related states) are found to be stable in a large regime with intermediate frustration, while at the classical level they are limited to the point $J_2/J_1=0.5$. For large frustration, the ${120}^{\circ }$ state is stabilized for finite values of $J_2/J_1$ in both models.

We have investigated NbN-DyN-NbN junctions with 1 to 10 nm thick DyN barriers. A crossover from diffusive (hopping) to tunneling-type transport was found in these junctions as the DyN thickness is reduced below $\sim$4 nm. We have also made a detailed study of magnetic and electrical properties of thicker DyN thin films deposited under similar conditions; DyN films were found to be ferromagnetic with $T_{\text{Curie}}\sim 35\pm 5$ K. Electrical transport of the junctions with $\sim$10 nm DyN was understood in terms of Shklovskii-Efros (SE)-type variable range hopping (VRH) at low temperature between 90 and 35 K. We estimated localization length $\xi =5.6$ nm in this temperature range. Temperature dependence of resistance was found to deviate from SE-VRH below 35 K along with large suppression of resistance with magnetic field. This is correlated with onset of magnetism below 35 K. Large butterfly-shaped MR up to $\sim$40$\%$ was found for the $\sim$10 nm thick DyN junction at 2 K. In the tunneling regime, barrier height of the tunnel junction was estimated $\sim$50 meV from the Simmons model. Signatures of spin filtering was found in temperature dependence of resistance in tunnel junction with $\sim$3 nm thick DyN. Cooper pair tunneling in these junctions below $T_C$ ($\sim$10.8 K) of NbN was understood according to S-I-S tunneling current model. We found coherent tunneling of Cooper pairs through a $\sim$1 nm thick DyN tunnel barrier with critical current $I_C\sim 12$ $\mu$A. The critical current also showed modulation with magnetic field.

We used Raman and terahertz spectroscopies to investigate lattice and magnetic excitations and their cross coupling in the hexagonal YMnO${}_3$ multiferroic. Two phonon modes are strongly affected by the magnetic order. Magnon excitations have been identified thanks to comparison with neutron measurements and spin-wave calculations but no electromagnon has been observed. In addition, we evidenced two additional Raman-active peaks. We have compared this observation with the anticrossing between magnon and acoustic-phonon branches measured by neutron. These optical measurements underlie the unusual strong spin-phonon coupling.

The stoichiometric 114-layered material YBaCo${}_4$O${}_7$ exhibits long-range antiferromagnetic order below a Néel temperature of 106 K. Nonstoichiometric YBaCo${}_4$O${}_{7.1}$, which contains a relatively small amount (1.4%) of interstitial oxygen, has recently been shown to have drastically modified magnetic properties compared to the parent compound. The present experiments have used magnetization, ac susceptibility, and zero applied field NMR to study the spin configuration and spin dynamics in a single crystal of YBaCo${}_4$O${}_{7.1}$ as a function of temperature below 100 K. Evidence has been obtained for a magnetic transition at 80 K corresponding to some form of spin freeze-out. Based on previous results for the stoichiometric material, it is likely that the freezing process involves spins in the triangular layers in this frustrated antiferromagnet. At lower temperatures, dynamic effects persist and below 50 K a fraction of the spins, located primarily in the kagome layers, constitute what may be termed a viscous spin liquid component. For $T$ < 10 K, a disordered or glasslike spin structure, with a large distribution of spin correlation times, emerges as the low-temperature state of the spin system.

We study the plaquette valence-bond solid phase of the spin-$1/2$$J_1$-$J_2$ antiferromagnet Heisenberg model on the square lattice within the bond-operator theory. We start by considering four $S=1/2$ spins on a single plaquette and determine the bond operator representation for the spin operators in terms of singlet, triplet, and quintet boson operators. The formalism is then applied to the $J_1$-$J_2$ model and an effective interacting boson model in terms of singlets and triplets is derived. The effective model is analyzed within the harmonic approximation and the previous results of Zhitomirsky and Ueda [Phys. Rev. B 54, 9007 (1996)] are recovered. By perturbatively including cubic (triplet-triplet-triplet and singlet-triplet-triplet) and quartic interactions, we find that the plaquette valence-bond solid phase is stable within the parameter region $0.34<J_2/J_1<0.59$, which is narrower than the harmonic one. Differently from the harmonic approximation, the excitation gap vanishes at both critical couplings $J_2=0.34J_1$ and $J_2=0.59J_1$. Interestingly, for $J_2<0.48J_1$, the excitation gap corresponds to a singlet-triplet excitation at the $\Gamma$ point while, for $J_2>0.48J_1$, it is related to a singlet-singlet excitation at the $\mathbf{X}=(\pi /2,0)$ point of the tetramerized Brillouin zone.

Every singlet state of a quantum spin-$1/2$ system can be decomposed into a linear combination of valence bond basis states. The range of valence bonds within this linear combination as well as the correlations between them can reveal the nature of the singlet state and are key ingredients in variational calculations. In this work, we study the bipartite valence bond distributions and their correlations within the ground state of the Heisenberg antiferromagnet on bipartite lattices. In terms of field theory, this problem can be mapped to correlation functions near a boundary. In dimension $d\ge 2$, a nonlinear $\sigma$ model analysis reveals that at long distances the probability distribution $P\left(\mathbf{r}\right)$ of valence bond lengths decays as ${\left|r\right|}^{-d-1}$ and that valence bonds are uncorrelated. By a bosonization analysis, we also obtain $P\left(\mathbf{r}\right)\propto {\left|r\right|}^{-d-1}$ in $d=1$ despite the different mechanism. On the other hand, we find that correlations between valence bonds are important even at large distances in $d=1$, in stark contrast to $d\ge 2$. The analytical results are confirmed by high-precision quantum Monte Carlo simulations in $d=1$, 2, and 3. We develop a single-projection loop variant of the valence bond projection algorithm, which is well designed to compute valence bond probabilities and for which we provide algorithmic details.

We have studied the influence of an externally applied magnetic field on the photon and dark count rates of meander-type niobium nitride superconducting nanowire single-photon detectors. Measurements have been performed at a temperature of 4.2 K, and magnetic fields up to 250 mT have been applied perpendicularly to the meander plane. While photon count rates are field independent at weak applied fields, they show a strong dependence at fields starting from approximately ±25 mT. This behavior, as well as the magnetic field dependence of the dark count rates, is in good agreement with the recent theoretical model of vortex-assisted photon detection and spontaneous vortex crossing in narrow superconducting lines. However, the local reduction of the superconducting free energy due to photon absorption, which is the fitting parameter in the model, increases much slower with the photon energy than the model predicts. Furthermore, changes in the free-energy during photon counts and dark counts depend differently on the current that flows through the meander. This indicates that photon counts and dark counts occur in different parts of the meander.

We study the magnetic response of a superconducting double strip, i.e., two parallel coplanar thin strips of width $2w$, thickness $d\ll w$, and infinite length, separated by a gap of width $2s$ and subject to a perpendicular magnetic field $H$. The magnetic properties of this system are governed by the presence of a geometric energy barrier for vortex penetration which we investigate as a function of applied field $H$ and gap parameter $s$. The new results deal with the case of a narrow gap $s\ll w$, where the field penetration from the inner edges is facilitated by large flux focusing. Upon reducing the gap width $2s$, we observe a considerable rearrangement of the screening currents, leading to a strong reduction of the penetration field and the overall magnetization loop, with a suppression factor reaching $\sim$${(d/w)}^{1/2}$ as the gap drops below the sample thickness, $2s<d$. We compare our results with similar systems of different shapes (elliptic and rectangular platelet) and include effects of surface barriers as well. Furthermore, we verify that corrections arising from the magnetic response of the Shubnikov phase in the penetrated state are small and can be omitted. Extending the analysis to multiple strips, we determine the specific sequence of flux penetrations into the different strips. Our studies are relevant for the understanding of platelet-shaped samples with cracks or the penetration into layered superconductors at oblique magnetic fields.

We theoretically demonstrate that a type of robust two-particle bound state embedded in the continuum (BIC), which we call Floquet-Hubbard (FH) BIC, can be induced in a homogeneous (i.e., defect-free) Hubbard semilattice by an intense oscillating electric field. While single-particle BIC states are fragile states that exist solely for specially tailored potentials, FH BIC states are found in a wide range of parameter space, do not require fulfillment of resonance conditions, and are thresholdless. Analytical results are derived in the high-frequency limit of field modulation by a multiple-time-scale asymptotic analysis of the ac-driven Hubbard Hamiltonian in the two-particle sector. A FH BIC mode basically corresponds to a molecular state, in which the two particles bind together, undergoing correlated tunneling on the lattice. Localization of the molecular state is induced by the external oscillating field, which effectively attracts the molecular state at the edge of the semi-infinite lattice. Our results can pave the way for the study and interpretation of strong-field phenomena in correlated-particles physics.

One-dimensional polariton condensates (PoCos) in a photonic wire are generated through nonresonant laser excitation, by which also a reservoir of background carriers is created. Interaction with this reservoir may affect the coherence of the PoCo, which is studied here by injecting a condensate locally and monitoring the coherence along the wire. While the incoherent reservoir is mostly present within the excitation laser spot, the condensate can propagate ballistically through the wire. Photon correlation measurements show that far from the laser spot the second-order correlation function approaches unity value, as expected for the coherent condensed state. When approaching the spot, however, the correlation function increases up to values of 1.2 showing the addition of noise to the emission due to interaction with the reservoir. This finding is substantiated by measuring the first-order coherence by a double-slit experiment, which shows a reduced visibility of interference at the excitation laser spot.