In magnetic materials, the exchange is the strongest quantum interaction due to the Pauli exclusion principle. For that reason it can induce high-frequency modes $f_{\mathrm{exch}}$ of the magnetization precession. In this work we investigate these modes over a wide range of temperatures ($50–300\text{K}$) and magnetic fields up to $10T$ in a bismuth-doped garnet with perpendicular magnetic anisotropy by performing femtosecond magneto-optical pump-probe experiments. Near the compensation temperature $T_M$ the divergence of $1/f_{\mathrm{exch}}\left(T\right)$ allows identifying unambiguously $f_{\mathrm{exch}}$ with the rare-earth $\leftrightarrow$ iron exchange mode. In addition, at low temperature $f_{\mathrm{exch}}$ is independent of the field as usually observed. In contrast, we find that near $T_M,f_{\mathrm{exch}}$ decreases linearly with an increasing magnetic field. This behavior is explained in the context of the ferromagnetic resonance theory by including the perturbation term linear in the external applied field $H_{\mathrm{ext}}$.

Magnetoelectric multiferroics hold great promise for electrical control of magnetism or magnetic control of ferroelectricity. However, single phase ferroelectric materials with a sizeable ferromagnetic magnetization are rare. Here, we demonstrate that a single-phase orthorhombic $\mathrm{LuMn}{\mathrm{O}}_3$ thin film features coexisting magnetic and ferroelectric orders. The temperature dependence of the different order parameters are presented with ferromagnetic order appearing below 100 K and thus at much higher temperatures than ferroelectricity or antiferromagnetism $(T_{\mathrm{N}},T_{\mathrm{FE}}\le 40\mathrm{K})$.

We report synthesis and detailed characterization of the noncentrosymmetric superconductor ${\mathrm{Re}}_6\mathrm{Hf}$ using powder x-ray diffraction (XRD), magnetization, transport, and thermodynamic measurements. XRD confirmed the noncentrosymmetric, $\alpha$-Mn cubic structure in ${\mathrm{Re}}_6\mathrm{Hf}$ with the cubic cell parameter $a=9.6850\left(3\right)$ Å. Resistivity, DC, and AC magnetization measurements confirmed the type-II superconductivity in ${\mathrm{Re}}_6\mathrm{Hf}$ with the transition temperature $T_c^{\mathrm{onset}}\sim 5.96$ K, having the lower critical field $H_{c1}\left(0\right)5.6$ mT and upper critical field $H_{c2}\left(0\right)12.2$ T. The electronic specific heat data fits well with the single-gap BCS model. The Sommerfeld coefficient $\left(\gamma \right)$ also shows linear relation with the magnetic field. All above results suggest $s$-wave superconductivity in ${\mathrm{Re}}_6\mathrm{Hf}$.

Quantum dynamics of magnetic order in a chiral multiferroic chain is studied. We consider two different scenarios: ultrashort terahertz excitations or a sudden electric field quench. Performing analytical and numerical exact diagonalization calculations, we trace the pulse induced spin dynamics and extract quantities that are relevant to quantum information processing. In particular, we analyze the dynamics of the system chirality, the von Neumann entropy, and the pairwise and many-body entanglement. If the characteristic frequencies of the generated states are noncommensurate, then a partial loss of pair concurrence occurs. Increasing the system size, this effect becomes even more pronounced. Many-particle entanglement and chirality are robust and persist in the incommensurate phase. To analyze the dynamical quantum transitions for the quenched and pulsed dynamics we combined the Weierstrass factorization technique for entire functions and the Lanczos exact diagonalization method. For a small system we obtained analytical results including the rate function of the Loschmidt echo. Exact numerical calculations for a system up to 40 spins confirm phase transition. Quench-induced dynamical transitions have been extensively studied recently. Here we show that related dynamical transitions can be achieved and controlled by appropriate electric field pulses.

We study the crossed Andreev effects in two-dimensional conductor/superconductor hybrid systems under a perpendicular magnetic field. Both a graphene/superconductor hybrid system and an electron gas/superconductor one are considered. It is shown that an exclusive crossed Andreev reflection, with other Andreev reflections being completely suppressed, is obtained in a high magnetic field because of the chiral edge states in the quantum Hall regime. Importantly, the exclusive crossed Andreev reflection not only holds for a wide range of system parameters, e.g., the size of system, the width of central superconductor, and the quality of coupling between the graphene and the superconductor, but also is very robust against disorder. When the applied bias is within the superconductor gap, a robust Cooper-pair splitting process with high-efficiency can be realized in this system.

Quantum Monte Carlo simulations of fermions are hampered by the notorious sign problem whose most striking manifestation is an exponential growth of sampling errors with the number of particles. With the sign problem known to be an NP-hard problem and any generic solution thus highly elusive, the Monte Carlo sampling of interacting many-fermion systems is commonly thought to be restricted to a small class of model systems for which a sign-free basis has been identified. Here we demonstrate that entanglement measures, in particular the so-called Rényi entropies, can intrinsically exhibit a certain robustness against the sign problem in auxiliary-field quantum Monte Carlo approaches and possibly allow for the identification of global ground-state properties via their scaling behavior even in the presence of a strong sign problem. We corroborate these findings via numerical simulations of fermionic quantum phase transitions of spinless fermions on the honeycomb lattice at and below half filling.

We study the effects of electron-electron interaction in a Floquet topological superconducting chain using the exact diagonalization and the dynamics simulation of quenching process. It is found that a weak interaction can lead to a multitude of tiny avoided crossings in quasienergy spectra, indicating the emergence of chaos, and that the Floquet topological superconducting state can be robust against the weak chaos. With increasing the interaction strength, there is a crossover from weak to strong chaos. On the other hand, the survival probability of Majorana edge modes vanishes and so the Floquet system is transformed into a topologically trivial phase.

The Coulombic quantum spin liquid in quantum spin ice is an exotic quantum phase of matter that emerges on the pyrochlore lattice and is currently actively searched for. Motivated by recent experiments on the Yb-based breathing pyrochlore material ${\mathrm{Ba}}_3{\mathrm{Yb}}_2{\mathrm{Zn}}_5{\mathrm{O}}_{11}$, we theoretically study the phase diagram and magnetic properties of the relevant spin model. The latter takes the form of a quantum spin ice Hamiltonian on a breathing pyrochlore lattice, and we analyze the stability of the quantum spin liquid phase in the absence of the inversion symmetry which the lattice breaks explicitly at lattice sites. Using a gauge mean-field approach, we show that the quantum spin liquid occupies a finite region in parameter space. Moreover, there exists a direct quantum phase transition between the quantum spin liquid phase and featureless paramagnets, even though none of theses phases break any symmetry. At nonzero temperature, we show that breathing pyrochlores provide a much broader finite-temperature spin liquid regime than their regular counterparts. We discuss the implications of the results for current experiments and make predictions for future experiments on breathing pyrochlores.

In previous studies, we have shown that gold deposited on a monolayer (ML) of graphene on SiC(0001) is intercalated below the ML after an annealing procedure and affects the band structure of graphene. Here we prove experimentally and theoretically that some of the gold forms a dispersed phase composed of single adatoms, being intercalated between the ML and the buffer layer and in a hollow position with respect to C atoms of the ML on top. They are freestanding and negatively charged, due to the partial screening of the electron transfer between SiC and the ML, without changing the intrinsic n-type doping of the ML. As these single atoms decouple the ML from the buffer layer, the quasiparticles of graphene are less perturbed, thus increasing their Fermi velocity. Moreover, the hollow position of the intercalated single Au atoms might lead to spin-orbit coupling in the graphene layer covering IC domains. This effect of spin-orbit coupling has been recently observed experimentally in Au-intercalated graphene on SiC(0001) [D. Marchenko, A. Varykhalov, J. Sánchez-Barriga, Th. Seyller, and O. Rader, Appl. Phys. Lett. 108, 172405 (2016)] and has been theoretically predicted for heavy atoms, like thallium, in a hollow position on graphene [C. Weeks, J. Hu, J. Alicea, M. Franz, and R. Wu, Phys. Rev. X 1, 021001 (2011); A. Cresti, D. V. Tuan, D. Soriano, A. W. Cummings, and S. Roche, Phys. Rev. Lett. 113, 246603 (2014)].

We investigate the influences of exchange field and perpendicular electric field on the subgap transport in silicene-based ferromagnetic/superconducting (FS) and ferromagnetic/superconducting/ferromagnetic (FSF) junctions. Owing to the unique buckling structure of silicene, the Andreev reflection and subgap conductance can be effectively modulated by a perpendicular electric field. It is revealed that the subgap conductance in the FS junction can be distinctly enhanced by an exchange field. Remarkably, resorting to the tunable band gap of silicene, an exclusive crossed Andreev reflection (CAR) process in the FSF junction can be realized within a wide range of related parameters. Moreover, in the FSF junction the exclusive CAR and exclusive elastic cotunneling processes can be switched by reversing the magnetization direction in one of the ferromagnetic regions.

Indirect transitions of electrons in graphene and graphite are investigated by means of angle-resolved photoemission spectroscopy (ARPES) with several different incident photon energies and light polarizations. The theoretical calculations of the indirect transition for graphene and for a single crystal of graphite are compared with the experimental measurements for highly-oriented pyrolytic graphite and a single crystal of graphite. The dispersion relations for the transverse optical (TO) and the out-of-plane longitudinal acoustic (ZA) phonon modes of graphite and the TO phonon mode of graphene can be extracted from the inelastic ARPES intensity. We find that the TO phonon mode for $\mathbf{k}$ points along the $\mathrm{\Gamma }\text{−}K$ and $K\text{−}M\text{−}{K}^{\prime }$ directions in the Brillouin zone can be observed in the ARPES spectra of graphite and graphene by using a photon energy $\approx 11.1$ eV. The relevant mechanism in the ARPES process for this case is the resonant indirect transition. On the other hand, the ZA phonon mode of graphite can be observed by using a photon energy $\approx 6.3$ eV through a nonresonant indirect transition, while the ZA phonon mode of graphene within the same mechanism should not be observed.

We examine the excitonic nature of the $E_{33}$ optical transition of the individual free-standing index-identified $(23,7)$ single-walled carbon nanotube by means of the measurements of its radial-breathing-mode and $G$-mode Raman excitation profiles. We confirm that it is impossible to determine unambiguously the nature of its $E_{33}$ optical transition (excitonic vs band to band) based only on the excitation profiles. Nevertheless, by combining Raman scattering, Rayleigh scattering, and optical absorption measurements on strictly the same individual $(23,7)$ single-walled carbon nanotube, we show that the absorption, Rayleigh spectra, and Raman excitation profiles of the longitudinal and transverse $G$ modes are best fitted by considering the nature of the $E_{33}$ transition as excitonic. The fit of the three sets of data gives close values of the transition energy $E_{33}$ and damping parameter ${\mathrm{\Gamma }}_{33}$. This comparison shows that the fit of the Raman excitation profiles provides with good accuracy the energy and damping parameter of the excitonic optical transitions in single-walled carbon nanotubes.

A cumulant-based $GW$ approximation for the retarded one-particle Green's function is proposed, motivated by an exact relation between the improper Dyson self-energy and the cumulant generating function. Qualitative aspects of this method are explored within a simple one-electron independent phonon model, where it is seen that the method preserves the energy moment of the spectral weight while also reproducing the exact Green's function in the weak-coupling limit. For the three-dimensional electron gas, this method predicts multiple satellites at the bottom of the band, albeit with inaccurate peak spacing. However, its quasiparticle properties and correlation energies are more accurate than both previous cumulant methods and standard $G_0{W}_0$. Our results point to features that may be exploited within the framework of cumulant-based methods and suggest promising directions for future exploration and improvements of cumulant-based $GW$ approaches.

We show that the spin-lattice relaxation in $n$-type insulating GaAs is dramatically accelerated at low magnetic fields. The origin of this effect, which cannot be explained in terms of well-known diffusion-limited hyperfine relaxation, is found in the quadrupole relaxation, induced by fluctuating donor charges. Therefore, quadrupole relaxation, which governs low field nuclear spin relaxation in semiconductor quantum dots, but was so far supposed to be harmless to bulk nuclei spins in the absence of optical pumping, can be studied and harnessed in the much simpler model environment of $n$-GaAs bulk crystal.

The spin-$\frac{1}{2}$ XXZ chain with easy-plane anisotropy in a transverse field describes well the thermodynamic properties of the material ${\mathrm{Cs}}_2{\mathrm{CoCl}}_4$ in a wide range of temperatures and fields including the region close to the spin-flop Ising quantum phase transition. For a comparison with prospective inelastic neutron scattering experiments on this compound, we present results of an extensive numerical study of its dynamic structure factor ${\mathcal{S}}^{\alpha \beta }(k,\omega )$ using matrix-product-state (MPS) techniques. Close to criticality, the dynamic part of the correlator ${\mathcal{S}}^{xx}$ longitudinal to the applied field is incoherent and possesses a small total weight as the ground state is already close to saturation. The transverse correlator ${\mathcal{S}}^{zz}$, on the other hand, is dominated by a coherent single-particle excitation with additional spectral weight at higher energies that we tentatively attribute to a repulsively bound pair of particles. With increasing temperature, the latter quickly fades and spectral weight instead accumulates close to zero wave vector just above the single-particle energy. On a technical level, we compare the numerical efficiency of real-time evolution to an MPS-based Chebyshev expansion in the present context, finding that both methods yield results of similar quality at comparable numerical costs.

In this work we examine critically the electronic structure of the rare-earth elements by use of the so-called Hubbard I approximation. From the theoretical side all measured features of both occupied and unoccupied states are reproduced, without significant deviations between observations and theory. We also examine cohesive properties like the equilibrium volume and bulk modulus, where we find, in general, a good agreement between theory and measurements. In addition, we have reproduced the spin and orbital moments of these elements as they are reflected from measurements of the saturation moment. We have also employed the Hubbard I approximation to extract the interatomic exchange parameters of an effective spin Hamiltonian for the heavy rare earths. We show that the Hubbard I approximation gives results which are consistent with calculations where $4f$ electrons are treated as core states for Gd. The latter approach was also used to address the series of the heavy/late rare earths. Via Monte Carlo simulations we obtained ordering temperatures which reproduce measurements within about $20\%$. We have further illustrated the accuracy of these exchange parameters by comparing measured and calculated magnetic configurations for the heavy rare earths and the magnon dispersion for Gd. The Hubbard I approximation is compared to other theories of the electronic structure, and we argue that it is superior. We discuss the relevance of our results in general and how this makes it possible to treat the electronic structure of materials containing rare-earth elements, such as permanent magnets, magnetostrictive compounds, photovoltaics, optical fibers, topological insulators, and molecular magnets.

Commuting transfer matrices for linear chains of interacting non-Abelian anyons from the two-dimensional irreducible representation of the dihedral group $D_3$ [or, equivalently, the integer sector of the $su{\left(2\right)}_4$ spin-1 chain] are constructed using the spin-anyon correspondence to a $D_3$-symmetric formulation of the XXZ Heisenberg spin chain. The spectral problem is solved using discrete inversion identities satisfied by these transfer matrices and functional Bethe ansatz methods. The resulting spectrum can be related to that of the XXZ spin-1/2 Heisenberg chain with boundary conditions depending on the topological sector of the anyon chain. The properties of this model in the critical regime are studied by finite size analysis of the spectrum. In particular, points in the phase diagram where the anyon chain realizes some of the rational ${\mathbb{Z}}_2$ orbifold theories are identified.