The isobaric transformation behavior of unannealed (uHDA) and expanded (eHDA) high-density amorphous ice at pressures up to 0.20 GPa is compared using powder x-ray diffraction and dilatometry. eHDA shows high thermal stability and crystallizes to a single ice phase only, whereas uHDA shows much lower thermal stability and always crystallizes to a mixture of ice phases. Unexpectedly, at low temperatures hexagonal ice grows first from uHDA, whereas this phase never crystallizes from eHDA. This leads us to conclude that hidden structural order in the form of nanocrystalline domains is present in uHDA, which triggers growth of hexagonal ice. By contrast, these ordered domains are absent in eHDA, which appears to be a homogeneous material and, thus, could be considered as a candidate for the low-temperature proxy of the proposed high-density liquid phase of water. The present work provides the basis for further experimental studies aiming at investigating this possibility since it establishes that the well-studied uHDA is not the right material to be studied in this context, whereas the more recently discovered eHDA is.

We have succeeded in synthesizing single crystals of the verdazyl radical $\beta$-3-(2,6-dichlorophenyl)-1,5-diphenylverdazyl. The $ab$ $initio$ molecular orbital calculation indicates the formation of an $S=1/2$ Heisenberg antiferromagnetic chain with fourfold magnetic periodicity consisting of three types of exchange interactions. We successfully explain the magnetic and thermodynamic properties based on the expected spin model by using the quantum Monte Carlo method. Furthermore, we reveal that the alternating and unique Ising ferromagnetic chains become effective in the specific field regions and observe a cooperative phenomenon caused by the magnetic order and quantum fluctuations. These results demonstrate that the verdazyl radical could form an unconventional spin model with interesting quantum behavior and provide a way to study a variety of quantum spin systems.

Magnetic lateral multilayers have been fabricated on weak perpendicular magnetic anisotropy amorphous Nd-Co films in order to perform a systematic study on the conditions for controlled nucleation of topological defects within their magnetic stripe domain pattern. A lateral thickness modulation of period $w$ is defined on the nanostructured samples that, in turn, induces a lateral modulation of both magnetic stripe domain periods $\lambda$ and average in-plane magnetization component $M_{\text{in-plane}}$. Depending on lateral multilayer period and in-plane applied field, thin and thick regions switch independently during in-plane magnetization reversal and domain walls are created within the in-plane magnetization configuration coupled to variable angle grain boundaries and disclinations within the magnetic stripe domain patterns. This process is mainly driven by the competition between rotatable anisotropy (that couples the magnetic stripe pattern to in-plane magnetization) and in-plane shape anisotropy induced by the periodic thickness modulation. However, as the structural period $w$ becomes comparable to magnetic stripe period $\lambda$, the nucleation of topological defects at the interfaces between thin and thick regions is hindered by a size effect and stripe domains in the different thickness regions become strongly coupled.

We examine superconductivity in the mesoscopically mixed antiferromagnetic (AF) and superconducting (SC) phases of ternary iron selenides K${}_y$Fe${}_{2-x}$Se${}_2$. It is shown that the interlayer hopping and AF order are key factors to determine $T_c$ of the SC phase. In general, the hopping will produce deformed Fermi surfaces (FSs) that tend to suppress superconductivity. However, contrary to the common expectation, we find that larger AF order actually results in larger SC order, which explains the observed relatively high $T_c$ in these phases. Furthermore, our results indicate that by reducing the interlayer hopping appropriately, phase-separated K${}_y$Fe${}_{2-x}$Se${}_2$ may exhibit its intrinsic SC phase in the two-dimensional limit with a much higher $T_c$ ($\sim 65\text{K}$) than what has been observed.

We have studied the shearing forces in a superconducting vortex system with artificial pinning sites using the Corbino geometry. Current was injected into the center of a Nb disc and propagated radially outward to produce a force in the azimuthal direction on the vortices, with strength proportional to 1/$r$. We investigated the magnetoresistance properties of the vortex lattice as a function of temperature, current, and pinning lattice configuration. The measurements show steps instead of minima at the matching field positions, indicating an unexpected influence of the pinning array on the motion of the vortex lattice. The results imply that these steps are due to a shearing transition of the vortices.

The origins of enhanced piezoelectric coupling along nonpolar crystallographic directions in ferroelectric BaTiO${}_3$ are investigated using in situ neutron spectroscopy. It is observed that an electric field applied away from the equilibrium polarization direction causes a stiffening of the transverse acoustic (TA) phonon branch and consequently increases interaction between the TA and the transverse optic soft mode for a range of wave vectors extending from the Brillouin zone center. This provides a direct lattice dynamics mechanism for enhanced electromechanical coupling, and could act as a guide for designing improved piezoelectric materials.

We studied the doping and temperature ($T$) dependence of the infrared (IR) response of Ba$($Fe${}_{1-x}$Co${}_x{)}_2$As${}_2$ single crystals. We show that a weak band around 1000 cm${}^{-1}$, that was previously interpreted in terms of interaction of the charge carriers with magnetic excitations or of a pseudogap, is rather related to low-energy interband transitions. Specifically, we show that this band exhibits a similar doping and $T$ dependence as the hole pockets seen by angle resolved photoemission spectroscopy (ARPES). Notably, we find that it vanishes as a function of doping near the critical point where superconductivity is suppressed in the overdoped regime. Our IR data thus provide bulk specific information (complementary to the surface sensitive ARPES) for a Lifshitz transition. Our IR data also reveal a second low-energy band around 2300 cm${}^{-1}$ which further emphasizes the necessity to consider the multiband nature of these iron arsenides in the analysis of the optical response.

We present a comprehensive macroscopic thermodynamic study of the quasi-one-dimensional (1D) $s=\frac{1}{2}$ frustrated spin-chain system linarite. Susceptibility, magnetization, specific heat, magnetocaloric effect, magnetostriction, and thermal-expansion measurements were performed to characterize the magnetic phase diagram. In particular, for magnetic fields along the $b$ axis five different magnetic regions have been detected, some of them exhibiting short-range-order effects. The experimental magnetic entropy and magnetization are compared to a theoretical modeling of these quantities using density matrix renormalization group (DMRG) and transfer matrix renormalization group (TMRG) approaches. Within the framework of a purely 1D isotropic model Hamiltonian, only a qualitative agreement between theory and the experimental data can be achieved. Instead, it is demonstrated that a significant symmetric anisotropic exchange of about 10% is necessary to account for the basic experimental observations, including the three-dimensional (3D) saturation field, and which in turn might stabilize a triatic (three-magnon) multipolar phase.

Spin injection properties of ferromagnetic metals are studied and are compared by using highly reproducible cobalt/copper and permalloy/copper lateral spin valves (LSVs) with transparent contacts, fabricated with a careful control of the interface and the purity of copper. Spin polarization of permalloy and cobalt are obtained as a function of temperature. Analysis of the temperature dependence of both the spin polarization and the conductivity of permalloy confirms that the two-channel model for ferromagnetic metals is valid to define the current spin polarization and shows that a correction factor of ∼2 is needed for the values obtained by LSV experiments. The spin transport properties of copper, which also are studied as a function of temperature, are not affected by the used ferromagnetic material. The low-temperature maximum in the spin-diffusion length of copper is attributed to the presence of diluted magnetic impurities intrinsic from the copper.

The magnetic and structural phase diagram of equiatomic FeCo has been studied up to 45 GPa using $K$-edge x-ray magnetic circular dichroism, x-ray absorption near edge spectroscopy, x-ray diffraction, and supporting density-functional-theory-based calculations. FeCo foils with different degrees of chemical order were obtained by magnetron sputtering. Our results show that Fe${}_{0.5}$Co${}_{0.5}$ undergo the bcc ferromagnetic to hcp nonferromagnetic transition in the 30–45 GPa pressure range. Interestingly, the chemical order, i.e., the relative arrangements of Fe and Co atoms, plays a major role in affecting the high-pressure structural and magnetic phase diagram of these alloys. This result is confirmed by first-principles modeling of different structures of equiatomic FeCo alloy. Moreover, the total-energy analysis reveals a strong competition between different magnetic hcp states upon compression. A possible emergence of antiferromagnetism is emphasized and requires further experimental investigation.

We study the variational solution of generic interacting fermionic lattice systems using fermionic Gaussian states and show that the process of gaussification, leading to a nonlinear closed equation of motion for the covariance matrix, is locally optimal in time by relating it to the time-dependent variational principle. By linearizing our nonlinear equation of motion around the ground-state fixed point we describe a method to study low-lying excited states leading to a variational method to determine the dispersion relations of generic interacting fermionic lattice systems. This procedure is applied to study the attractive and repulsive Hubbard model on a two-dimensional lattice, as well as the stability of the Hofstadter butterfly structure in the presence of interactions.

We perform linear and nonlinear photon absorption calculations in ultrathin films of a topological insulator on a substrate. Due to the unique band structure of the coupled topological surface states, novel features are observed for suitable photon frequencies, including a divergent edge singularity in the one-photon absorption process and a significant enhancement in the two-photon absorption process. The resonant frequencies can be controlled by tuning the energy difference and coupling the top and bottom surface states. Such unique linear and nonlinear optical properties make ultrathin films of topological insulators promising material building blocks for tunable high-efficiency nanophotonic devices.

The surface band bending tunes considerably the surface band structures and transport properties in topological insulators. We present a direct measurement of the band bending on the Bi${}_2$Se${}_3$ by using the bulk sensitive angular-resolved hard x-ray photospectroscopy (HAXPES). We tracked the depth dependence of the energy shift of Bi and Se core states. We estimate that the band bending extends up to about 20 nm into the bulk with an amplitude of 0.23–0.26 eV, consistent with profiles previously deduced from the binding energies of surface states in this material.

We study the real-time dynamics of spin chains driven out of thermal equilibrium by an initial temperature gradient $T_L\ne T_R$ using density matrix renormalization group methods. We demonstrate that the nonequilibrium energy current saturates fast to a finite value if the linear-response thermal conductivity is infinite, i.e., if the Drude weight $D$ is nonzero. Our data suggest that a nonintegrable dimerized chain might support such dissipationless transport ($D>0$). We show that the steady-state value $J_E$ of the current for arbitrary $T_L\ne T_R$ is of the functional form $J_E=f\left(T_L\right)-f\left(T_R\right)$, i.e., it is completely determined by the linear conductance. We argue for this functional form, which is essentially a Stefan-Boltzmann law in this integrable model; for the XXX ferromagnet, $f$ can be computed via the thermodynamic Bethe ansatz in good agreement with the numerics. Inhomogeneous systems exhibiting different bulk parameters as well as Luttinger liquid boundary physics induced by single impurities are discussed briefly.

We propose a theory of spin relaxation of electrons and holes in two-dimensional hexagonal crystals such as atomic layers of transition-metal dichalcogenides (MoS${}_2$, WSe${}_2$, etc.). We show that, even in intrinsically defect-free crystals, their flexural deformations are able to generate spin relaxation of carriers. Based on symmetry analysis, we formulate a generic model for spin-lattice coupling between electrons and flexural deformations and use it to determine temperature- and material-dependent spin lifetimes in atomic crystals in ambient conditions.

Introducing a periodic array of holes, i.e., an antidot lattice, in a graphene sheet has been suggested as a route towards the tantalizing objective of “opening the gap” in this otherwise zero-gap semiconductor. Combining density-functional and mean-field Hubbard tight-binding methods, we study the effect of spin polarization on graphene antidot lattices (GALs). Focusing on GALs with extended zigzag edges, we systematically investigate the geometry dependence of spin polarization, electronic structure, and band gaps. A scaling law for the band gap is established, demonstrating marked deviations from that of circular holes without spin polarization. Furthermore, we estimate the robustness of the magnetic ordering against raised temperature and doping and, finally, consider how the optical properties are modified by spin polarization. Our results demonstrate that large, stable band gaps are expected for a range of geometries.

Three-dimensional (3D) topological insulators in general need to be protected by certain kinds of symmetries other than the presumed $U\left(1\right)$ charge conservation. A peculiar exception is the Hopf insulators which are 3D topological insulators characterized by an integer Hopf index. To demonstrate the existence and physical relevance of the Hopf insulators, we construct a class of tight-binding model Hamiltonians which realize all kinds of Hopf insulators with arbitrary integer Hopf index. These Hopf insulator phases have topologically protected surface states and we numerically demonstrate the robustness of these topologically protected states under general random perturbations without any symmetry other than the $U\left(1\right)$ charge conservation that is implicit in all kinds of topological insulators.

We show that the Si${}_x$Ge${}_{1-x}$ random alloys in the honeycomb monolayer structure are nontrivial topological insulators with the same topological classification $Z_2=1$ as pristine silicene and germanene. The application of an external electric field decreases the band gap up to its closure at a critical value of the field, $|E_c|$. For absolute values of the field larger than $|E_c|$ the band gap increases again, and the systems are now trivial insulators. Thus, similarly to what happens for zinc-blend Si${}_x$Ge${}_{1-x}$ alloys, these materials will bring new possibilities in the design of devices, such as band gap and mobility engineering, with the added feature that they will exhibit a quantum spin Hall effect due to the gap generated by the spin-orbit coupling.

Polariton condensates have proved to be model systems to investigate topological defects, as they allow for direct and nondestructive imaging of the condensate complex order parameter. The fundamental topological excitations of such systems are quantized vortices. In specific configurations, further ordering can bring the formation of vortex lattices. In this work we demonstrate the spontaneous formation of ordered vortical states, consisting in geometrically self-arranged vortex-antivortex pairs. A mean-field generalized Gross-Pitaevskii model reproduces and supports the physics of the observed phenomenology.

Hexagonal lattice systems (e.g., triangular, honeycomb, kagome) possess a multidimensional irreducible representation corresponding to $d_{x^2-y^2}$ and $d_{xy}$ symmetry. Consequently, various unconventional phases that combine these $d$-wave representations can occur, and in so doing may break time-reversal and spin-rotation symmetries. We show that hexagonal lattice systems with extended repulsive interactions can exhibit instabilities in the particle-hole channel to phases with either $d_{x^2-y^2}+d_{xy}$ or $d+id$ symmetry. When lattice translational symmetry is preserved, the phase corresponds to nematic order in the spin channel with broken time-reversal symmetry, known as the $\beta$ phase. On the other hand, lattice translation symmetry can be broken, resulting in various $d_{x^2-y^2}+d_{xy}$ density wave orders. In the weak-coupling limit, when the Fermi surface lies close to a van Hove singularity, instabilities of both types are obtained in a controlled fashion.