Using a combination of photoemission and x-ray magnetic circular dichroism (XMCD), we characterize the growth and the electronic as well as magnetic structure of cobalt layers intercalated in between graphene and Ir(111). We demonstrate that magnetic ordering exists beyond one monolayer intercalation, and determine the Co orbital and spin magnetic moments. XMCD from the carbon edge shows an induced magnetic moment in the graphene layer, oriented antiparallel to that of cobalt. The XMCD experimental data are discussed in comparison to our results of first-principles electronic structure calculations. It is shown that good agreement between theory and experiment for the Co magnetic moments can be achieved when the local-spin-density approximation plus the Hubbard $U$ ($\text{LSDA}+U$) is used.

We determined the density fluctuations spectrum $\left[S\right(Q,\omega \left)\right]$ of vitreous and polycrystalline ${\text{GeO}}_2$ by inelastic x-ray scattering in the wave vector ($Q$) range between 3 and $13{\mathrm{nm}}^{-1}$ at pressures ($P$) between ambient and 7.1 GPa. The polycrystalline sample displays the expected $P$ evolution of the $Q$-dispersion relation of longitudinal acoustic (LA) modes. On the contrary, the dispersion relation of the glassy sample for $P\ge 1$ GPa shows an excess energy of the LA modes, while at ambient pressure follows the trend expected from ultrasonic and light scattering data. These findings point towards a pressure-induced positive sound dispersion, with an associated high-frequency sound velocity comparable to that of the polycrystalline sample at the same $P$.

We show an explicit mechanism of large superspin formation in double perovskites ${\mathrm{PbFe}}_{1/2}{M}_{1/2}{\mathrm{O}}_3$ ($M=\text{Nb}$, Ta, and Sb). First, we demonstrate that antiferromagnetically interacting ${\mathrm{Fe}}^{3+} \left(S=5/2\right)$ ions may be arranged in a periodic superstructure that exhibits a long-range ferrimagnetic ordering below $T_{\mathrm{fe}}\approx 5.6J_1$ ($J_1/k_B\sim 50$ K), which is close to room temperature. The ferrimagnetism of this superstructure is due to its geometry (so-called Lieb-Mattis ferrimagnetism), in contrast to the “usual” ferrimagnetism, which is due to the coupling of ions with different spin values. Next, we show that small clusters of the same structure exhibit a superparamagnetic behavior at $T\lesssim T_{\mathrm{fe}}$. The possibility of formation of such clusters explains the room-temperature (superpara)magnetism in $3d$-metal based oxides.

${\mathrm{CoAl}}_2{\mathrm{O}}_4$ spinel with magnetic ${\mathrm{Co}}^{2+}$ ions on the diamond lattice is known to be magnetically frustrated. We compare neutron single-crystal diffraction patterns measured in zero and applied magnetic fields with the ones obtained from classical Monte Carlo models. In simulations we test the influence of various parameters on diffraction patterns: the ratio of nearest-, $J_1$, and next-nearest-, $J_2$, neighbor interactions, magnetic field applied along the principal crystallographic directions, and random disorder on the $A\left({\mathrm{Co}}^{2+}\right)$ and $B\left({\mathrm{Al}}^{3+}\right)$ sites. We conclude that the models considered so far explain the broadening of magnetic Bragg peaks in zero magnetic field and their anisotropic response to applied magnetic field only partly. As bulk properties of our single crystal are isotropic, we suggest that its microstructure, specifically $\langle 111\rangle$-twin boundaries, could be a reason for the nonconventional magnetic order in ${\mathrm{CoAl}}_2{\mathrm{O}}_4$.

The intermetallic compound ${\mathrm{Nd}}_7{\mathrm{Rh}}_3$, which shows two magnetic transitions, one at 32 K and another around 16 K, has been known to exhibit a magnetic-field induced first-order magnetic phase transition (FOPT) at low temperatures. Using neutron diffraction we tracked the evolution of the magnetic features as a function of temperature and external magnetic field across the two transition temperatures. We provide evidence for the existence of both antiferromagnetism and ferromagnetism below 20 K. Notably the results reveal concrete evidence for the partial persistence of the high-field magnetic state at 2 K after cycling through the magnetic-field-induced magnetic transition, thereby offering microscopic evidence for magnetic coexistence phenomenon in this intermetallic compound.

It is now well established, both theoretically and experimentally, that very small changes in the size of isolated nanograins lead to substantial nonmonotonic variations, and sometimes enhancement, of the mean-field spectroscopic gap of conventional superconductors. A natural question to ask, of broad relevance for the theory and applications of superconductivity, is whether these size effects can also enhance the critical temperature of a bulk granular material composed of such nanograins. Here we answer this question affirmatively. We combine mean-field, semiclassical, and percolation techniques to show that engineered nanoscale granularity in conventional superconductors can enhance the critical temperature by up to a few times compared to the nongranular bulk limit. This prediction is valid for three-dimensional and also quasi-two-dimensional samples, provided the thickness is much larger than the grain size. Our model takes into account an experimentally realistic distribution of grain sizes in the array, charging effects, tunneling by quasiparticles, and limitations related to the proliferation of thermal fluctuations for sufficiently small grains.

It has been predicted theoretically that an unconventional odd-frequency spin-triplet component of a superconducting order parameter can be induced in multilayered ferromagnetic structures with noncollinear magnetization. In this work, we study experimentally nanoscale devices, in which a ferromagnetic spin valve is embedded into a Josephson junction. We demonstrate two ways of in situ analysis of such Josephson spin valves: via magnetoresistance measurements and via in situ magnetometry based on flux quantization in the junction. We observe that supercurrent through the device depends on the relative orientation of magnetizations of the two ferromagnetic layers and is enhanced in the noncollinear state of the spin valve. We attribute this phenomenon to controllable generation of the spin-triplet superconducting component in a ferromagnet.

We perform a detailed analysis of how an amplified interferometer can be used to enhance the quality of a dispersive qubit measurement, such as one performed on a superconducting transmon qubit, using homodyne detection on an amplified microwave signal. Our modeling makes a realistic assessment of what is possible in current circuit-QED experiments; in particular, we take into account the frequency dependence of the qubit-induced phase shift for short microwaves pulses. We compare the possible signal-to-noise ratios obtainable with (single-mode) SU(1,1) interferometers with the current coherent measurement and find a considerable reduction in measurement error probability in an experimentally accessible range of parameters.

We investigate the near-field heat transfer between a semi-infinite medium and a nanoparticle made of composite materials. We show that in the effective medium approximation, the heat transfer can be greatly enhanced by considering composite media, being maximal at the percolation transition. Specifically, for titanium inclusions embedded in a polystyrene sphere, this enhancement can be up to thirty times larger than in the case of the corresponding homogeneous titanium sphere. We demonstrate that our findings are robust against material losses, to changes in the shape of inclusions and materials, and apply for different effective medium theories. These results suggest the use of composite media as a new, versatile material platform to enhance, optimize, and tailor near-field heat transfer in nanostructures.

The structure, phase stability, and mechanical properties of ternary alloys of the Zr-Ta-N system are investigated by combining thin-film growth and ab initio calculations. ${\mathrm{Zr}}_{1-x}{\mathrm{Ta}}_x\mathrm{N}$ films with $0\le x\le 1$ were deposited by reactive magnetron cosputtering in $\mathrm{Ar}+{\mathrm{N}}_2$ plasma discharge and their structural properties characterized by x-ray diffraction. We considered both ordered and disordered alloys, using supercells and special quasirandom structure approaches, to account for different possible metal atom distributions on the cation sublattice. Density functional theory within the generalized gradient approximation was employed to calculate the electronic structure as well as predict the evolution of the lattice parameter and key mechanical properties, including single-crystal elastic constants and polycrystalline elastic moduli, of ternary ${\mathrm{Zr}}_{1-x}{\mathrm{Ta}}_x\mathrm{N}$ compounds with cubic rocksalt structure. These calculated values are compared with experimental data from thin-film measurements using Brillouin light scattering and nanoindentation tests. We also study the validity of Vegard's empirical rule and the effect of growth-dependent stresses on the lattice parameter. The thermal stability of these ${\mathrm{Zr}}_{1-x}{\mathrm{Ta}}_x\mathrm{N}$ films is also studied, based on their structural and mechanical response upon vacuum annealing at 850 °C for 3 h. Our findings demonstrate that ${\mathrm{Zr}}_{1-x}{\mathrm{Ta}}_x\mathrm{N}$ alloys with Ta fraction $0.51⩽x⩽0.78$ exhibit enhanced toughness, while retaining high hardness ∼30 GPa, as a result of increased valence electron concentration and phase stability tuning. Calculations performed for disordered or ordered structures both lead to the same conclusion regarding the mechanical behavior of these nitride alloys, in agreement with recent literature findings [H. Kindlund, D. G. Sangiovanni, L. Martinez-de-Olcoz, J. Lu, J. Jensen, J. Birch, I. Petrov, J. E. Greene, V. Chirita, and L. Hultman, APL Materials 1, 042104 (2013)].

The Potts model plays an essential role in classical statistical mechanics, illustrating many fundamental phenomena. One example is the existence of partially long-range-ordered states, in which some degrees of freedom remain disordered. This situation may arise from frustration of the interactions, but also from an irregular but unfrustrated lattice structure. We study partial long-range order in a range of antiferromagnetic $q$-state Potts models on different two-dimensional lattices and for all relevant values of $q$. We exploit the power of tensor-based numerical methods to evaluate the partition function of these models and hence to extract the key thermodynamic properties (entropy, specific heat, magnetization, and susceptibility) giving deep insight into the phase transitions and ordered states of each system. Our calculations reveal a range of phenomena related to partial ordering, including different types of entropy-driven phase transition, the role of lattice irregularity, very large values of the critical $q_c$, and double phase transitions.

We measured the acquisition of magnetic remanence of iron-nickel alloys (${\mathrm{Fe}}_{64}{\mathrm{Ni}}_{36}$, ${\mathrm{Fe}}_{58}{\mathrm{Ni}}_{42}$, and ${\mathrm{Fe}}_{50}{\mathrm{Ni}}_{50}$) and pure Ni under pressures up to 23 GPa at room temperature. Magnetization decreases markedly for ${\mathrm{Fe}}_{64}{\mathrm{Ni}}_{36}$ between 5 and 7 GPa yet remains ferromagnetic until at least 16 GPa. Magnetization rises by a factor of 2–3 for the other compositions during compression to the highest applied pressures. Immediately upon decompression, magnetic remanence increases for all Fe-Ni alloys while magnetic coercivity remains fairly constant at relatively low values (5–20 mT). The amount of magnetization gained upon complete decompression correlates with the maximum pressure experienced by the sample. Martensitic effects best explain the increase in remanence rather than grain-size reduction, as the creation of single domain sized grains would raise the coercivity. The magnetic remanence of low Ni Invar alloys increases faster with pressure than for other body-centered-cubic compositions due to the higher magnetostriction of the low Ni Invar metals. Thermal demagnetization spectra of ${\mathrm{Fe}}_{64}{\mathrm{Ni}}_{36}$ measured after pressure release broaden as a function of peak pressure, with a systematic decrease in Curie temperature. Irreversible strain accumulation from the martensitic transition likely explains the broadening of the Curie temperature spectra, consistent with our x-ray diffraction analyses.

Simulating superfluid turbulence using the localized induction approximation allows neighboring parallel vortices to proliferate. In many circumstances a turbulent tangle becomes unsustainable, degenerating into a series of parallel, noninteracting vortex lines. Calculating with the fully nonlocal Biot-Savart law prevents this difficulty but also increases computation time. Here we use a truncated Biot-Savart integral to investigate the effects of nonlocality on homogeneous turbulence. We find that including the nonlocal interaction up to roughly the spacing between nearest-neighbor vortex segments prevents the parallel alignment from developing, yielding an accurate model of homogeneous superfluid turbulence with less computation time.

We carry out $T=0$ first-principles total energy calculations in the entire set of alkaline 122-pnictides ($A{\mathrm{Fe}}_2{\mathrm{As}}_2$; $A$ = alkaline-earth element Ca, Sr, Ba, Ra) as a function of hydrostatic pressure. We find multiple distinct transitions to occur, namely an enthalpic transition in which the zero-pressure striped antiferromagnetic orthorhombic (OR-AFM) phase becomes thermodynamically less stable than a competing tetragonal (T) phase, a magnetic transition in which the OR-AFM phase loses its magnetism and orthorhombicity, and a lattice parameter anomaly in which the tetragonal c-axis collapses and a collapsed tetragonal (cT) phase becomes stable. Our results for energy band dispersions and spectra, lattice parameters, enthalpies, magnetism, and elastic constants over a wide range of hydrostatic pressure provide a coherent understanding of these experimentally observed transitions. In particular, the T-cT transition and anomalies in lattice parameters and elastic properties, observed at finite temperatures, are interpreted as arising from proximity to $T=0$ Lifshitz transitions, wherein pressure causes nontrivial changes in the Fermi surface topology in these materials.

We collect optical reflectivity data as a function of temperature across the structural tetragonal-to-orthorhombic phase transition at $T_s$ on ${\mathrm{Ba}(\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ for $x=0\%$, 2.5%, and 4.5%, with uniaxial and in situ tunable applied pressure in order to detwin the sample and to exert on it an external symmetry-breaking field. At $T<T_s$, we discover a remarkable hysteretic optical anisotropy as a function of the applied pressure at energies far away from the Fermi level. Such an anisotropy turns into a reversible linear pressure dependence at $T\ge T_s$. Moreover, the optical anisotropy gets progressively depleted with increasing Co content in the underdoped regime, consistent with the doping dependence of the orthorhombicity but contrary to the nonmonotonic behavior observed for the dc anisotropy.

A low-energy theory for the helical metallic states, residing on the surface of cubic topological Kondo insulators, is derived. Despite our analysis being primarily focused on a prototype topological Kondo insulator, samarium hexaboride $\left({\mathrm{SmB}}_6\right)$, the surface theory derived here can also capture key properties of other heavy fermion topological compounds with a similar underlying crystal structure. Starting from an effective mean-field eight-band model in the bulk, we arrive at a low-energy description of the surface states, pursuing both analytical and numerical approaches. In particular, we show that helical Dirac excitations occur near the $\overline{\Gamma }$ point and the two $\overline{X}$ points of the surface Brillouin zone and generally the energies of the Dirac points exhibit offset relative to each other. We calculate the dependence of several observables (such as bulk insulating gap, energies of the surface Dirac fermions, their relative position to the bulk gap, etc.) on various parameters in the theory. We also investigate the effect of a spatial modulation of the chemical potential on the surface spectrum and show that this band bending generally results in “dragging down” of the Dirac points deep into the valence band and strong enhancement of Fermi velocity of surface electrons. Comparisons with recent ARPES and quantum oscillation experiments are drawn.

Here we calculate the electron transmission phase through a quantum point contact (QPC). The QPC is considered in the saddle-point approximation in the single-electron picture. We show that when the electron energy is close to the height of the potential barrier, the transmission phase depends linearly on the energy. The coefficient in the linear dependence is logarithmically enhanced by the ratio of the height over the curvature of the barrier potential. We compare the calculated transmission phase with the first experimental measurements of the phase.

The electron-phonon coupling constant $\left(\lambda \right)$ of carbon nanotubes $\left(\lambda =0.006\right)$ is much smaller than that of alkali-metal-doped ${\mathrm{C}}_{60}$ crystals $\left(\lambda =0.6\right)$. This difference may be due to the shape of the $\pi$-electron conjugated system: the former has a flat, whereas the latter has a sphere. In order to confirm the shape effects in $\lambda$, we have examined the magnitude of $\lambda$ for a one-dimensional (1D) uneven peanut-shaped ${\mathrm{C}}_{60}$ polymer that has an intermediate shape of the $\pi$-electron conjugated system between a carbon nanotube and a ${\mathrm{C}}_{60}$ system, using femtosecond (fs) time-resolved pump-probe spectroscopy, because it can be expected to have an intermediate value of $\lambda$ between them. Theoretical analysis of fs-transient refractivity obtained experimentally found the magnitude of $\lambda$ of the 1D ${\mathrm{C}}_{60}$ polymer film to be 0.02 as our expectation. This indicates that the shape of the $\pi$-electron conjugated system affects the magnitude of $\lambda$ for nanocarbon allotropes significantly.

We study the mutual interaction between two identical quantum dots coupled to the normal modes of two-site photonic crystal molecules in a planar waveguide geometry, i.e., photonic crystal dimers. We find that the radiative coupling between the two quantum emitters is maximized when they are in resonance with either the bonding or the antibonding modes of the coupled-cavity system. Moreover, we find that such effective interdot coupling is sizable, in the range of $\sim 1$ meV, and almost independent from the cavities' distance, as long as a normal-mode splitting exceeding the radiative linewidth can be established (strong cavity-cavity coupling condition). In realistic and high-quality-factor photonic crystal cavity devices, such distance can largely exceed the emission wavelength, which is promising for long-distance entanglement generation between two qubits in an integrated nanophotonic platform. We show that these results are robust against position disorder of the two quantum emitters within their respective cavities.

We consider holes confined to Ge/Si core/shell nanowires subject to strong Rashba spin-orbit interaction and screened Coulomb interaction. Such wires can, for instance, serve as host systems for Majorana bound states. Starting from a microscopic model, we find that the Coulomb interaction strongly influences the properties of experimentally realistic wires. To show this, a Luttinger liquid description is derived based on a renormalization group analysis. This description in turn allows us to calculate the scaling exponents of various correlation functions as a function of the microscopic system parameters. It furthermore permits us to investigate the effect of Coulomb interaction on a small magnetic field, which opens a strongly anisotropic partial gap.