Condensed Matter

Recent heat-capacity experiments show quite unambiguously the existence of a liquid 3 He phase adsorbed on graphite. This liquid is stable at an extremely low density, possibly one of the lowest found in Nature. Previous theoretical calculations of the same system, and in strictly two dimensions, agree with the result that this liquid phase is not stable and the system is in the gas phase. We calculated the phase diagram of normal 3 He adsorbed on graphite at T=0 using quantum Monte Carlo methods. Considering a fully corrugated substrate we observe that at densities lower that 0.006 Å −2 the system is a very dilute gas, that at that density is in equilibrium with a liquid of density 0.014 Å −2 . Our prediction matches very well the recent experimental findings on the same system. On the contrary, when a flat substrate is considered, no gas-liquid coexistence is found, in agreement with previous calculations. We also report results on the different solid structures, and the corresponding phase transitions that appear at higher densities.

Below the phase transition temperature Tc??10 ?? K He-3B has a mixture of normal and superfluid components. Turbulence in this material is carried predominantly by the superfluid component. We explore the statistical properties of this quantum turbulence, stressing the differences from the better known classical counterpart. To this aim we study the time-honored Hall-Vinen-Bekarevich-Khalatnikov coarse-grained equations of superfluid turbulence. We combine pseudo-spectral direct numerical simulations with analytic considerations based on an integral closure for the energy flux. We avoid the assumption of locality of the energy transfer which was used previously in both analytic and numerical studies of the superfluid He-3B turbulence. For T<0.37 Tc, with relatively weak mutual friction, we confirm the previously found "subcritical" energy spectrum E(k), given by a superposition of two power laws that can be approximated as E(k) k ?�x with an apparent scaling exponent 5/3 <x(k)< 3. For T>0.37 Tc and with strong mutual friction, we observed numerically and confirmed analytically the scale-invariant spectrum E(k) k ?�x with a (k-independent) exponent x > 3 that gradually increases with the temperature and reaches a value x?? for T??.72Tc . In the near-critical regimes we discover a strong enhancement of intermittency which exceeds by an order of magnitude the corresponding level in classical hydrodynamic turbulence.

Dynamics of simplest quantum vortex knots of torus type in a superfluid at zero temperature has been simulated within a regularized Biot-Savart law (the torus radii R 0 and r 0 for initial vortex configuration were large in comparison with a vortex core width ξ ). Computations of evolution times of knots until their significant deformation were carried out with a small step on parameter B 0 = r 0 / R 0 for different values of parameter Λ=log( R 0 /ξ) . It has been found that at Λ≳3 , bands of quasi-stability appear in a region of B 0 ≲0.2 , which correspond to long knot lifetimes and to very large traveling distances --- up to several hundreds of R 0 . This result is new and quite unexpected, because previously it was believed that maximal lifetime of torus knots until reconnection does not exceed several typical periods. The opening of quasi-stable 'windows' at increasing Λ is due to narrowing main parametric resonances of the dynamical system on parameter B 0 .

We study the long-range electron and energy transfer mediated by a polaron on an α -helix polypeptide chain coupled to donor and acceptor molecules at opposite ends of the chain. We show that for specific parameters of the system, an electron initially located on the donor can tunnel onto the α -helix, forming a polaron which then travels to the other extremity of the polypeptide chain where it is captured by the acceptor. We consider three families of couplings between the donor, acceptor and the chain, and show that one of them can lead to a 90\% efficiency of the electron transport from donor to acceptor. We also show that this process remains stable at physiological temperatures in the presence of thermal fluctuations in the system.

We have performed an x-ray holotomography study of a three-dimensional (3D) photonic band gap crystal. The crystals was made from silicon by CMOS-compatible methods. We manage to obtain the 3D material density throughout the fabricated crystal. We observe that the structural design is for most aspects well-realized by the fabricated nanostructure. One peculiar feature is a slight shear-distortion of the cubic crystal structure. We conclude that 3D X-ray tomography has great potential to solve many future questions on optical metamaterials.

We have studied the ground-state properties of para-hydrogen in one dimension and in quasi-one-dimensional configurations using the path integral ground state Monte Carlo method. This method produces zero-temperature exact results for a given interaction and geometry. The quasi-one-dimensional setup has been implemented in two forms: the inner channel inside a carbon nanotube coated with H 2 and a harmonic confinement of variable strength. Our main result is the dependence of the Luttinger parameter on the density within the stable regime. Going from one dimension to quasi-one dimension, keeping the linear density constant, produces a systematic increase of the Luttinger parameter. This increase is however not enough to reach the superfluid regime and the system always remain in the quasi-crystal regime, according to Luttinger liquid theory.

The transport of a steel sphere on top of two dimensional periodic magnetic patterns is studied experimentally. Transport of the sphere is achieved by moving an external permanent magnet on a closed loop around the two dimensional crystal. The transport is topological i.e. the steel sphere is transported by a primitive unit vector of the lattice when the external magnet loop winds around specific directions. We experimentally determine the set of directions the loops must enclose for nontrivial transport of the steel sphere into various directions.

CrCl3 is a layered insulator that undergoes a crystallographic phase transition below room temperature and orders antiferromagnetically at low temperature. Weak van der Waals bonding between the layers and ferromagnetic in-plane magnetic order make it a promising material for obtaining atomically thin magnets and creating van der Waals heterostructures. In this work we have grown crystals of CrCl3, revisited the structural and thermodynamic properties of the bulk material, and explored mechanical exfoliation of the crystals. We find two distinct anomalies in the heat capacity at 14 and 17 K confirming that the magnetic order develops in two stages on cooling, with ferromagnetic correlations forming before long range antiferromagnetic order develops between them. This scenario is supported by magnetization data. A magnetic phase diagram is constructed from the heat capacity and magnetization results. We also find an anomaly in the magnetic susceptibility at the crystallographic phase transition, indicating some coupling between the magnetism and the lattice. First principles calculations accounting for van der Waals interactions also indicate spin-lattice coupling, and find multiple nearly degenerate crystallographic and magnetic structures consistent with the experimental observations. Finally, we demonstrate that monolayer and few-layer CrCl3 specimens can be produced from the bulk crystals by exfoliation, providing a path for the study of heterostructures and magnetism in ultrathin crystals down to the monolayer limit.

The total magnetic energy of Lithium ferrite thin films was determined using the classical Heisenberg Hamiltonian. The short range magnetic dipole interactions between spins within one unit cell and the interactions between spins in two adjacent unit cells have been determined in order to find the total magnetic energy of lithium ferrite films. Only the spin pairs with separation less than cell constant have been taken into account to calculate dipole interaction and spin exchange interaction. Theoretically several easy and hard directions of lithium ferrite film were found for one set of energy parameters included in our modified Heisenberg Hamiltonian. The dependence of total magnetic energy of a lithium ferrite film on number of unit cells, spin exchange interaction, dipole interaction, second order magnetic anisotropy, fourth order magnetic anisotropy, internal applied magnetic field and stress induced magnetic anisotropy has been explained in this manuscript.

Matrix diagonalization has long been a setback in the numerical simulation of the magnetic resonance spectra of multispin systems since the dimension of the Hilbert space of such systems grows exponentially with the number of spins -- a problem commonly referred to as the "curse of dimensionality". In this paper, we propose two mathematical instruments which, when harmoniously combined, could greatly help surmount to a fair degree and in a systematic manner the curse of dimensionality. These are: 1) the Holstein-Primakoff bosons and 2) what we have termed the "index compression maps". These two allow a bijective mapping of (multi)spin states to integers. Their combination leads to the block diagonalization of the multispin Hamiltonian, thus a computationally exact way of diagonalizing the latter but which also reduces significantly the computational cost. We also show that the eigenvectors and eigenvalues of the Liouvillian operator can be easily determined once those of the related multispin Hamiltonian are known. Interestingly, the method also enables an analytical characterization of the multispin Hilbert space -- a feat hardly attainable with other approaches. We illustrate the method here by showing how a general static isotropic multispin Hamiltonian could be exactly diagonalized with very less computational cost. Nonetheless, we emphasize that the method could be applied to study numerous quantum systems defined on finite Hilbert spaces and embodied with at most pairwise interactions.