arXiv subjects

E. V. Thuneberg

Publications and source records attributed to E. V. Thuneberg.

30 records · Page 2Linked to original sources

arXiv2003

Structure of surface vortex sheet between two rotating 3He superfluids

We study a two-phase sample of superfluid 3He where vorticity exists in one phase (3He-A) but cannot penetrate across the interfacial boundary to a second coherent phase (3He-B). We calculate the bending of the vorticity into a surface vortex sheet on the interface and solve the internal structure of this new type of vortex sheet. The compression of the vorticity from three to two dimensions enforces a structure which is made up of half-quantum units, independently of the structure of the source vorticity in the bulk. These results are consistent with our NMR measurements.

cond-mat↗

arXiv2002

Equilibrium simulations of 2D weak links in p-wave superfluids

A two-dimensional Ginzburg-Landau theory of weak links in a p-wave superfluid is presented. First we consider the symmetry properties of the energy functionals, and their relation to the conserved supercurrents which play an essential role in the weak link problem. In numerical studies, we use the A and B phases of superfluid 3He. The phases on the two sides of the weak link can be chosen separately, and very general soft degrees of freedom may be imposed as boundary conditions. We study all four inequivalent combinations of A and B which are possible for a hole in a planar wall, including weak links with a pinned A-B interface. In all cases, some illustrative current-phase relations (CPR's) are calculated and the critical currents are mapped. Phase diagrams covering the relevant phase space in zero magnetic field are constructed. The numerical methods are also described in some detail.

cond-mat.supr-con↗

arXiv2001

Vortex lines or sheets - what is formed in dynamic drives?

In isotropic macroscopic quantum systems vortex lines can be formed while in anisotropic systems also vortex sheets are possible. Based on measurements of superfluid 3He-A, we present the principles which select between these two competing forms of quantized vorticity: sheets displace lines if the frequency of the external field exceeds a critical limit. The resulting topologically stable state consists of multiple vortex sheets and has much faster dynamics than the state with vortex lines.

cond-mat↗

arXiv2001

Pinhole calculations of the Josephson effect in 3He-B

We study theoretically the dc Josephson effect between two volumes of superfluid 3He-B. We first discuss how the calculation of the current-phase relationships is divided into a mesoscopic and a macroscopic problem. We then analyze mass and spin currents and the symmetry of weak links. In quantitative calculations the weak link is assumed to be a pinhole, whose size is small in comparison to the coherence length. We derive a quasiclassical expression for the coupling energy of a pinhole, allowing also for scattering in the hole. Using a selfconsistent order parameter near a wall, we calculate the current-phase relationships in several cases. In the isotextural case, the current-phase relations are plotted assuming a constant spin-orbit texture. In the opposite anisotextural case the texture changes as a function of the phase difference. For that we have to consider the stiffness of the macroscopic texture, and we also calculate some surface interaction parameters. We analyze the experiments by Marchenkov et al. We find that the observed pi states and bistability hardly can be explained with the isotextural pinhole model, but a good quantitative agreement is achieved with the anisotextural model.

cond-mat.supr-con↗

arXiv2001

Hydrostatic theory of superfluid 3He-B

The determination of the texture of the order parameter is important for understanding many experiments in superfluid 3He. In addition to reviewing the theory of textures in superfluid 3He-B we give several new results, in particular on the surface parameters in the Ginzburg-Landau region and bulk parameters at arbitrary temperature. Special attention is paid to separate the results that are valid at all temperatures from those which are limited to the Ginzburg-Landau region. We study the validity of a trivial strong-coupling model, where the energy gap of the weak-coupling theory is scaled by a temperature dependent factor. We compare the theory with several experiments. For some quantities the theory seems to work fine and we extract the dipole-dipole interaction parameter from the measurements.

cond-mat.supr-con↗

arXiv2001

Soliton-limited Superflow in 3He-A between Parallel Plates

We have studied theoretically the flow of superfluid 3He-A in parallel-plate geometry. The equilibrium order-parameter texture is calculated numerically in two spatial dimensions consisting of the coordinates along the flow direction and perpendicular to the plates. The calculations have been done using the hydrostatic theory in the Ginzburg-Landau region and assuming a large external magnetic field perpendicular to the plane of calculation. We have studied a uniform texture and a dipole-unlocked splay soliton as initial configurations. In the former case we find the Freedericksz transition and a helical instability with increasing flow. In the latter case we find instability in the soliton. This instability is closely related to the critical velocity in the presence of a vortex sheet. Also, the transverse NMR frequency shift at the soliton has been calculated.

cond-mat↗

arXiv2000

One-Dimensional Textures and Critical Velocity in Superfluid 3He-A

We study theoretically the stability of flow in superfluid 3He-A. The calculations are done using a one-dimensional model where the order parameter depends only on the coordinate in the direction of the superfluid velocity v_s. We concentrate on the case that the external magnetic field H is perpendicular to v_s, where only few results are available analytically. We calculate the critical velocity v_c at which the superflow becomes unstable against the formation of continuous vortices. The detailed dependence of v_c on the temperature and on the form of the underlying orbital texture l(r) is investigated. Both uniform and helical textures and two types of domain-wall structures are studied. The results are partially in agreement with experiments made in a rotating cylinder.

cond-mat↗

arXiv1999

Theory of the pi state in 3He Josephson junctions

The flow of superfluid 3He-B through a 65 x 65 array of nanometer size apertures has been measured recently by Backhaus et al. They find in the current--phase relation a new branch, so-called pi state. We study two limiting cases which show that the pi state arises from coupling of the phase degree of freedom to the spin-orbit rotation. The pi state exists in a single large aperture, but is difficult to observe because of hysteresis. A better correspondence with experiments is obtained by assuming a thin wall, where the Josephson coupling between the two sides arises from a dense array of pin-holes.

cond-mat↗

arXiv1999

Periodic Vortex Structures in Superfluid 3He-A

We discuss the general properties of periodic vortex arrangements in rotating superfluids. The different possible structures are classified according to the symmetry space-groups and the circulation number. We calculate numerically several types of vortex structures in superfluid 3He-A. The calculations are done in the Ginzburg-Landau region, but the method is applicable at all temperatures. A phase diagram of vortices is constructed in the plane formed by the magnetic field and the rotation velocity. The characteristics of the six equilibrium vortex solutions are discussed. One of these, the locked vortex 3, has not been considered in the literature before. The vortex sheet forms the equilibrium state of rotating 3He-A at rotation velocities exceeding 2.6 rad/s. The results are in qualitative agreement with experiments.

cond-mat↗

arXiv1998

Ginzburg-Landau Theory for Impure Superfluid 3He

Liquid 3He at low temperatures is an ideal substance to study because of its natural purity. The superfluid state, which appears at temperatures below 3 mK, has many unusual and exciting properties. These have extensively been studied experimentally, and the results are in many cases well understood theoretically using quasiclassical approximation. In this work we study the case that liquid 3He is inside of aerogel. Aerogel is a very porous substance, where 98% of the volume can be empty. The purpose is to understand how the properties of the superfluid are modified when the quasiparticles are scattered from an impurity, such as aerogel. We study extensively the "homogeneous scattering model". From it we derive the Ginzburg-Landau theory of impure superfluid 3He. We give expressions for measurable quantities and compare them with experiments. We consider random anisotropy and its effect on the NMR properties. More sophisticated scattering models are briefly discussed.

cond-mat↗

arXiv1997

Models for Superfluid $^3$He in Aerogel

Several recent experiments find evidence of superfluidity of 3He in 98%-porous aerogel. The primary effect of the aerogel is that it scatters the quasiparticles of 3He. We find that many experimental findings are quantitatively understood by a relatively simple model that takes into account strong inhomogeneity of the scattering on a length scale of 100 nm.

cond-mat↗

arXiv1997

Critical Velocity of Vortex Nucleation in Rotating Superfluid 3He-A

We have measured the critical velocity v_c at which 3He-A in a rotating cylinder becomes unstable against the formation of quantized vortex lines with continuous (singularity-free) core structure. We find that v_c is distributed between a maximum and minimum limit, which we ascribe to a dependence on the texture of the orbital angular momentum l(r) in the cylinder. Slow cool down through T_c in rotation yields l(r) textures for which the measured v_c's are in good agreement with the calculated instability of the expected l texture.

cond-mat.soft↗