arXiv subjects

Erkki Thuneberg

Publications and source records attributed to Erkki Thuneberg.

13 recordsLinked to original sources

arXiv2026

Surface-induced vortex core restructuring in a spin-triplet superfluid

Observing the structure of quantized vortices can provide evidence for the pairing nature of a superfluid or superconductor and pinpoint its order parameter. Spin-triplet superfluid $^3$He supports a variety of vortices, calculated and identified so far in bulk fluid. We show numerically that the vortex core in $^3$He is strongly altered near a surface, resulting in a structure inhomogeneous along the vortex line. The effect is asymmetric with respect to the relative orientation of the core order parameter anisotropy axis and the surface normal. In a wide range of external conditions, the vortex structure at the surface is found to be completely different from that in bulk. The effect originates from the combination of spin-orbit interaction in triplet pairing with the symmetry breaking by the surface. As an implication, surface-limited vortex core observations in a triplet-candidate system may not reflect the bulk structure. We propose an experimental verification of the effect by measuring a transition in the vortex structure in thin slabs of superfluid $^3$He-B.

cond-mat.supr-con↗

arXiv2024

Structure of a single-quantum vortex in $^3$He-A

We have performed numerical calculations of the structure of the single-quantum vortex in superfluid $^3$He-A. The GPU-accelerated large-scale numerical simulation is performed in the Ginzburg-Landau model and resolves length scales of both coherence-length-sized hard core and dipolar-length-sized soft core of the vortex. The calculations support previously suggested qualitative structure of the vortex, recently named as eccentric fractional skyrmion, and provide numerical values for the vortex energy, sizes and locations of the hard and soft cores and highly-asymmetric flow profile of the vortex.

cond-mat.other↗

arXiv2024

Square-well model for superconducting pair-potential

We study Andreev reflection in a one-dimensional square-well pair-potential. We discuss the history of the model. The current-phase relation is presented as a sum over Matsubara frequencies. How the current arises from bound and continuum levels is found by analytic continuation. We discuss two limiting cases of the square-well potential, the zero-length well and the infinite well. The model is quantitatively valid in some cases but forms the basis for understanding a wide range of problems in inhomogeneous superconductivity and superfluidity.

cond-mat.supr-con↗

arXiv2023

Comment on "Ballistic SNS sandwich as a Josephson junction"

The old problem of supercurrent in a long ballistic SNS Josephson junction is interpreted using a simple picture of the fermion energy levels in the normal and superconducting regions. We argue that a recent paper on the topic by Sonin contains erroneous results.

cond-mat.supr-con↗

arXiv2014

Hard domain walls in superfluid 3He-B

We study theoretically planar interfaces between two domains of superfluid 3He-B. The structure of the B-B walls is determined on the scale of the superfluid condensation energy, and thus the domain walls have thickness on the order of the Ginzburg-Landau coherence length. We study the stability and decay schemes of five inequivalent structures of such domain walls using one-dimensional Ginzburg-Landau simulation. We find that only one of the structures is stable against small perturbations. We also argue that B-B interfaces could result from adiabatic A to B transition and study textures at B-B interfaces. The B-B interface has a strong orienting effect on spin-orbit rotation producing textures similar as caused by external walls. We study the B-B interface in a parallel-plate geometry and find that the conservation of spin current sets an essential condition on the structure. The stable B-B interface gives rise to half-quantum circulation.

cond-mat.supr-con↗

arXiv2013

Charge qubit driven via the Josephson nonlinearity

We study the novel nonlinear phenomena that emerge in a charge qubit due to the interplay between a strong microwave flux drive and a periodic Josephson potential. We first analyze the system in terms of the linear Landau-Zener-Stückelberg model, and show its inadequacy in a periodic system with several Landau-Zener crossings within a drive period. Experimentally, we probe the quasienergy levels of the driven qubit with an LC-cavity, which requires the use of linear response theory. We also show that our numerical calculations are in good agreement with the experimental data.

cond-mat.mes-hall↗

arXiv2013

Probe spectroscopy of quasienergy states

The present qubit technology, in particular in Josephson qubits, allows an unprecedented control of discrete energy levels. This motivates a new study of the old pump-probe problem, where a discrete quantum system is driven by a strong drive and simultaneously probed by a weaker one. The strong drive is included by the Floquet method and the resulting quasienergy states are then studied with the probe. We study a qubit where the harmonic drive has a significant longitudinal component relative to the static equilibrium state of the qubit. Both analytical and numerical methods are used to solve the problem. We present calculations with realistic parameters and compare the results with recent experimental results. A short introduction to the Floquet method and the probe absorption is given.

quant-ph↗

arXiv2011

Fermi liquid theory applied to vibrating wire measurements in 3He-4He mixtures

We use Fermi liquid theory to study the mechanical impedance of 3He-4He mixtures at low temperatures. The theory is applied to the case of vibrating wires, immersed in the liquid. We present numerical results based on a direct solution of Landau-Boltzmann equation for the 3He quasiparticle distribution for the full scale of the quasiparticle mean-free-path. The two-fluid nature of mixtures is taken into account in the theory, and the effect of Fermi liquid interactions and boundary conditions are studied in detail. The results are in fair quantitative agreement with experimental data. In particular, we can reproduce the anomalous decrease in inertia, observed in vibrating wire experiments reaching the ballistic limit. The essential effect of the experimental container and second-sound resonances is demonstrated.

cond-mat.str-el↗

arXiv2010

Pendulum in Fermi liquid

The Fermi liquid theory formulated by Landau is a basic paradigm of the behavior of an interacting many-body system. We present a new application of this theory to calculate "Landau force" on a macroscopic object. We show that immersing a pendulum in Fermi liquid can increase its oscillation frequency, and evidence of this has been observed in mixtures of 3He and 4He.

cond-mat.other↗

arXiv2010

Stark effect and generalized Bloch-Siegert shift in a strongly driven two-level system

A superconducting qubit was driven in an ultrastrong fashion by an oscillatory microwave field, which was created by coupling via the nonlinear Josephson energy. The observed Stark shifts of the `atomic' levels are so pronounced that corrections even beyond the lowest-order Bloch-Siegert shift are needed to properly explain the measurements. The quasienergies of the dressed two-level system were probed by resonant absorption via a cavity, and the results are in agreement with a calculation based on the Floquet approach.

cond-mat.mes-hall↗

arXiv2009

Interband transitions and interference effects in superconducting qubits

We investigate phase-sensitive interference effects in a periodically $\sin(2πf_{\rm rf} t)$-driven, artificial two-state system connected to a microwave resonator at $f_{LC} \simeq 800$ MHz. We observe two kinds of multiphoton transitions in the two-state system, accompanied by: 1) Several quanta from the drive at $f_{\rm rf}$ and 2) one quantum at $f_{\rm rf}$ and several at $f_{LC}$. The former are described using phase-sensitive Landau-Zener transitions, while the latter are discussed in terms of vibronic transitions in diatomic molecules. Interference effects in the vibronic transitions governed by Franck-Condon coefficients are also considered.

cond-mat.mes-hall↗

arXiv2008

Vibronic spectroscopy of an artificial molecule

With advanced fabrication techniques it is possible to make nanoscale electronic structures that have discrete energy levels. Such structures are called artificial atoms because of analogy with true atoms. Examples of such atoms are quantum dots in semiconductor heterostructures and Josephson-junction qubits. It is also possible to have artificial atoms interacting with each other. This is an artificial molecule in the sense that the electronic states are analogous to the ones in a molecule. In this letter we present a different type of artificial molecule that, in addition to electronic states, also includes the analog of nuclear vibrations in a diatomic molecule. Some of the earlier experiments could be interpreted using this analogy, including qubits coupled to oscillators and qubits driven by an intense field. In our case the electronic states of the molecule are represented by a Josephson-junction qubit, and the nuclear separation corresponds to the magnetic flux in a loop containing the qubit and an LC oscillator. We probe the vibronic transitions, where both the electronic and vibrational states change simultaneously, and find that they are analogous to true molecules. The vibronic transitions could be used for sideband cooling of the oscillator, and we see damping up to sidebands of order 10.

cond-mat.mes-hall↗

arXiv2005

Theory of Josephson Phenomena in Superfluid 3He

Quite detailed theoretical description of superfluid 3He is possible on length scales that are much larger than the atomic scale. We discuss weak links between two bulk states of 3He-B. The current through the weak link is determined by the bound states at the link. The bound state energies are spin split depending on the order parameters in the bulk. As a result, unusual current-phase relations with pi state appear. For not too weak links, the order parameter in the bulk is modified because of the Josephson coupling. This leads to a stronger pi state and to an additional current at constant pressure bias. The theoretical results are compared with experiments.

cond-mat.supr-con↗