arXiv subjects

Katrine Golubkov

Publications and source records attributed to Katrine Golubkov.

4 recordsLinked to original sources

arXiv2025

Observation of stationary spontaneous Hawking radiation and the time evolution of an analogue black hole

The emission of Hawking radiation from a black hole was predicted to be stationary, which is necessary for the correspondence between Hawking radiation and black-body radiation. Spontaneous Hawking radiation was observed in analogue black holes in atomic Bose-Einstein condensates, although the stationarity was not probed. Here, we confirm that the spontaneous Hawking radiation is stationary, by observing such a system at six different times. Furthermore, we follow the time evolution of Hawking radiation and compare and contrast it with predictions for real black holes. We observe the ramp up of Hawking radiation followed by stationary spontaneous emission, similar to a real black hole. The end of the spontaneous Hawking radiation is marked by the formation of an inner horizon, which is seen to cause stimulated Hawking radiation as predicted. We find that the stimulated Hawking and partner particles are directly observable, and that the stimulated emission evolves from multi-mode to monochromatic. Numerical simulations suggest that Bogoliubov-Cherenkov-Landau stimulation predominates, rather than black-hole lasing.

gr-qc↗

arXiv2022

Coupled Surface Diffusion and Mean Curvature Motion: Axisymmetric Steady States with Two Grains and a Hole

The evolution of two grains, which lie on a substrate and are in contact with each other, can be roughly described by a model in which the exterior surfaces of the grains evolve by surface diffusion and the grain boundary, namely the contact surface between the adjacent grains, evolves by motion by mean curvature. We consider an axi-symmetric two grain system, contained within an inert bounding semi-infinite cylinder with a hole along the axis of symmetry. Boundary conditions are imposed reflecting the considerations of W.W. Mullins, 1958. We focus here on the steady states of this system. At steady state, the exterior surfaces have constant and equal mean curvatures, and the grain boundary has zero mean curvature; the exterior surfaces are nodoids and the grain boundary surface is a catenoid. The physical parameters in the model can be expressed via two angles $β$ and $θ_c$, which depend on the surface free energies. Typically if a steady state solution exists for given values of $(β, θ_c)$, then there exists a continuum of such solutions. In particular, we prove that there exists a continuum of solutions with $θ_c=π$ for any $β\in (π/2, π)$.

math.AP↗

arXiv2020

Stiffnessometer, a magnetic-field-free superconducting stiffness meter and its application

We provide a detailed account for a new method to measure superconducting stiffness $ρ_{s}$, critical current density $j_c$, and coherence length $ξ$, in one apparatus, without subjecting the sample to magnetic field or attaching leads. The method is based on the London equation $\mathbf{j}=-ρ_{s}\mathbf{A}$, where ${\bf j}$ is the current density and ${\bf A}$ is the vector potential. Using a rotor free $\bf{A}$ and a measurement of $\bf{j}$ via the magnetic moment of a superconducting ring, we determine $ρ_{s}$. By increasing $\mathbf{A}$ until the London equation fails we determine $j_c$ and $ξ$. The method is sensitive to very small stiffness, which translates to penetration depth $λ\lesssim 1$~mm. It is also sensitive to low critical current density $j_c \sim 10^3$ Amm$^{-2}$ or long coherence length $ξ\sim 1$~$μ$m. Naturally, the method does not suffer from demagnetization factor complications, the presence of vortices, or out-of-equilibrium conditions. Therefore, the absolute values of the different parameters can be determined. We demonstrate the application of this method to La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.17$.

cond-mat.supr-con↗

arXiv2018

Observation of thermal Hawking radiation at the Hawking temperature in an analogue black hole

We measure the correlation spectrum of the Hawking radiation emitted by an analogue black hole and find it to be thermal at the Hawking temperature implied by the analogue surface gravity. The Hawking radiation is in the regime of linear dispersion, in analogy with a real black hole. Furthermore, the radiation inside of the black hole is seen to be composed of negative-energy partners only. This work confirms the prediction of Hawking's theory regarding the value of the Hawking temperature, as well as the thermality of the spectrum. The thermality of Hawking radiation is the root of the information paradox. The correlations between the Hawking and partner particles imply that the analogue black hole has no analogue firewall.

gr-qc↗