arXiv subjects

T. Koch

Publications and source records attributed to T. Koch.

12 recordsLinked to original sources

arXiv2025

Self-organized adaptive branching in frangible matter

Soft and frangible materials that remodel under flow can give rise to branched patterns shaped by material properties, boundary conditions, and the time scales of forcing. We present a general theoretical framework for emergent branching in these frangible (or threshold) materials that switch abruptly from resisting flow to permitting flow once local stresses exceed a threshold, relevant for examples as varied as dielectric breakdown of insulators and the erosion of soft materials. Simulations in 2D and 3D show that branching is adaptive and tunable via boundary conditions and domain geometry, offering a foundation for self-organized engineering of functional transport architectures.

cond-mat.soft↗

arXiv2025

Kinetic inductance coupling for circuit QED with spins

In contrast to the commonly used qubit resonator transverse coupling via the $\sigma_{xy}$-degree of freedom, longitudinal coupling through $\sigma_z$ presents a tantalizing alternative: it does not hybridize the modes, eliminating Purcell decay, and it enables quantum-non-demolishing qubit readout independent of the qubit-resonator frequency detuning. Here, we demonstrate longitudinal coupling between a {Cr$_7$Ni} molecular spin qubit ensemble and the kinetic inductance of a granular aluminum superconducting microwave resonator. The inherent frequency-independence of this coupling allows for the utilization of a 7.8 GHz readout resonator to measure the full {Cr$_7$Ni} magnetization curve spanning 0-600 mT, corresponding to a spin frequency range of $f_\text{spin}=$0-15 GHz. For 2 GHz detuning from the readout resonator, we measure a $1/e$ spin relaxation time $\tau=$0.38 s, limited by phonon decay to the substrate. Based on these results, we propose a path towards longitudinal coupling of single spins to a superconducting fluxonium qubit.

quant-ph↗

arXiv2014

Thermoelectric effects in molecular quantum dots with contacts

We consider the steady-state thermoelectric transport through a vibrating molecular quantum dot that is contacted to macroscopic leads. For moderate electron-phonon interaction strength and comparable electronic and phononic timescales, we investigate the impact of the formation of a local polaron on the thermoelectric properties of the junction. We apply a variational Lang-Firsov transformation and solve the equations of motion in the Kadanoff-Baym formalism up to second order in the dot-lead coupling parameter. We calculate the thermoelectric current and voltage for finite temperature differences in the resonant and inelastic tunneling regimes. For a near resonant dot level, the formation of a local polaron can boost the thermoelectric effect because of the Franck-Condon blockade. The line shape of the thermoelectric voltage signal becomes asymmetrical due to the varying polaronic character of the dot state and in the nonlinear transport regime, vibrational signatures arise.

cond-mat.mes-hall↗

arXiv2012

Phonon-affected steady-state transport through molecular quantum dots

We consider transport through a vibrating molecular quantum dot contacted to macroscopic leads acting as charge reservoirs. In the equilibrium and nonequilibrium regime, we study the formation of a polaron-like transient state at the quantum dot for all ratios of the dot-lead coupling to the energy of the local phonon mode. We show that the polaronic renormalization of the dot-lead coupling is a possible mechanism for negative differential conductance. Moreover, the effective dot level follows one of the lead chemical potentials to enhance resonant transport, causing novel features in the inelastic tunneling signal. In the linear response regime, we investigate the impact of the electron-phonon interaction on the thermoelectrical properties of the quantum dot device.

cond-mat.str-el↗

arXiv2011

Non-equilibrium transport through molecular junctions in the quantum regime

We consider a quantum dot, affected by a local vibrational mode and contacted to macroscopic leads, in the non-equilibrium steady-state regime. We apply a variational Lang-Firsov transformation and solve the equations of motion of the Green functions in the Kadanoff-Baym formalism up to second order in the interaction coefficients. The variational determination of the transformation parameter through minimization of the thermodynamic potential allows us to calculate the electron/polaron spectral function and conductance for adiabatic to anti-adiabatic phonon frequencies and weak to strong electron-phonon couplings. We investigate the qualitative impact of the quasi-particle renormalization on the inelastic electron tunneling spectroscopy signatures and discuss the possibility of a polaron induced negative differential conductance. In the high-voltage regime we find that the polaron level follows the lead chemical potential to enhance resonant transport.

cond-mat.str-el↗

arXiv2010

Transport through a vibrating quantum dot: Polaronic effects

We present a Green's function based treatment of the effects of electron-phonon coupling on transport through a molecular quantum dot in the quantum limit. Thereby we combine an incomplete variational Lang-Firsov approach with a perturbative calculation of the electron-phonon self energy in the framework of generalised Matsubara Green functions and a Landauer-type transport description. Calculating the ground-state energy, the dot single-particle spectral function and the linear conductance at finite carrier density, we study the low-temperature transport properties of the vibrating quantum dot sandwiched between metallic leads in the whole electron-phonon coupling strength regime. We discuss corrections to the concept of an anti-adiabatic dot polaron and show how a deformable quantum dot can act as a molecular switch.

cond-mat.mes-hall↗

arXiv2008

A purely dipolar quantum gas

We report on experiments exploring the physics of dipolar quantum gases using a Chromium Bose-Einstein condensate (BEC). By means of a Feshbach resonance, it is possible to reduce the effects of short range interactions and reach a regime where the physics is governed by the long-range, anisotropic dipole-dipole interaction between the large ($6 μ_{\rm B}$) magnetic moments of Chromium atoms. Several dramatic effects of the dipolar interaction are observed: the usual inversion of ellipticity of the condensate during time-of flight is inhibited, the stability of the dipolar gas depends strongly on the trap geometry, and the explosion following the collapse of an unstable dipolar condensate displays d-wave like features.

cond-mat.other↗

arXiv2007

Stabilizing a purely dipolar quantum gas against collapse

We report on the experimental observation of the dipolar collapse of a quantum gas which sets in when we reduce the contact interaction below some critical value using a Feshbach resonance. Due to the anisotropy of the dipole-dipole interaction, the stability of a dipolar Bose-Einstein condensate depends not only on the strength of the contact interaction, but also on the trapping geometry. We investigate the stability diagram and find good agreement with a universal stability threshold arising from a simple theoretical model. Using a pancake-shaped trap with the dipoles oriented along the short axis of the trap, we are able to tune the scattering length to zero, stabilizing a purely dipolar quantum gas.

cond-mat.other↗

arXiv2007

A two-frequency acousto-optic modulator driver to improve the beam pointing stability during intensity ramps

We report on a scheme to improve the pointing stability of the first order beam diffracted by an acousto-optic modulator (AOM). Due to thermal effects inside the crystal, the angular position of the beam can change by as much as 1 mrad when the radio-frequency power in the AOM is reduced to decrease the first order beam intensity. This is done for example to perform forced evaporative cooling in ultracold atom experiments using far-off-resonant optical traps. We solve this problem by driving the AOM with two radio-frequencies $f_1$ and $f_2$. The power of $f_2$ is adjusted relative to the power of $f_1$ to keep the total power constant. Using this, the beam displacement is decreased by a factor of twenty. The method is simple to implement in existing experimental setups, without any modification of the optics.

physics.optics↗

arXiv2006

Damagnetization cooling of a gas

We demonstrate demagnetization cooling of a gas of ultracold $^{52}$Cr atoms. Demagnetization is driven by inelastic dipolar collisions which couple the motional degrees of freedom to the spin degree. By that kinetic energy is converted into magnetic work with a consequent temperature reduction of the gas. Optical pumping is used to magnetize the system and drive continuous demagnetization cooling. Applying this technique, we can increase the phase space density of our sample by one order of magnitude, with nearly no atom loss. This method can be in principle extended to every dipolar system and could be used to achieve quantum degeneracy via optical means.

cond-mat.other↗

arXiv2006

Expansion dynamics of a dipolar Bose-Einstein condensate

Our recent measurements on the expansion of a chromium dipolar condensate after release from an optical trapping potential are in good agreement with an exact solution of the hydrodynamic equations for dipolar Bose gases. We report here the theoretical method used to interpret the measurement data as well as more details of the experiment and its analysis. The theory reported here is a tool for the investigation of different dynamical situations in time-dependent harmonic traps.

cond-mat.other↗

arXiv2005

Observation of dipole-dipole interaction in a degenerate quantum gas

We have investigated the expansion of a Bose-Einstein condensate (BEC) of strongly magnetic chromium atoms. The long-range and anisotropic magnetic dipole-dipole interaction leads to an anisotropic deformation of the expanding Cr-BEC which depends on the orientation of the atomic dipole moments. Our measurements are consistent with the theory of dipolar quantum gases and show that a Cr-BEC is an excellent model system to study dipolar interactions in such gases.

cond-mat.other↗