Our research focus is on the following topics
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Ground-State Cooling of a Mechanical Oscillator by Interference in Andreev Reflection
Pascal Stadler, Wolfgang Belzig, and Gianluca Rastelli
Phys. Rev. Lett. 117, 197202 (2016)
Reaching the quantum ground state of a nanomechanical oscillator consisting of millions of atoms would allow to study the weirdness of quantum mechanics in a new regime. Suspended carbon nanotubes can oscillate at different frequencies similar to a guitar string. In this paper, we propose a novel way to cool such a system towards the absolute zero of temperature, when these modes are in their quantum ground state with the minimal possible energy. Just like in an ordinary refrigerator electric current is used to extract the energy. By attaching one normal (N) and one superconducting (S) lead to the nanotube the electron transport process couples the hot environment to the zero-temperature reservoir of Cooper pairs in the superconductor allowing to achieve an unprecedented cooling efficiency. In this manner several mechanical modes can be brought into the quantum ground state, which was not possible in previous proposals usually limited to one particular mode. Our method paves the way to sophisticated quantum manipulation of carbon nanotube oscillators like a coherent superposition of modes which might be used as quantum limited sensor of weight or motion.
Probing non-integral spin bosons via noise
Super-Poissonian Shot Noise of Squeezed-Magnon Mediated Spin Transport
Akashdeep Kamra and Wolfgang Belzig
Phys. Rev. Lett. 116, 146601 (2016)
Microscopic particles called electrons carry the charge currents which underlie the modern electronic devices. A novel technological paradigm based on magnets relies upon spin currents carried by fundamentally different particles - magnons, carrying a quantum of spin. In this paper, we show that interactions lead to a new exotic particle called squeezed-magnon, consisting of a quantum conglomerate of several magnons and, thus, carrying an angular momentum different from the fundamental quantum. To test such a property we suggest to generalize the observation initially made by Schottky in 1918 that the electronic charge can be measured through the current fluctuations. Spin current fluctuations are predicted to carry the information about the squeezed magnons. Hence, the experimentally feasible investigation of the fluctuations of spin current in a spin pumping setup might be the key to reveal the quantum nature of exotic quasiparticles like the squeezed magnon.
Electron and electron-hole quasiparticle states in a driven quantum contact
Mihajlo Vanević, Julien Gabelli, Wolfgang Belzig, and Bertrand Reulet
Phys. Rev. B 93, 041416(R) (2016)
We study the many-body electronic state created by a time-dependent drive of a mesoscopic contact. The many-body state is expressed manifestly in terms of single-electron and electron-hole quasiparticle excitations with the amplitudes and probabilities of creation which depend on the details of the applied voltage. We experimentally probe the time dependence of the constituent electronic states by using an analog of the optical Hong-Ou-Mandel correlation experiment where electrons emitted from the terminals with a relative time delay collide at the contact. The electron wave packet overlap is directly related to the current noise power in the contact. We have confirmed the time dependence of the electronic states predicted theoretically by measurements of the current noise power in a tunnel junction under harmonic excitation.
Left: Theoretical and experimental predictions for the overlap of the wave packets. Right: Hong-Ou-Mandel setup to detect the overlap of two wave packets with delay tau interferometrically.
Ultrafast pseudospin dynamics in graphene
M. Trushin, A. Grupp, G. Soavi, A. Budweg, D. De Fazio, U. Sassi, A. Lombardo, A. C. Ferrari, W. Belzig, A. Leitenstorfer, D. Brida
Phys. Rev. B 92, 165429 (2015)
We address a long standing problem of pseudospin control in graphene from both experimental and theoretical point of view. The outcome of this work is twofold.
First, we provide a conclusive evidence of the anisotropic photocarrier occupation (i. e. pseudospin polarization) in graphene caused by interaction between pseudospin and linearly polarized light, see left figure. This peculiar phenomenon has long been discussed in numerous theoretical papers but its direct observation in a truly single layer graphene (rather than in multilayer stacks utilized before) was, up to now, missing.
Second, we clarify the mechanisms responsible for the photocarrier occupation isotropization (i. e. pseudospin randomization) using an entirely analytical model specially developed for our experimental settings. The main driving isotropization mechanism is discovered to be optical phonon emission rather than electron-electron scattering, further distinguishing graphene from conventional semiconductors. The predictive power of the model is demonstrated in the right figure, where the pseudospin occupation (i. e. the ratio between optical transmittance for opposite polarization configurations)
is shown as a function of radiation fluence.
Ground state cooling of a carbon nano-mechanical resonator by spin-polarized current
Pascal Stadler, Wolfgang Belzig and Gianluca Rastelli
Phys.Rev. Lett. 113, 047201 (2014) [ arXiv:1404.0485]
We study the non-equilibrium regime of a mechanical resonator at low temperature realized with a suspended carbon nanotube quantum dot contacted to two ferromagnets. Due to spin- orbit interaction and/or an external magnetic gradient, the spin on the dot couples directly to the flexural eigenmodes. Owing to this interaction, the nanomechanical motion induces spin-flips of the electrons passing through the nanotube. When a finite voltage is applied, a spin-polarized current causes either heating or active cooling of the mechanical modes, depending on the gate voltage. Optimal cooling is achieved at resonance transport realized when the energy splitting between two dot levels of opposite spin equals the resonator frequency. We show that weak interaction coupling strength and moderate polarization can achieve ground state cooling.
"Smile"-gap in the density of states of a cavity between superconductors
J. Reutlinger, L. Glazman, Yu. V. Nazarov, W. Belzig
Phys. Rev. Lett. 112, 067001 (2014) [arXiv:1308.2529]
The density of Andreev levels in a normal metal (N) in contact with two superconductors (S) is known to exhibit an induced minigap related to the inverse dwell time. We predict a small secondary gap just below the superconducting gap edge - a feature that has been overlooked so far in numerous microscopic studies of the density of states in SNS structures. In a generic structure with N being a chaotic cavity, the secondary gap is the widest at zero phase bias. It closes at some finite phase bias, forming the shape of a "smile". Asymmetric couplings give even richer gap structures near the phase difference π. All the features found should be amendable to experimental detection in high-resolution low-temperature tunneling spectroscopy.
Left: DOS in the central region at zero phase difference and E_{Th}=Δ showing the usual minigap around E = 0 and additionally a secondary gap below E = Δ. Center: Quantum circuit theory diagram of the system under investigation. A pseudo-terminal labelled with E_{Th} accounts for random phase shifts between electron and hole components of the quasiparticle wave functions (not implying an electric connection to the ground) Right: DOS near Δ illustrating the phase dependence of the secondary gap. |
New Guest Scientist in the Group
Narasimha Raju Chebrolu from the University of Hyberabad joined the group for 6 months. Welcome.
Pascal Stadler has defended his Thesis
His work entitled "Quantum transport in quantum dots with spin-effects and electron-vibration interaction" was accepted as Doctoral Thesis with Distinction. Congratulations.
New Guest Scientists in the Group.
Dominik Maile from Tübingen, Victor Fernandez from Madrid and Angelo Di Marco from Chalmers joined the group for several months to collaborate with us. Welcome.
Johannes Reutlinger has defended his Thesis
His work entitled "Topology, 'Smile'-Gaps and Level Fluctuations in the Density of States of superconducting Proximity Systems" was accepted as Doctoral Thesis with Distinction. Congratulations.
Carl-Zeiss Fellowship for Robert Hussein
Robert will join the group in December 2016 as postdoc and work on noise and full counting statistics of charge and heat in mesoscopic systems
628. Wilhelm and Else Heraeus Seminar
"Trends in Mesoscopic Superconductivity"
November 14-18, 2016 in Bad Honnef / Germany
Third Funding Period for SFB 767
The DFG has granted 8.6M € for the Collaborative Research Center on Controlled Nanosystems headed by Wolfgang Belzig for the years 2016-2019
Humboldt-Fellowship for Akashdeep Kamra
Akash will work as Humboldt Fellow in the group for the years 2016/2017 on the interface between Magnetism and Superconductivity
Wolfgang Belzig elected for DPG Parliament
For the period 2015-2017 of the main committee of the German Physical Society (DPG-Vorstandrat) Wolfgang Belzig will act as representative of the university members