Direct evidence for Cooper pairing without a spectral gap in a disordered superconductor above .

The idea that preformed Cooper pairs could exist in a superconductor at temperatures higher than its zero-resistance critical temperature () has been explored for unconventional, interfacial, and disordered superconductors, but direct experimental evidence is lacking. We used scanning tunneling noise spectroscopy to show that preformed Cooper pairs exist up to temperatures much higher than in the disordered superconductor titanium nitride by observing an enhancement in the shot noise that is equivalent to a change of the effective charge from one to two electron charges. We further show that the spectroscopic gap fills up rather than closes with increasing temperature.

Shielded cantilever with on-chip interferometer circuit for THz scanning probe impedance microscopy.

We have realized a microstrip based terahertz (THz) near field cantilever that enables quantitative measurements of the impedance of the probe tip at THz frequencies (0.3 THz). A key feature is the on-chip balanced hybrid coupler that serves as an interferometer for passive signal cancellation to increase the readout circuit sensitivity despite extreme impedance mismatch at the tip.

Hybrid rf SQUID qubit based on high kinetic inductance.

We report development and microwave characterization of rf SQUID (Superconducting QUantum Interference Device) qubits, consisting of an aluminium-based Josephson junction embedded in a superconducting loop patterned from a thin film of TiN with high kinetic inductance. Here we demonstrate that the systems can offer small physical size, high anharmonicity, and small scatter of device parameters. The work constitutes a non-tunable prototype realization of an rf SQUID qubit built on the kinetic inductance of a superconducting nanowire, proposed in Phys.

Transport regimes of a split gate superconducting quantum point contact in the two-dimensional LaAlO/SrTiO superfluid.

One of the hallmark experiments of quantum transport is the observation of the quantized resistance in a point contact in GaAs/AlGaAs heterostructures. Being formed with split gate technology, these structures represent in an ideal manner equilibrium reservoirs which are connected only through a few electron mode channel. It has been a long standing goal to achieve similar experimental conditions also in superconductors.

Transport Properties of an Electron-Hole Bilayer in Contact with a Superconductor Hybrid Junction.

We investigate the transport properties of a junction consisting of an electron-hole bilayer in contact with normal and superconducting leads. The electron-hole bilayer is considered as a semimetal with two electronic bands. We assume that in the region between the contacts the system hosts an exciton condensate described by a BCS-like model with a gap Γ in the quasiparticle density of states.

Proximity-Induced Shiba States in a Molecular Junction.

Superconductors containing magnetic impurities exhibit intriguing phenomena derived from the competition between Cooper pairing and Kondo screening. At the heart of this competition are the Yu-Shiba-Rusinov (Shiba) states which arise from the pair breaking effects a magnetic impurity has on a superconducting host. Hybrid superconductor-molecular junctions offer unique access to these states but the added complexity in fabricating such devices has kept their exploration to a minimum.

Gapless Andreev bound states in the quantum spin Hall insulator HgTe.

In recent years, Majorana physics has attracted considerable attention because of exotic new phenomena and its prospects for fault-tolerant topological quantum computation. To this end, one needs to engineer the interplay between superconductivity and electronic properties in a topological insulator, but experimental work remains scarce and ambiguous. Here, we report experimental evidence for topological superconductivity induced in a HgTe quantum well, a 2D topological insulator that exhibits the quantum spin Hall (QSH) effect.

Coherent Excited States in Superconductors due to a Microwave Field.

We describe theoretically the depairing effect of a microwave field on diffusive s-wave superconductors. The ground state of the superconductor is altered qualitatively in analogy to the depairing due to a dc current. In contrast to dc depairing, the density of states acquires, for microwaves with frequency ω_{0}, steps at multiples of the photon energy Δ±nℏω_{0} and shows an exponential-like tail in the subgap regime.

Superconducting Coplanar Waveguide Filters for Submillimeter Wave On-Chip Filterbank Spectrometers.

We show the first experimental results which prove that superconducting NbTiN coplanar-waveguide resonators can achieve a loaded factor in excess of 800 in the 350 GHz band. These resonators can be used as narrow band pass filters for on-chip filter bank spectrometers for astronomy. Moreover, the low-loss coplanar waveguide technology provides an interesting alternative to microstrip lines for constructing large scale submillimeter wave electronics in general.

4π-periodic Josephson supercurrent in HgTe-based topological Josephson junctions.

The Josephson effect describes the generic appearance of a supercurrent in a weak link between two superconductors. Its exact physical nature deeply influences the properties of the supercurrent. In recent years, considerable efforts have focused on the coupling of superconductors to the surface states of a three-dimensional topological insulator.

Electronic Transport and Possible Superconductivity at Van Hove Singularities in Carbon Nanotubes.

Van Hove singularities (VHSs) are a hallmark of reduced dimensionality, leading to a divergent density of states in one and two dimensions and predictions of new electronic properties when the Fermi energy is close to these divergences. In carbon nanotubes, VHSs mark the onset of new subbands. They are elusive in standard electronic transport characterization measurements because they do not typically appear as notable features and therefore their effect on the nanotube conductance is largely unexplored.

Ballistic Josephson junctions in edge-contacted graphene.

Hybrid graphene-superconductor devices have attracted much attention since the early days of graphene research. So far, these studies have been limited to the case of diffusive transport through graphene with poorly defined and modest-quality graphene/superconductor interfaces, usually combined with small critical magnetic fields of the superconducting electrodes. Here, we report graphene-based Josephson junctions with one-dimensional edge contacts of molybdenum rhenium.

Heterodyne detection at near-infrared wavelengths with a superconducting NbN hot-electron bolometer mixer.

We report on the development of a highly sensitive optical receiver for heterodyne IR spectroscopy at the communication wavelength of 1.5 μm (200 THz) by use of a superconducting hot-electron bolometer. The results are important for the resolution of narrow spectral molecular lines in the near-IR range for the study of astronomical objects, as well as for quantum optical tomography and fiber-optic sensing.

Evidence of a nonequilibrium distribution of quasiparticles in the microwave response of a superconducting aluminum resonator.

In a superconductor, absorption of photons with an energy below the superconducting gap leads to redistribution of quasiparticles over energy and thus induces a strong nonequilibrium quasiparticle energy distribution. We have measured the electrodynamic response, quality factor, and resonant frequency of a superconducting aluminium microwave resonator as a function of microwave power and temperature. Below 200 mK, both the quality factor and resonant frequency decrease with increasing microwave power, consistent with the creation of excess quasiparticles due to microwave absorption.

Fluctuations in the electron system of a superconductor exposed to a photon flux.

In a superconductor, in which electrons are paired, the density of unpaired electrons should become zero when approaching zero temperature. Therefore, radiation detectors based on breaking of pairs promise supreme sensitivity, which we demonstrate using an aluminium superconducting microwave resonator. Here we show that the resonator also enables the study of the response of the electron system of the superconductor to pair-breaking photons, microwave photons and varying temperatures.

Probing dynamics of an electron-spin ensemble via a superconducting resonator.

We study spin relaxation and diffusion in an electron-spin ensemble of nitrogen impurities in diamond at low temperature (0.25-1.2 K) and polarizing magnetic field (80-300 mT).

Strongly disordered TiN and NbTiN s-wave superconductors probed by microwave electrodynamics.

We probe the effects of strong disorder (2.4 View Article and Find Full Text PDF

Number fluctuations of sparse quasiparticles in a superconductor.

We have directly measured quasiparticle number fluctuations in a thin film superconducting Al resonator in thermal equilibrium. The spectrum of these fluctuations provides a measure of both the density and the lifetime of the quasiparticles. We observe that the quasiparticle density decreases exponentially with decreasing temperature, as theoretically predicted, but saturates below 160 mK to 25-55/μm(3).

Phase locking of a 2.7 THz quantum cascade laser to a microwave reference.

We demonstrate the phase locking of a 2.7 THz metal-metal waveguide quantum cascade laser (QCL) to an external microwave signal. The reference is the 15th harmonic, generated by a semiconductor superlattice nonlinear device, of a signal at 182 GHz, which itself is generated by a multiplier chain (x12) from a microwave synthesizer at approximately 15 GHz.

Quasiparticle relaxation in optically excited high-Q superconducting resonators.

The quasiparticle relaxation time in superconducting films has been measured as a function of temperature using the response of the complex conductivity to photon flux. For tantalum and aluminum, chosen for their difference in electron-phonon coupling strength, we find that at high temperatures the relaxation time increases with decreasing temperature, as expected for electron-phonon interaction. At low temperatures we find in both superconducting materials a saturation of the relaxation time, suggesting the presence of a second relaxation channel not due to electron-phonon interaction.

Surface plasmon quantum cascade lasers as terahertz local oscillators.

We characterize a heterodyne receiver based on a surface-plasmon waveguide quantum cascade laser (QCL) emitting at 2.84 THz as a local oscillator, and an NbN hot electron bolometer as a mixer. We find that the envelope of the far-field pattern of the QCL is diffraction-limited and superimposed onto interference fringes, which are similar to those found in narrow double-metal waveguide QCLs.

Adiabatic quantum pumping at the Josephson frequency.

We analyze theoretically adiabatic quantum pumping through a normal conductor that couples the normal regions of two superconductor - normal-metal - superconductor Josephson junctions. By using the phases of the superconducting order parameter in the superconducting contacts as pumping parameters, we demonstrate that a nonzero pumped charge can flow through the device. The device exploits the evolution of the superconducting phases due to the ac Josephson effect, and can therefore be operated at very high frequency, resulting in a pumped current as large as a few nanoamperes.

Antenna model for wire lasers.

An antenna model is proposed for long (L >> A) lasers with subwavelength cross sections (wire lasers). It is shown that the far-field pattern of the wire lasers is determined by the ratio of the wavelength to the length. The radiation of the wire laser is predicted to be concentrated in a narrow beam theta approximately radical(2lambda/L) for laser modes where the longitudinal phase velocity is in synchronism with the velocity of light in air.

Critical voltage of a mesoscopic superconductor.

We study the influence of a voltage-driven nonequilibrium of quasiparticles on the properties of short mesoscopic superconducting wires. We employ a numerical calculation based upon the Usadel equation. Going beyond linear response, we find a nonthermal energy distribution of the quasiparticles caused by the applied bias voltage.

Magnetization of a strongly interacting two-dimensional electron system in perpendicular magnetic fields.

We measure the thermodynamic magnetization of a low-disordered, strongly correlated two-dimensional electron system in silicon in perpendicular magnetic fields. A new, parameter-free method is used to directly determine the spectrum characteristics (Landé g factor and the cyclotron mass) when the Fermi level lies outside the spectral gaps and the interlevel interactions between quasiparticles are avoided. Intralevel interactions are found to strongly modify the magnetization, without affecting the determined g* and m*.