Christian Hoffmann

Enhanced electron–phonon coupling and its irrelevence to high Tc superconductivity

It is argued that the origin of the buckling of the CuO2 planes in certain cuprates as well as the strong electron–phonon coupling of the B1g phonon is due to the electric field across the planes induced by atoms with different valence above and below. The magnitude of the electric field is deduced from new Raman results on YBa2Cu3O6+x and Bi2Sr2(Ca1−xYx)Cu2O8 with different O and Y doping, respectively. In the latter case it is shown that the symmetry breaking by replacing Ca partially by Y enhances the coupling by an order of magnitude, while the superconducting Tc drops to about two third of its original value.

In situ measurement of the permittivity of helium using microwave NbN resonators

By measuring the electrical transport properties of superconducting NbN quarter-wave resonators in direct contact with a helium bath, we have demonstrated a high-speed and spatially sensitive sensor for the permittivity of helium. In our implementation an ∼10−3mm3 sensing volume is measured with a bandwidth of 300kHz in the temperature range of 1.8–8.8K. The minimum detectable change in the permittivity of helium is calculated to be ∼6×10−11ϵ0∕Hz1∕2 with a sensitivity of the order of 10−13ϵ0∕Hz1∕2 easily achievable. Potential applications include operation as a fast, localized helium thermometer and as a transducer in superfluid hydrodynamic experiments.

Use of quantum-point-contact high-electron-mobility-transistors for time domain multiplexing of large arrays of high impedance low temperature bolometers.

The use of a multiplexing readout for an array of bolometers simplifies the electronics and wiring, so making the readout of large arrays of bolometers (>100) feasible. Here we describe a time domain multiplexing technique and its performance based on the use of quantum-point-contact high-electron-mobility-transistors as low temperature (to approximately 100 mK) switches for measuring high impedance (5...70 MOmega) resistances and sensors. The presented system is well matched to ground based millimetric astronomy demands.

Pseudogap and superconducting gap in the electronic Raman spectra of underdoped cuprates

Abstract Raman spectra of underdoped cuprate superconductors have been studied for different polarizations and temperatures. Strong anisotropies show up in both the normal and superconducting state. In the normal state below T *≈200K, a pseudogap Δ* is found near ( π ,0) with an energy scale close to that of the magnetic exchange interaction J . Superconducting gap excitations are found only at B 2 g symmetry and suggest d x 2 − y 2 pairing with an amplitude Δ 0 of 8 kT c , similar to all other doping levels.

Pseudogap and Superconducting Gap in Y-123: A Raman Study

We present results of electronic Raman-scattering experiments in differently doped YBa2Cu3O6+x. In B2gsymmetry, an analysis of the data in terms of a memory function approach is presented and dynamical relaxation rates Γ (ω, T) and mass-enhancement factors 1 + λ(ω, T) for the carriers are obtained. Starting from temperatures T > 180K, Γ(ω, T) and 1 + λ(ω,T) are extrapolated to lower temperatures and used to re-calculate Raman spectra. By comparison with our data, we find a loss of spectral weight between Tc< T < T* at all doping levels x. T* is comparable to the pseudogap temperature found in other experiments. Below Tc, the superconducting gap is observed. It depends on x and scales with Tcwhereas the energy scale of the pseudogap remains the same.

Pseudogap and Superconducting Gap in YBa2Cu3o6+x: A Raman Study

We present results of electronic Raman-scattering experiments in differently doped YBa2Cu3O6+x. In B2gsymmetry, an analysis of the data in terms of a memory function approach is presented and dynamical relaxation rates Γ (ω, T) and mass-enhancement factors 1 + λ(ω, T) for the carriers are obtained. Starting from temperatures T > 180K, Γ(ω, T) and 1 + λ(ω,T) are extrapolated to lower temperatures and used to re-calculate Raman spectra. By comparison with our data, we find a loss of spectral weight between Tc< T < T* at all doping levels x. T* is comparable to the pseudogap temperature found in other experiments. Below Tc, the superconducting gap is observed. It depends on x and scales with Tcwhereas the energy scale of the pseudogap remains the same.

Pseudogap and superconducting gap in YBa{sub 2}Cu{sub 3}O{sub 6+x}: A Raman study

We present results of electronic Raman-scattering experiments in differently doped YBa2Cu3O6+x. In B2gsymmetry, an analysis of the data in terms of a memory function approach is presented and dynamical relaxation rates Γ (ω, T) and mass-enhancement factors 1 + λ(ω, T) for the carriers are obtained. Starting from temperatures T > 180K, Γ(ω, T) and 1 + λ(ω,T) are extrapolated to lower temperatures and used to re-calculate Raman spectra. By comparison with our data, we find a loss of spectral weight between Tc< T < T* at all doping levels x. T* is comparable to the pseudogap temperature found in other experiments. Below Tc, the superconducting gap is observed. It depends on x and scales with Tcwhereas the energy scale of the pseudogap remains the same.