On the observation of superconducting phonon structure in proximity tunneling

Unusually clear phonon structure from superconducting Nb is observed in tunneling into the normal side of ultrathin AlNb (NS) proximity sandwiches. Contrary to recent reports, this phonon structure is not inverted. The possible utility of such spectra in deducing the properties of the superconducting state in the underlying S electrode is pointed out.     

Vacuum phonon tunneling

Field-induced phonon tunneling, a previously unknown mechanism of interfacial thermal transport, has been revealed by ultrahigh vacuum inelastic scanning tunneling microscopy (STM). Using thermally broadened Fermi-Dirac distribution in the STM tip as in situ atomic-scale thermometer we found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is driven by interfacial electric field and thermally vibrating image charges, and its rate is enhanced by surface electron-phonon interaction.     

Line-shapes of localized impurity phonon modes of boron acceptors in silicon

The Raman scattering cross-section of localized phonon modes due to impurities has been measured. Interference effects between the localized phonon and electronic continuum scattering amplitudes are signalled by the onset line-shape asymmetries. Qualitative analysis of the results is given in terms of the Fano theory.     

Search for E(2g) phonon modes in MgB2 single crystals by point-contact spectroscopy

The electron-phonon interaction in magnesium diboride MgB2 single crystals is investigated by point-contact (PC) spectroscopy. For the first time the electron coupling with E(2g) phonon modes is resolved in the PC spectra. The correlation between intensity of the extremely broad E(2g) modes in the PC spectra and value of the superconducting gap is established. Our observations favor current theoretical models for electron-phonon mediated superconductivity in MgB2, and they better match the harmonic phonon model.     

Sources of loss processes in phonon generation and detection experiments with superconducting tunneling junctions

Observing the phonon yield, i.e. the ratio of the experimental phonon signal amplitude and the corresponding calculated value, phonon losses within the generation-detection system can be localized and determined quantitatively. With tin junctions on pure silicon substrates immersed in liquid helium the phonon yield is 3–5%. Under vacuum conditions the yield rises to 10–12% indicating strong phonon transmission to the helium bath. The experimental lifetime for 280 GHz phonons in the silicon substrate is longer than 65 µs indicating negligible volume losses and losses at the free substrate surface. It is further shown, that volume losses inside the phonon generator and detector are small compared to the total loss of about 90%. By phonon reverberation measurements we find evidence that the main sources for phonon losses are localized at the boundaries of the tunneling junctions to the substrate. This is supported by an increase of the phonon yield with improved polishing from about 9% (mechanical), 10% (chemical) to 12% (sputter etching). A SIMS analysis indicates the presence of carbonhydrates and probably of water in the boundaries. This layer of extraneous molecules together with the nonideal surface structure of the substrate and the evaporated films weakens the mechanical bonding between the tunnel junctions and the substrate and is possibly causing strong phonon splitting by anharmonic forces.     

Decoherence in superconducting quantum bits by phonon radiation

We discuss a fundamental limitation for the coherent operation of superconducting quantum bits originating from phonon radiation generated in the Josephson junctions of the device. The time dependent superconducting phase across the junction produces an electric field that couples to the underlying crystal lattice via the piezoelectric effect. We determine the radiation resistance of the junction due to phonon emission and derive substantial decoherence rates for the quantum bits, which are compatible with quality factors measured in recent experiments.     

Desorption kinetics mediated by surface phonon modes

A quantum statistical theory of physisorption kinetics is formulated taking proper account of the surface phonon modes of a semi-infinite solid. Desorption times are shorter than those calculated with bulk phonons by a factor two to three. For mobile physisorption the rates are dominated by the Rayleigh mode, for localized physisorption by the bulk-like shear horizontal mode. An effective surface Debye temperature is deduced that is about 0.65 to 0.8 times the bulk Debye temperature.     

Gap anisotropy in superconducting alloys with localized states within the energy gap

The effect of paramagnetic impurities on the gap function of an anisotropic superconducting alloy is studied within the framework of BCS and Shiba Rusinov theory. It is found that the anisotropy of the gap function increases with increasing impurity concentration but the increase is additionally affected by the parameter ?₀ which describes the relative position of the localized state within the gap.     

Unconventional phonon spectra and obstructed edge phonon modes

Based on the elementary band representations(EBR), many topologically trivial materials are classified as unconventional ones(obstructed atomic limit), where the EBR decomposition for a set of electronic states is not consistent with atomic valenceelectron band representations. In the work, we identify that the unconventional nature can also exist in phonon spectra, where the EBR decomposition for a set of well-separated phonon modes is not consistent with atomic vibration band representations(A...     

Effect of hydrostatic pressure on the phonon spectrum of PbIn alloys using electron tunneling

The effect of hydrostatic pressure on the phonon spectrum of PbIn alloys has been studied. A shift to higher frequencies of the transverse, longitudinal and impurity band peaks is observed as well as a reduction in amplitude.     

Anomalous phonon thermal resistivity in superconducting Ti55Nb45 alloys

The thermal conductivity of cold-worked and heat-treated Ti₅₅Nb₄₅ alloys has been measured in the temperature range between 0.6 and 1.5 K. Anomalously strong scattering of phonons for every sample and the remarkable enhancement in the thermal resistivity caused by the annealing at 500°C on cold-worked alloys have been found. These behaviors indicate the existence of an anomalous phonon scattering mechanism besides dislocation scattering in Ti₅₅Nb₄₅ alloys. Electron diffraction patterns show the circular diffuse streaks characteristic of the precursory lattice distortion for the ω atomic configuration. Two kinds of models which correlate the anomalies in the phonon scattering with the lattice instability of the β phase of Ti-Nb matrix are described.     

Dispersive phonon modes by resonant Raman scattering in AgBr

Dispersive Raman modes are observed in AgBr at low temperatures when the incident laser light is tuned across the indirect Γ-L^′₃ exciton absorption. These modes occur as sidebands of the resonantly enhanced 2 TO(L), TO+LA(L) and 2 LA(L) Raman peaks and are interpreted as being due to three- and four-phonon processes, respectively. They involve longitudinal acoustic phonons near the Brillouin zone center that exhibit strong dispersion. From analysing the spectra the relaxation of the intermediate exciton state can be studied in detail. The model applied enables us to obtain directly the average effective mass of the ls indirect exciton as 1.5 electron masses.     

Strong coupling theory of superconducting alloys with localized excited states in the energy gap

Modified Eliashberg equations are derived to describe the spin exchange scattering from paramagnetic impurities. The electron-impurity scattering is described by theT-matrix element first derived by Rusinov and all scattering modes are allowed. For a Pb–Mn alloy the electronic density of statesN()/N(0) and the tunneling conductanceg(V) are calculated and compared with experimental data. Experimental data and theoretical predictions are found to agree very satisfying.Research supported by Fonds zur Förderung der wissenschaftlichen Forschung, project number: 4440     

Anomalous tunneling into magnetically ordered dilute alloys which have been induced into the superconducting state

Unusual tunneling characteristics for junctions consisting of Al, oxide and magnetic alloys, which are backed by Pb, are presented. The Pb induces the alloys to be superconducting. The alloys studied are 1500 ppm Fe in Au and 1200 ppm Co in Pd.     

Phase controlled superconducting proximity effect probed by tunneling spectroscopy

Using a dual-mode STM-AFM microscope operating below 50 mK we measured the local density of states along small normal wires connected at both ends to superconductors with different phases. We observe that a uniform minigap can develop in the whole normal wire and in the superconductors near the interfaces. The minigap depends periodically on the phase difference. The quasiclassical theory of superconductivity applied to a simplified 1D model geometry accounts well for the data.     

Soft modes and phonon interactions in studied by neutron scattering

The low frequency lattice dynamics and its relationship to the second order paraelectric-to-ferroelectric transition in Sn₂P₂S₆ is studied. The dispersion branches of the acoustic and lowest lying optical phonons in the a^*-c^* plane have been obtained in the ferroelectric phase, for x-polarized phonons. Close to the phase transition a considerable softening is found for the lowest optical mode (P_x), comparable to the behaviour observed in previous Raman investigations. As found previously in Sn₂P₂Se₆, a strong coupling between the TO(P_x) and TA(u_xz) phonons is observed, although, apparently, not strong enough to lead to an incommensurate phase. The soft TO(P_x) mode at the zone center is observed. The temperature dependence of its frequency and damping shows that the transition is not entirely displacive. At low temperatures an unusual apparent negative LO-TO splitting is observed which is shown to arise from the coupling of the x-polarized soft mode to the nearby z-polarized optical phonon. For comparison, the soft TO(P_x) dispersion in the a^*-b^* plane is measured in both the paraelectric and ferroelectric phases. Consistent frequency changes and LO-TO splitting are observed, revealing a significant interaction between the TA(u_yx) and LA(u_xx) acoustics branches and the TO and LO soft optic branches, respectively. In contrast, the nearby y-polarized optic branch shows almost no temperature dependence. Finally, the influence of piezoelectric effects on the limiting acoustic slopes in the ferroelectric phase is discussed. Received: 11 May 1998 / Revised and Accepted: 15 June 1998     

Crystal field effects in superconducting tunneling

A comparison of the tunnel curves of superconducting $\text{La}$Pr, $\text{La}$Y and $\text{La}$Gd reveals that the character of the tunneling density of states of $\text{La}$Pr is between that of the BCS- and that of the Abrikosov-Gorkov theory. This is expected on theoretical grounds for pair-breaking caused by ions in a singlet ground state. The predicted crystal field induced structures in the tunnel curves could not be detected.