Hiroyuki Tsujii

Study of ferromagnetic transition in Pd nanometer-scale constrictions using a mechanically controllable break junction technique

We measured the magnetoresistance (MR) of Pd nanometer-scale constrictions while changing the constriction size by stretching the Pd wire using a mechanically controllable break junction technique. In large constrictions, the resistance is constant for |H|≤ 400 Oe, reflecting bulk Pd paramagnetic properties. As size decreases to ∼10 nm, the MR effect with hysteresis is observed with a sweeping field, indicating the ferromagnetic transition in Pd constrictions. We proposed this technique to detect ferromagnetic transitions caused by downsizing in paramagnetic metals and to study magnetic properties after the transitions.

Ultrasound Study of the Solid–Liquid Transition and Solid–Liquid Interface of 4He in Aerogels

The freezing and melting of 4 He in various aerogels with porosities ranging from 92 to 97% were studied using longitudinal ultrasound. The freezing pressure, detected from changes in velocity and attenuation, was elevated compared with that of the bulk by about 0.3 MPa, and showed a weak dependence on aerogel porosity. Inside the aerogels, there was a liquid phase, a solid phase or a coexisting state depending on temperature and pressure of the sample. We report the measurement of the transmission of sound through the solid–liquid interface in aerogel for the first time, which was independent of temperature and unaffected by a small addition of 3 He, unlike that at the bulk interface. This indicates that sound attenuation at the interface is due to the disorder, originating from silica strands. Small amounts of 3 He in the solid significantly decreased attenuation, because the resultant pinning of dislocations by 3 He suppressed phonon scattering, as is observed in bulk solid 4 He.

Fe magnetic impurity effect in Au atomic sized conductor

We have studied the electrical conductance in Au nanowire containing 0.07 at.%Fe ions (AuFe nanowire) with mechanically controllable break junction technique to investigate the magnetic impurity effects in the atomic-sized conductance. At room temperature, we observe not only conductance steps at the integer multiples of G0 = 2e2/h, but also steps deviating from them in AuFe nanowire. Moreover, these features persist down to T = 4.2 K. The scattering between the conduction electrons and Fe magnetic ions may lift the spin degeneracy of the transmission probability, which is responsible for the deviation. Zero bias anomaly is observed in the AuFe nanowire with the contact diameter larger than ~3 nm in the current-voltage (I-V) measurements at T = 4.2 K, which may be caused by Kondo effect.

Spectroscopic study of low-temperature hydrogen absorption in palladium

We report real-time detection of hydrogen (H) absorption in metallic palladium (Pd) nano-contacts immersed in liquid H2 using inelastic electron spectroscopy (IES). After introduction of liquid H2, the spectra exhibit the time evolution from the pure Pd to the Pd hydride, indicating that H atoms are absorbed in Pd nano-contacts even at the temperature where the thermal process is not expected. The IES time and bias voltage dependences show that H absorption develops by applying bias voltage 30 ∼ 50 mV, which can be explained by quantum tunneling. The results represent that IES is a powerful method to study the kinetics of high density H on solid surface.

Electron tunneling measurements in atomic scale gap filled with liquid 4He below 4.2K

We report the tunneling spectroscopy investigation in an atomic scale gap filled with liquid 4He using mechanically controllable break junction (MCBJ) technique. In order to assure the filling of liquid 4He into the gap, we construct a cryostat with an inner chamber for the tunneling spectroscopy inside the vacuum jacket of the liquid 4He bath. MCBJ apparatus is installed in the inner chamber with a flexible bellows. After filling inner chamber with liquid 4He below 4.2 K, Au electrical electrodes were stretched by the mechanical force generated by a piezo device. We observed the increase of the tunnel conductance through liquid 4He compared to that in the vacuum environment.

Low-Temperature hydrogen absorption into v and Nb metals from liquid hydrogen

We report experimental study on low-temperature hydrogen (H) absorption in vanadium (V) and niobium (Nb) nanocontacts below T = 20 K using a point-contact spectroscopy (PCS) technique. When a small bias voltage is applied between both sides of nanocontacts immersed in liquid H2, the differential conductance (dI/dV) and the second derivative (d2 I/dV 2) are changed from those for pure V and Nb nanocontacts. Further, the spectra approach to those for a high concentrated phase of H with increasing the bias voltage. The results indicate that in-situ investigation of H absorption process from liquid H2 is possible through dI/dV and d2 I/dV 2 measurements using the PCS technique.

Local conductance spectra of itinerant ferromagnetic SrRuO3 through break junction

We have measured the local differential conductance spectra (dI/dV–V) of an itinerant ferromagnet composed of polycrystalline SrRuO3 using the mechanically controllable break junction technique. Below the materials Curie temperature (T C = 160 K), characteristic peak or dip conductance spectra are observed. The characteristic energy scale is comparable to the exchange spin splitting energy that is based on ferromagnetic band calculations. Both the peak and dip spectral shapes are explained based on the itinerant ferromagnetic characteristics of SrRuO3 in terms of spin-dependent transmission, which is similar to the giant magnetoresistance mechanism.

Magnetic and superconducting properties of vanadium nanoconstrictions

We study the magnetic and superconducting properties in a paramagnetic vanadium (V) nanoconstriction with changing its size using a mechanically controllable break junction technique. In the normal state the magnetoresistance effect is observed below the diameter d ~ 8 nm. Moreover, a Fano resonance appears around zero-bias voltage in the differential conductance for the atomic-size contacts and changes shape as the size of the constriction changes. On the other hand, below the superconducting critical temperature the superconducting gap features in V contacts are largely different from those in Pb contacts which exhibit typical features expected in superconducting point contacts. Only a single Andreev anomaly at 2Δ, where Δ is the superconducting energy gap, is observed in the spectra of the V contacts, while two anomalies at Δ and 2Δ appear in the case of the Pb contacts. In the tunnel conductance regime, the structure of the superconducting quasiparticle tunneling density of states is not seen in the V spectra in contrast to the Pb spectra. The origin of these features is discussed.

Control of nanosize ferromagnetic electrodes by magnetostriction

We show the relation between the conductance and the magnetostriction by the applied field in the atomic-sized Ni wire. The magnetostriction is available for the fine control of the displacement between the two Ni electrodes.

Magnetic impurity effect in atomic sized conductor

We have studied the electrical conductance in Au nanowire containing 0.07 at.% Fe ions and Cu nanowire containing 13 at.% Mn ions with mechanically controllable break junction technique to investigate the magnetic impurity effects in the atomic-sized conductance. We observe not only conductance steps at the integer multiples of G0 = 2e2/h, but also steps deviating from them. The scattering between the conduction electrons and impurity ions lifts the spin degeneracy of the transmission probability, which may be responsible for the deviation.