Koichiro Ienaga

Orbital Selectivity in Scanning Tunneling Microscopy: Distance-Dependent Tunneling Process Observed in Iron Nitride

In scanning tunneling microscopy, orbital selectivity of the tunneling process can make the topographic image dependent on a tip-surface distance. We have found reproducible dependence of the images on the distance for a monatomic layer of iron nitride formed on a Cu(001) surface. Observed atomic images systematically change between a regular dot array and a dimerized structure depending on the tip-surface distance, which turns out to be the only relevant parameter in the image variation. An accompanied change in the weight of Fe-3d local density of states to a tunneling background was detected in dI/dV spectra. These have been attributed to a shift in surface orbitals detected by the tip from the d states to the s/p states with increasing the tip-surface distance, consistent with an orbital assignment from first-principles calculations.

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.

New phase diagram of PrPb3 in [100] magnetic filed direction

We measured the magnetic field dependence of the specific heat in PrPb 3 in the field direction H ∥[100] up to H =8 T. At H =0 T, an antiferro-quadrupole ordering occurs at T Q =0.43 K, which is higher than T Q =0.4 K in previous measurements, and is evidence of the high quality of the sample. We describe a new T – H phase diagram in H ∥[100]. Note that a high-field phase appears above the low-field phase with the O 2 0 order parameter, which is likely the O 2 2 phase assisted by the octupole moment.

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.

Magnetic phase diagram of DMACuCl3

Result of specific heat measurements on the single crystal of DMACuCl3 is presented and compared with previous one obtained for the powder crystal. Possible reason for the marked difference between them is discussed on the basis of competition between critical fluctuation in the magnon BEC process and short correlation length in the powder crystal.

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.

Specific heat study of geometrically frustrated magnet clinoatacamite Cu2Cl(OH)3

We have performed the specific heat study in a new geometrically frustrated system, clinoatacamite Cu2Cl(OH)3 with the corner-sharing tetrahedron structure of the Cu2+ ions with 5=1/2 Heisenberg spin. At H=0 T, two anomalies are observed at T2=18.1 K and at T2=6.2 K. The specific heat decreases rapidly below T2 and shows no anomaly down to T=150 mK despite the existence of the spin fluctuation shown in the μSR experiments. As the magnetic field is increased, the sharp peak at T2 is broadened and shows a small reentrant behavior in the T – H phase diagram. On the other hand, the peak at T1 shows no obvious change up to H=5 T. The entropy at T1 is estimated as ~0.35Rln2 at H=0 T. These features may be caused by the two dimensional nature of the kagome antiferromagnets which are weakly coupled via Cu2+ ions at the triangular sites located in between the kagome layers.

Modulation of Electron–Phonon Coupling in One-Dimensionally Nanorippled Graphene on a Macrofacet of 6H-SiC

Local electron-phonon coupling of a one-dimensionally nanorippled graphene is studied on a SiC(0001) vicinal substrate. We have characterized local atomic and electronic structures of a periodically nanorippled graphene (3.4 nm period) prepared on a macrofacet of the 6H-SiC crystal using scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES). The rippled graphene on the macrofacets distributes homogeneously over the 6H-SiC substrate in a millimeter scale, and thus replica bands are detected by the macroscopic ARPES. The STM/STS results indicate the strength of electron-phonon coupling to the out-of-plane phonon at the K̅ points of graphene is periodically modified in accordance with the ripple structure. We propose an interface carbon nanostructure with graphene nanoribbons between the surface rippled graphene and the substrate SiC that periodically modifies the electron-phonon coupling in the surface graphene.

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.