Publications

*Fuyuki Nabeshima, Kosuke Nagasawa, and Atsutaka Maeda,
Superconducting fluctuations in FeSe0.5Te0.5 thin films probed via microwave spectroscopy,
Physical Review B97, 024504/1-6 (2018).

[Summary] We investigated the microwave conductivity spectrum of FeSe0.5Te0.5 epitaxial films on CaF2 in the vicinityof the superconducting transition. We observed the critical behavior of the superconducting fluctuations in thesefilms with a dimensional crossover from two dimensional (2D) to three dimensional (3D) as the film thicknessincreased. From the temperature dependence of the scaling parameters we conclude that the universality classof the superconducting transition in FeSe0.5Te0.5 is that of the 3D-XY model. The lower limit of the onsettemperature of the superconducting fluctuations T onset determined by our measurementswas 1.1Tc, suggesting that the superconducting fluctuations of FeSe0.5Te0.5 are at least as large as those of optimally and overdoped cuprates.

*G. N. Phan, Kousuke Nakayama, K. Sugawara, T. Sato, T. Urata, Yoichi Tanabe, Katsumi Tanigaki, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda, and Takashi Takahashi,
Effects of strain on the electronic structure, superconductivity, and nematicity in FeSe studiedby angle-resolved photoemission spectroscopy,
Physical Review B95, 224507/1-6 (2017).

[Summary] One of central issues in iron-based superconductors is the role of structural change to the superconductingtransition temperature (Tc). Itwas found in FeSe that the lattice strain leads to a drastic increase in Tc, accompaniedby suppression of nematic order. By angle-resolved photoemission spectroscopy on tensile- or compressivestrainedand strain-free FeSe, we experimentally show that the in-plane strain causes a marked change in theenergy overlap (Eh−e) between the hole and electron pockets in the normal state. The change in Eh−e modifiesthe Fermi-surface volume, leading to a change in Tc. Furthermore, the strength of nematicity is also found to becharacterized by Eh−e. These results suggest that the key to understanding the phase diagram is the fermiologyand interactions linked to the semimetallic band overlap.

*Masamichi Nakajima, K. Yanase, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda, and Setsuko Tajima,
Gradual Fermi-surface modification in orbitally ordered state of FeSe revealedby optical spectroscopy,
Physical Review B95, 184502/1-5 (2017).

[Summary] We performed optical spectroscopy on a thin film of FeSe, in which orbital ordering shows up in theorthorhombic phase below Ts. The optical conductivity spectrum for FeSe exhibits no gap feature in the orbitallyordered state, in contrast to those for iron pnictides showing the gap opening in the magnetostructurally orderedphase. Instead,we observed a gradual suppression of the coherent carrier density with temperature. This highlightsa peculiar metallic state in FeSe that the Fermi surfaces are gradually modified below Ts. We also found ananomalous behavior of the optical phonon mode, indicating the intimate connection between the orbital andlattice degrees of freedom. The result implies the orbital origin of the structural transition.

*Yoshinori Imai, Yuichi Sawada, Fuyuki Nabeshima, Daisuke Asami, Masataka Kawai, Atsutaka Maeda,
Control of structural transiton in FeSe1-xTex thin films by changing substrate materials,
Scientific Reports 7, 46653/1-5 (2017).

[Summary] Iron chalcogenide superconductors FeSe1−xTex are important materials for investigating the relationbe-tween the superconductivity and the orbital and/or electronic nematic order, because the endmember material FeSe exhibits a structural transition without a magnetic phase transition. However,the phase separation occurs in the region of 0.1 ≤ x ≤ 0.4 for bulk samples, and it prevents the completeunderstanding of this system. Here, we report the successful fabrication of epitaxial thin films ofFeSe1−xTex with 0 ≤ x ≤ 0.7, which includes the phase-separation region, on LaAlO3 substrates viapulsed laser deposition. In the temperature dependences of differential resistivity for these films with0 ≤ x ≤ 0.3, the dip- or peak- anomalies, which are well-known to be originated from the structuraltransition in FeSebulk samples, are observed at the characteristic temperatures, T*. The dopingtemperature(x–T) phase diagram of FeSe1−xTex films clearly shows that T* decreases with increasingx, and that Tc suddenly changes at a certain Te content where T* disappears, which turns out to becommonly observed for both films on LaAlO3 and CaF2. These indicate the importance of controlling thestructural transition to achieve high Tc in iron chalcogenides.

*Fuyuki Nabeshima, Yoshinori Imai, Ichiro Tsukada, and Atsutaka Maeda,
Growth and transport properties of FeSe/FeTe superlattice thin films,
Japanese Journal of Applied Physics 56, 020308 (2017).

[Summary] Superlattice thin films composed of iron chalcogenides, FeSe and FeTe, were grown via pulsed laser deposition. The X-ray diffraction patterns show clear satellite peaks demonstrating periodic stacking structures of FeSe and FeTe. The FeTe layers have the a-axis lengths identical to those of the FeSe layers, indicating that the FeTe layers are coherently strained to the underlying FeSe. The superlattice films show superconducting transition temperatures higher than FeSe, and more importantly the superconductivity emerged in several-unit-cell-thick layers. Our results demonstrate that the strained superlattice technique is a useful tool to control superconducting properties of Fe(Se,Te) thin films.

*Yuichi Sawada, Fuyuki Nabeshima, Daisuke Asami, Ryo Ogawa, Yoshinori Imai,
Transport properties of FeSe_1-xTe_x thin films under magnetic fields up to 8 T,
Physica C 530, 27-30 (2016).

[Summary] We investigated transport properties under magnetic fields up to 8 T for FeSe1−x1−xTex thin films on CaF2. For x=0.2--0.4,x=0.2--0.4, where Tc’s are the highest, resistive broadening was observed, while the width of superconducting transition was almost the same with increasing magnetic fields in x=0--0.1,x=0--0.1, indicating differences in the nature of superconductivity between these two groups.

*Yoshinori Imai, Yuichi Sawada, Daisuke Asami, Fuyuki Nabeshima, Atsutaka Maeda,
Superconducting properties of FeSe_1-xTe_x films with x = 0－0.4,
Physica C 530, 24-26 (2016).

[Summary] We investigate superconducting properties of FeSe1−x1−xTex films with x=0−0.4,x=0−0.4, which are around the optimal Te contents, on CaF2 substrates. From the results of electrical resistivity measurements, the values of Tc in films with x=0.3,0.4x=0.3,0.4 are about 22 K. These values are very close to that of the film with x=0.2x=0.2 which was reported to be the optimal composition in FeSe1−x1−xTex films on CaF2 substrates. In addition, a sudden suppression of Tc is observed in very narrow region, that is, 0.1 < x < 0.15. This behavior is different from the dome-shaped phase diagram that is familiar in iron-based superconductors.

*Ichiro Tsukada, Ataru Ichinose, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda,
Origin of lattice compression of FeS_1-xTe_x thin films on CaF₂ substrates.,
AIP Advances 6, 095314/1-7 (2016).

[Summary] Microstructure of FeSe1-xTex thin films near the interface to CaF2 is investigated by means of transmission electron microscopy (TEM) and energy-dispersive X-ray analysis (EDX). TEM observation at the initial crystal-growth stage reveals that marked lattice compression occurs along the in-plane direction in the films with Se-rich composition, while the a-axis length of FeTe remains as its original value of bulk crystal. Subsequent EDX analysis demonstrates substantial diffusion of Se into the CaF2 substrate. Such diffusion is not prominent for Te. Thus, the formation of Se-deficient layer at the initial growth stage on CaF2 is concluded to be the main reason of the lattice compression in FeSe1-xTex thin films.

*Hideyuki Takahashi, Yoshinori Imai, Atsutaka Maeda,
Low-temperature-compatible tunneling-current-assisted scanning microwave microscope utilizing a rigid coaxial resonator,
Review of Scientific Instruments 87, 063706/1-6 (2016).

[Summary] We present a design for a tunneling-current-assisted scanning near-field microwave microscope. For stable operation at cryogenic temperatures, making a small and rigid microwave probe is important. Our coaxial resonator probe has a length of approximately 30 mm and can fit inside the 2-in. bore of a superconducting magnet. The probe design includes an insulating joint, which separates DC and microwave signals without degrading the quality factor. By applying the SMM to the imaging of an electrically inhomogeneous superconductor, we obtain the spatial distribution of the microwave response with a spatial resolution of approximately 200 nm. Furthermore, we present an analysis of our SMM probe based on a simple lumped-element circuit model along with the near-field microwave measurements of silicon wafers having different conductivities.

*Yuichi Sawada, Fuyuki Nabeshima, Yoshinori Imai, Atsutaka Maeda,
Investigation of Transport Properties for FeSe_1-xTe_x Thin Films under Magnetic Fields.,
Journal of the Physical Society of Japan 85, 073703/1-4 (2016).

[Summary] We investigated the transport properties under magnetic fields of up to 9 T for FeSe1−xTex thin films on CaF2. Measurements of the temperature dependence of the electrical resistivity revealed that for x = 0.2–0.4, where Tc is the highest, the width of the superconducting transition increased with increasing magnetic field, while the width was almost the same with increasing magnetic field for x = 0–0.1. In addition, the temperature dependence of the Hall coefficient drastically changed between x = 0.1 and 0.2 at low temperatures. These results indicate that clear differences in the nature of the superconductivity and electronic structure exist between x = 0–0.1 and x ≥ 0.2.

*Yuji Hirokane, Yasuhide Tomioka, Yoshinori Imai, Atsutaka Maeda, Yoshinori Onose,
Longitudinal and transverse thermoelectric transport in MnSi,
Physical Review B 93, 014436/1-5 (2016).

[Summary] We have investigated the longitudinal and transverse thermoelectric phenomena (Seebeck and Nernst effects) in a single crystal of intermetallic compound MnSi. The Seebeck coefficient is largely decreased (≈50%) by magnetic field around the helimagnetic transition temperature, which implies the large entropy release at the magnetic transition. We have discerned the anomalous and normal components of Nernst signal dependent on the magnetization and the magnetic field, respectively. For the anomalous Nernst effect, the intrinsic mechanism induced by the Berry phase of electrons seems dominant over the extrinsic mechanisms induced by scattering at least in the high temperature (T≥15 K) and resistive (ρ>4 μΩcm) region. The Nernst and Seebeck signals show little change in the course of the transition to skyrmion crystal.