ホーム » 活動状況 » 最近の発表論文

最近の発表論文

・K. Nakade et al., Scientific Reports, 6 23178 (2016)

Applications using high-Tc superconducting terahertz emitters

Using recently-developed THz emitters constructed from single crystals of the high-Tc superconductor Bi2Sr2CaCu2O8+δ, we performed three prototype tests of the devices to demonstrate their unique characteristic properties for various practical applications. The first is a compact and simple transmission type of THz imaging system using a Stirling cryocooler. The second is a high-resolution Michelson interferometer used as a phase-sensitive reflection-type imaging system. The third is a system with precise temperature control to measure the liquid absorption coefficient. The detailed characteristics of these systems are discussed.

・H. Minami et al., J. Phys.: Condens. Matter, 28 025701 (2016)

0.43 THz emission from high-Tc superconducting emitters optimized at 77 K

A liquid helium-free, compact and continuous sub-terahertz radiation system operating at 77 K has been developed using a rectangular mesa device made from a high Tc-superconducting Bi2Sr2CaCu2O8+δ single crystal, based on a different design of a stand-alone mesa sandwich structure to reduce the dc-current Joule heating effects. The mesa was thermally connected to sapphire plates through thin thermal grease embedded with diamond nano-crystals. When immersed in liquid N2 , the device emits intense radiation at 0.437 THz, the highest frequency ever achieved at 77 K, due to excitation of the TM(1,0) rectangular cavity mode. By varying the dc current–voltage bias and the bath temperature in a He-flow cryostat, the device’s emission frequency is broadly tunable from 0.31 THz at 79 K to 1.31 THz at 30 K.

・M. Tsujimoto et al., Optics Express, 24 4591 (2016)

Cavity mode identification for coherent terahertz emission from high-Tc superconductors

Stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ emit intense and coherent terahertz waves determined by the internal electromagnetic cavity resonance. We identify the excited transverse magnetic mode by observing the broadly tunable emissions from a nearly square stack and simulating the scattering spectrum. We employ a wedge-type interferometer to measure spatially-integrated power independently of the far-field pattern. The simulation results are in good agreement with observed resonance behaviors as a function of frequency.

・T. Kashiwagi et al., Phys. Rev. Applied, 4 054018.(2015)

Efficient Fabrication of Intrinsic-Josephson-Junction Terahertz Oscillators with Greatly Reduced Self-Heating Effects

The intrinsic Josephson junctions (IJJs) in the high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) are shown to have great potential for the construction of an oscillator emitting in the terahertz-frequency f regime. However, earlier devices with Bi2212 substrates exhibit strong self-heating effects detrimental to their operation and limiting the maximum f to approximately 1 THz. Here we describe an efficient fabrication procedure for a stand-alone-mesa IJJ terahertz oscillator with considerably reduced self-heating effects, greatly expanding the tunability and maximum value of f, potentially even to 15 THz. Their typical current-voltage characteristics, radiation tunability and power, and some practical uses are also presented.

・T. Kashiwagi et al., Applied Physics Letters, 107 082601 (2015)

A high-Tc intrinsic Josephson junction emitter tunable from 0.5 to 2.4 terahertz

Strong, monochromatic, coherent and continuous terahertz (THz) radiation was generated from the intrinsic Josephson junctions in a cylindrical stand-alone mesa sandwich structure fabricated from a single crystal of the high-temperature superconductor Bi2Sr2CaCu2O8+δ. By varying the base temperature and the dc bias current-voltage characteristic (IVC) points, the emission frequency is tunable from 0.5 to a record high 2.4 THz observed on two inner IVC branch points. Strong emission power peaks were observed at 1.0 THz and 1.6 THz. This device is hence an excellent candidate to fill the “THz gap” between ∼1.4 and 2.0 THz.

・T. Kashiwagi et al., Applied Physics Letters, 106 092601 (2015)

Generation of electromagnetic waves from 0.3 to 1.6 terahertz with a high-T c superconducting Bi2Sr2CaCu2O8+δ intrinsic Josephson junction emitter

To obtain higher power P and frequency f emissions from the intrinsic Josephson junctions in a high-T c superconducting Bi2Sr2CaCu2O8+δ single crystal, we embedded a rectangular stand-alone mesa of that material in a sandwich structure to allow for efficient heat exhaust. By varying the current-voltage (I-V) bias conditions and the bath temperature Tb, f is tunable from 0.3 to 1.6 THz. The maximum P of a few tens of μW, an order of magnitude greater than from previous devices, was found at Tb～55 K on an inner I-V branch at the TM(1,0) cavity resonance mode frequency. The highest f of 1.6 THz was found at Tb=10 K on an inner I–V branch, but away from cavity resonance frequencies.

・C. Watanabe et al., Applied Physics Letters, 106 042603 (2015)

Influence of the local heating position on the terahertz emission power from high-Tc superconducting Bi2Sr2CaCu2O8+δ mesas

Simultaneous measurements of spectroscopic terahertz emissions from and SiC photoluminescent local temperature T(r) distributions of high transition temperature Tc superconducting Bi2Sr2CaCu2O8+δ rectangular mesa devices were made. A local region with T(r) > Tc known as a hot spot can emerge with current bias changes. When the hot spot position was moved to a mesa end by locally heating the mesa surface with a laser beam, the intensity of the emission increased, but no changes to its frequency or line width were observed. These results suggest that higher power radiation is attainable by adjusting the hot spot position.

・T. Kitamura et al., Applied Physics Letters, 105 202603 (2014)

Broadly tunable, high-power terahertz radiation up to 73 K from a stand-alone Bi2Sr2CaCu2O8+δ mesa

High-power, continuous, broadly tunable THz radiation from 0.29 to 1.06 THz, was obtained from the outer current-voltage characteristic (IVC) branch of a single stand-alone mesa of the high-transition temperature Tc superconductor Bi2Sr2CaCu2O8+δ. The particular metallic film structures placed both beneath and atop the mesas resulted in more efficient heat dissipation, higher allowed applied dc voltages, larger IVC loops, wider emission temperature ranges, and much broader emission frequency tunability than obtained previously.

・C. Watanabe et al., J. Phys.: Condens. Matter 26 172201 (2014)

Spectral investigation of hot spot and cavity resonance effects on the terahertz radiation from high-Tc superconducting Bi2Sr2CaCu2O8+δ mesas

Terahertz (THz) electromagnetic radiation emitted from single and series-connected rectangular mesa devices of high-Tc superconducting Bi2Sr2CaCu2O8+δ is investigated spectroscopically during simultaneous temperature distribution observations using a microcrystalline SiC photoluminescence technique. In single mesas, a hot-spot region with its temperature T locally exceeding Tc was observed to jump suddenly in position under small current I-bias changes. Although these hot-spot position jumps cause large changes in the output power with small changes in I, as long as the voltage V per junction number N is kept constant, they do not affect the output frequency f, which is given by the ac Josephson frequency fJ. f can lock onto that of a particular mesa cavity resonance frequency fc , which enhances the emission power and serves as the primary mechanism for the synchronization of the emissions from each of the intrinsic Josephson junctions in the mesa.

・T. Kashiwagi et al., Applied Physics Letters, 104 082603 (2014)

Computed tomography image using sub-terahertz waves generated from a high-Tc superconducting intrinsic Josephson junction oscillator

A computed tomography (CT) imaging system using monochromatic sub-terahertz coherent electromagnetic waves generated from a device constructed from the intrinsic Josephson junctions in a single crystalline mesa structure of the high-Tc superconductor Bi2Sr2CaCu2O8+δ was developed and tested on three samples: Standing metallic rods supported by styrofoam, a dried plant (heart pea) containing seeds, and a plastic doll inside an egg shell. The images obtained strongly suggest that this CT imaging system may be useful for a variety of practical applications.

・H. Minami et al., Phys. Rev. B, 89 054503 (2014)

Local SiC photoluminescence evidence of hot spot formation and sub-THz coherent emission from a rectangular Bi2Sr2CaCu2O8+δ mesa

From the photoluminescence of SiC microcrystals uniformly covering a rectangular mesa of the high transition temperature Tc superconductor Bi2Sr2CaCu2O8+δ, the local surface temperature T(r) was directly measured during simultaneous sub-THz emission from the N ～ 103 intrinsic Josephson junctions (IJJs) in the mesa. At high bias currents I and low bath temperatures T bath ≦ 35 K, the center of a large elliptical hot spot with T (r) > Tc jumps dramatically with little current-voltage characteristic changes. The hot spot does not alter the ubiquitous primary and secondary emission conditions: the ac-Josephson relation and the electromagnetic cavity resonance excitation, respectively. Since the most intense sub-THz emission was observed for high T bath ≧ 50 K in the low I bias regime where hot spots are absent, hot spots cannot provide the primary mechanisms for increasing the output power, the tunability, or promoting the synchronization of the N IJJs for the sub-THz emission, but can at best coexist nonmutualistically with the emission. No T(r) standing waves were observed.

・T. Kashiwagi et al., Applied Physics Letters, 104 022601 (2014)

Reflection type of terahertz imaging system using a high-Tc superconducting oscillator

A reflection type of imaging system is shown at sub-terahertz frequencies generated from high-Tc superconducting intrinsic Josephson junction mesa structures fabricated by single crystalline Bi2Sr2CaCu2O8+δ to demonstrate how the sub-terahertz imaging technique using monochromatic radiation is powerful and unique for the variety of practical applications. Several examples are discussed in detail and are compared to other terahertz imaging systems.
・S. Sekimotoet al., Applied Physics Letters, 103 182601 (2013)

Continuous 30 μW terahertz source by a high-Tc superconductor mesa structure

Using a modified mesa structure of high-Tc superconducting Bi2Sr2CaCu2O8+δ with a thin underlaying base superconductor (~3 μm), the effective working temperature of the continuous and monochromatic terahertz emitter is extended up to 70 K, and the maximum power of ~30 μW at 0.44 THz is achieved at the relatively high temperature of T = 55 K in a low bias current retrapping region. The diverging behavior of the intensity occurring at 55 K in the low current regime without hot spot formation may provide us an important clue for the stronger THz radiation from intrinsic Josephson junction devices.
・S.V. Chonget al., EPL, 104 17002 (2013)

Large Low-Temperature Magnetoresistance in SrFe2As2 Single Crystals

We present the first report on a large low-temperature magnetoresistance (MR) of more than 1600% in a SrFe2As2 single crystal and 1300% in a low-energy Ca ion-implanted SrFe2As2 single crystal that occurs before the emergence of crystallographic strain-induced bulk superconductivity arising from a sample aging effect. In accordance to band structure calculations from the literature, which consitently show that more than 2 bands are involved in the transport, we have modeled this large MR at high fields using a 3-carrier scenario rather than solely on quantum linear MR model generally used to explain the MR in iron-pnictides. At and below 20 K the large MR may be due to 3-carrier transport in an inhomogeneous state where there are superconducting and metallic regions.
・K. Delfanazari et al., J Infrared Milli Terahz Waves DOI 10.1007/s10762-013-0027-y

Terahertz Oscillating Devices Based Upon the Intrinsic Josephson Junctions in a High Temperature Superconductor

Recent developments of coherent terahertz (THz) oscillators based on the intrinsic Josephson junctions (IJJs) in mesas of the high temperature superconductor Bi2Sr2CaCu2O8+δ are reviewed. Experimental and theoretical studies of the emission from equilateral, right-angled isosceles, and acute isosceles triangular mesas are compared with those obtained from rectangular, square, and disk mesas, in order to determine the role of the mesa geometry. The superconducting properties and emission frequency spectra are presented for a variety of triangular mesa geometries
・T. Benseman　et al., J. Appl. Phys. 113 133902 (2013)

Direct Imaging of Hot Spots in Bi2Sr2CaCu2O8+δ Mesa Terahertz Sources

Stacks of intrinsic Josephson junctions (IJJs) made from high-temperature superconductors such as Bi2Sr2CaCu2O8+δ (Bi-2212) (BSCCO) are a promising source of coherent continuous-wave terahertz radiation. It is thought that at electrical bias conditions under which THz-emission occurs, hot spots may form due to resistive self-heating, and that these spots may be highly beneficial for the generation of high levels of THz power. Here, we perform an imaging study of the temperature distribution at the surface of BSCCO stacks utilizing the temperature-dependent 612nm fluorescence line of Eu3+ in a europium chelate. The images directly reveal a highly non-uniform temperature distribution in which the temperature in the middle of the stack can exceed the superconducting transition temperature by tens of Kelvin under biasing conditions typical for THz-emission
・K. Delfanazari　et al., OPTICS EXPRESS. 21, 2171 (2013)

Tunable Terahertz Emission from the Intrinsic Josephson Junctions in Acute Isosceles Triangular Bi2Sr2CaCu2O8+δ Mesas

In order to determine if the mesa geometry might affect the properties of the coherent terahertz (THz) radiation emitted from the intrinsic Josephson junctions in mesas constructed from single crystals of the high-temperature superconductor, Bi2Sr2CaCu2O8+δ , we studied triangular mesas. For equilateral triangular mesas, the observed emission was found to be limited to the single mesa TM(1,0) mode. However, tunable radiation over the range from 0.495 to 0.934 THz was found to arise from an acute isosceles triangular mesa. This 47% tunability is the widest yet observed from the outer current-voltage characteristic branch of such mesas of any geometry. Although the radiation at a few of the frequencies in the tunable range appear to have been enhanced by cavity resonances, most frequencies are far from such resonance frequencies, and can only be attributed to the ac-Josephson effect.
・M. Tsujimoto　et al., J. Appl. Phys. 111, 123111 (2012)

Terahertz imaging system using high-Tc superconducting oscillation devices

Microwatt power oscillation devices at sub-terahertz frequency region between 0.3 and 1.0 terahertz (THz) were fabricated from high-Tc superconducting single crystalline Bi2Sr2CaCu2O8+δ and used as a source of the transmission terahertz imaging system. As test examples, terahertz images of coins and a razor blade placed inside the brownish paper envelopes with the spatial resolution of 1 mm are presented. The signal-to-noise ratio exceeds 130 in these images. Using a simple wedge-shaped interferometer and analysing the interference fringe pattern, the wavelength of the terahertz wave is calibrated within 0.1% accuracy. This interferometer also provides a simple method to measure the absorption coef?cient of the liquid sample. Two test measurements for distilled water and ethanol are demonstrated and their absorption coef?cients are obtained with 99.2% accuracy. This suggests that our terahertz imaging system can be applied to many practical applications, such as biological and biomedical imaging, environmental monitoring, microanalysis of impurities, structure and dynamical analyses of large molecules and ions in solution.
・M. Tsujimoto　et al., Phys. Rev. Lett. 108, 107006 (2012).

Broadly Tunable Sub-terahertz Emission from Internal Branches of the Current-voltage Characteristics of Superconducting Bi2Sr2CaCu2O8+δSingle Crystals

Abstract : Continuous, coherent subterahertz radiation arises when a dc voltage is applied across a stack of the many intrinsic Josephson junctions in a Bi2Sr2CaCu2O8+δsingle crystal. The active junctions produce an equal number of I-V characteristic branches. Each branch radiates at a slightly tunable frequency obeying the Josephson relation. The overall output is broadly tunable and nearly independent of heating effects and internal cavity frequencies. Amplification by a surrounding external cavity to allow for the development of a useful high-power source is proposed.
・H. Asai　et al., Phys. Rev. B 85, 064521 (2012).

Three-dimensional numerical analysis of terahertz radiation emitted from intrinsic Josephson junctions with hot spots

Abstract : In this study, we numerically investigate the terahertz radiation from mesa-structured intrinsic Josephson junctions (IJJs) using a three-dimensional calculation model. We assume an in-phase mode of the phase differences and calculate electromagnetic fields inside and outside of the IJJs simultaneously. We consider the appearance of a hot spot in the mesa where jc locally decreases and investigate the change of the radiation power with varying hot-spot positions. The radiation powers for three different hot-spot positions are calculated as functions of voltage. We observe strong radiation when the ac Josephson frequency satisfies the cavity resonance condition. Transverse-magnetic modes TMm,n whose indices m and n are even appear regardless of the positions of hot spots. Meanwhile, the TMm,n cavity modes whose m or n are odd appear only when the hot spots break the reflectional symmetry of the mesa structure. Moreover, we calculate the radiation patterns emitted by the IJJs at these cavity resonance conditions. The radiation patterns reflect the existence of two types of internal modes, that is, a uniform background mode and a cavity resonance mode.
・H. Minami　et al.,
　　IEICE (The Institute of Electronics, Information and Communication Engineers). E95-C, 347 (2012).

Terahertz radiation emitted from intrinsic Josephson junctions in high-Tc superconductor Bi2Sr2CaCu2O8+δ

Abstract :The present status of superconducting terahertz emitter using the intrinsic Josephson junctions in high-Tc superconductor Bi2Sr2CaCu2O8+δ is reviewed. Fabrication methods of the emitting device, electrical and optical characteristics of them, synchronizing operation will be discussed. After the description of fabrication techniques by an Argon ion milling with photolithography or metal masks and by a focused ion beam, optical properties of radiation spectra, the line width, polarization and the spatial distribution of emission are presented with some discussion on the operation mechanism. For electrical properties, reversible and irreversible operations at high and low electrical currents, respectively, and electrical modulation of the radiation intensity for terahertz imaging are presented.
・T. Kashiwagi　et al., Japanese Journal of Applied Physics 1, 010113 (2012).

High Temperature Superconductor Terahertz Emitters: Fundamental Physics and Its Applications

Abstract : Coherent and continuous radiation sources of the electromagnetic (EM) waves at terahertz (1 THz = 1012 c/s) frequencies using a mesa structure fabricated from high temperature superconducting Bi2Sr2CaCu2O8+δ single crystals are described with a special emphasis on the physics of the radiation mechanism and the applications. After the intensive studies of many mesas fabricated with different conditions, it is revealed that the ac-Josephson effect works as a primary driving mechanism of the radiation and the cavity resonance needed for stronger radiation plays an additional role to the mechanism, although both are working together while radiating. A prototype of the imaging machine for multipurpose uses has successfully been developed.
・T. Kashiwagi　et al., J. Phys. Soc. Jpn. 80, 094709 (2011).

Geometrical Full-Wavelength Resonance Mode Generating Terahertz Waves from a Single-Crystalline Bi2Sr2CaCu2O8+δ Rectangular Mesa

Abstract : The terahertz radiation from a rectangular mesa of single-crystalline Bi2Sr2CaCu2O8+δ with dimensions of 80 ×320 ×1.6 µm3 fabricated by Ar and focused ion bean milling techniques was studied by Fourier transform infrared (FTIR) spectroscopy and angular distribution measurement of the radiation intensity. The emission measured using the FTIR spectrometer is centered at 0.9219 THz, which corresponds to exactly twice of that previously observed from mesas of the same width. The spectral width was as narrow as 7.5 GHz, which is limited by the instrumental resolution. The radiation obeys the ac Josephson relation and has a tunability of about 7% in frequency. The angular distribution of the emission power was analyzed using the conventional patch antenna theory. Both results strongly suggest that the excitation mode with a standing full-wavelength may be observed across the mesa width as the fundamental mode, instead of the half-wavelength mode commonly observed previously.
・M. Tachiki　et al., Phys. Rev. B 83, 014508 (2011).

Emission of terahertz electromagnetic waves from intrinsic Josephson junction arrays embedded in resonance LCR circuits

Abstract : The emission of terahertz electromagnetic waves from an intrinsic Josephson junction array (IJJA) embedded in an LCR resonant circuit is studied theoretically. A bias current is applied to the electrodes at the top and bottom of the array. In the voltage state, the ac Josephson current generates a displacement current in the IJJA, and both the currents induce an oscillating current in the electrodes. We describe the whole system, including the array and the environment around it, in terms of an LCR resonant circuit. When the Josephson frequency is in the resonance frequency region of the LCR circuit, the amplitudes of the displacement current in the Josephson junction array and the oscillating current in the electrodes both are strongly enhanced by a feedback process. We calculate the emission power and the current-voltage (I-V) characteristic curve for the system. Inside the frequency region of the LCR circuit resonance, stable and intense emission occurs in both the increasing and decreasing processes of the high-bias current. In the emission region the I-V characteristic curve has a dip structure. These results are consistent with those of the emission observed in a high-bias current region by using mesa-shaped samples of Bi2Sr2CaCu2O8+δ . We also discuss the difference between the properties of the emission and the I-V characteristic curve for intrinsic Josephson junctions embedded in and shunted by the LCR resonant circuit.
・Pradip Das　et al., Phys. Rev. B 83, 220513 (2011).

Spin-triplet vortex state in the topological superconductor CuxBi2Se3

Abstract : We report on the observation of bulk superconductivity from dc magnetization measurements in a cylindrical single crystal of CuxBi2Se3 . The magnitude of the magnetization in the Meissner state is very small, and the magnetic-field dependence of the magnetization just above the lower critical field Hc1 is very different from those of usual type-II superconductors. We studied the character of the vortex state theoretically in a spin-triplet pairing superconductor and compared it with the experimental results. The results showed that the superconductivity observed in CuxBi2Se3 is consistent with the spin-triplet pairing superconductivity with odd parity. We also observed a rapid relaxation phenomenon of the superconducting diamagnetism.
・M. Tsujimoto et al., Phys. Rev. Lett. 105, 037005 (2010).

Geometrical Resonance Conditions for THz Radiation from the Intrinsic Josephson Junctions in Bi2Sr2CaCu2O8+δ

Abstract : Subterahertz radiation emitted from a variety of short rectangular-, square-, and disk-shaped mesas of intrinsic Josephson junctions fabricated from a Bi2Sr2CaCu2O8+δ single crystal was studied from the observed I-V characteristics, far-infrared spectra, and spatial radiation patterns. In all cases, the radiation frequency satisfies the conditions both for the ac Josephson effect and for a mesa cavity resonance mode. The integer higher harmonics observed in all spectra imply that the ac Josephson effect plays the dominant role in the novel dual-source radiation mechanism.

テラヘルツギャップを埋める新たな発振素子として注目されているメサ型固有ジョセフソン素子に関して、本研究ではメサの幾何学的共振条件を詳細に検討 した。我々は高温超伝導体Bi2Sr2CaCu2O8+δ の単結晶を矩形、正方形、および円盤メサ形状に微細加工し、試料のI-V特性、遠赤外スペクトル測定、および空 間放射パターン測定を行った。観測した発振周波数とメサの幾何学的形状の対応関係は発振原理として提案されている空洞共振効果を強く支持し、また異方 的な空間放射パターンは空間に一様な電磁波発振源の存在を示唆している。さらに円盤試料における整数倍の高調波の存在は、交流ジョセフソン電流が放射 源として大きく寄与することを示す重要な実験事実である。

・K. Kadowaki et al., J. Phys. Soc. Jpn. 79, 023703 (2010).

Evidence for a Dual-Source Mechanism of Terahertz Radiation from Rectangular Mesas of Single Crystalline Bi2Sr2CaCu2O8+δ Intrinsic Josephson Junctions

Abstract : The THz radiation emitted from rectangular mesas of single-crystal Bi2Sr2CaCu2O8+δ was studied using angular distribution measurements and Fourier transform infrared spectroscopy. Unlike the recent theoretical predictions, the results provide strong evidence for a dual-source mechanism in which the uniform and non-uniform parts of the ac-Josephson current act as electric and magnetic current sources, respectively. The latter synchronizes with cavity modes of the mesa with integral harmonics of the fundamental radiation.