Transparent conductors as solar energy materials: A panoramic review

Intermetallic compounds containing $f$-electron elements display a wealth of superconducting phases, which are prime candidates for unconventional pairing with complex order parameter symmetries. For instance, superconductivity has been found at the border of magnetic order as well as deep within ferromagnetically and antiferromagnetically ordered states, suggesting that magnetism may promote rather than destroy superconductivity. Superconducting phases near valence transitions or in the vicinity of magnetopolar order are candidates for new superconductive pairing interactions such as fluctuations of the conduction electron density or the crystal electric field, respectively. The experimental status of the study of the superconducting phases of $f$-electron compounds is reviewed.

Superconducting phases of f -electron compounds

We review pioneering and recent studies of the conductivity of solid state systems at terahertz frequencies. A variety of theoretical formalisms that describe the terahertz conductivity of bulk, mesoscopic and nanoscale materials are outlined, and their validity and limitations are given. Experimental highlights are discussed from studies of inorganic semiconductors, organic materials (such as graphene, carbon nanotubes and polymers), metallic films and strongly correlated electron systems including superconductors.

A Review of the Terahertz Conductivity of Bulk and Nano-Materials

Multipolar interactions in f -electron systems: The paradigm of actinide dioxides

In non-centrosymmetric superconductors, where the crystal structure lacks a centre of inversion, parity is no longer a good quantum number and an electronic antisymmetric spin-orbit coupling (ASOC) is allowed to exist by symmetry. If this ASOC is sufficiently large, it has profound consequences on the superconducting state. For example, it generally leads to a superconducting pairing state which is a mixture of spin-singlet and spin-triplet components. The possibility of such novel pairing states, as well as the potential for observing a variety of unusual behaviors, led to intensive theoretical and experimental investigations. Here we review the experimental and theoretical results for superconducting systems lacking inversion symmetry. Firstly we give a conceptual overview of the key theoretical results. We then review the experimental properties of both strongly and weakly correlated bulk materials, as well as two dimensional systems. Here the focus is on evaluating the effects of ASOC on the superconducting properties and the extent to which there is evidence for singlet-triplet mixing. This is followed by a more detailed overview of theoretical aspects of non-centrosymmetric superconductivity. This includes the effects of the ASOC on the pairing symmetry and the superconducting magnetic response, magneto-electric effects, superconducting finite momentum pairing states, and the potential for non-centrosymmetric superconductors to display topological superconductivity.

https://iopscience.iop.org/article/10.1088/1361-6633/80/3/036501/pdf

Superconductivity and spin-orbit coupling in non-centrosymmetric materials: a review.

Magnetic and transport properties of the cubic ternary compound ${\mathrm{Ce}}_{3}$${\mathrm{Pd}}_{20}$${\mathrm{Ge}}_{6}$ are herein reported. The electrical resistivity exhibits the Kondo lattice behavior and a large ${T}^{2}$ coefficient at low temperatures, indicating that this compound is a heavy-electron system with a Kondo temperature of a few degrees Kelvin. The low-temperature specific heat revealed two successive anomalies at ${T}_{1}=1.2$ K and ${T}_{2}=0.7$ K. The ${\ensuremath{\Gamma}}_{8}$ quartet crystalline-field ground state of Ce ions is conjectured from the thermal variation of magnetic entropy deduced from the specific-heat data. The anomaly at ${T}_{1}$ is probably a result of quadrupolar interaction between ${\ensuremath{\Gamma}}_{8}$ ground states, while the other one at ${T}_{2}$ is due to antiferromagnetic ordering.

https://ir.lib.hiroshima-u.ac.jp/files/public/1/19281/20141016133507534348/PhysRevB_53_5101.pdf

Possible quadrupolar ordering in a Kondo-lattice compound Ce3Pd20Ge6

Photoconductive antennas made on low-temperature-grown Be doped InxGa1−xAs (0.45⩽x⩽0.53) have been investigated focusing on the terahertz emission properties. In the antenna of x=0.45, the resistance as high as 3MΩ enabled us to increase the bias field up to 60kV∕cm, and the terahertz waves emitted from the antenna were significantly enhanced. In addition, terahertz waves with the spectral range over 2.5THz and the peak to noise ratio of 45dB were generated and detected using only 1.56μm pulses.

Terahertz wave emission and detection using photoconductive antennas made on low-temperature-grown InGaAs with 1.56μm pulse excitation

The authors have investigated the dc and terahertz-detection characteristics of the photoconductive antennas made on low-temperature-grown (LTG) InxGa1−xAs (0.4<x<0.53). It was found that the resistivity of the LTG In0.4Ga0.6As can be as high as 700Ωcm, with which the resistance of the antenna becomes higher than 3MΩ. Terahertz waves were detected by the antennas with the pulse excitation at 1.56μm, with a spectral range exceeding 3THz, and a dynamic range of about 55dB. The results also indicate that the photocarrier dynamics depend on the In content.

https://ir.lib.hiroshima-u.ac.jp/files/public/1/19274/20141016133457527576/ApplPhysLett_90_101119.pdf

Detection of terahertz waves using low-temperature-grown InGaAs with 1.56 μm pulse excitation

High-entropy alloys are a new class of alloys, and attract much attention due to their unique properties. Since the discovery of a superconducting high-entropy alloy (HEA) in 2014, the materials research on HEA superconductor is a hot topic. We have found that Hf21Nb25Ti15V15Zr24 body-centered-cubic (BCC) HEA is a new superconductor with the superconducting critical temperature T c of 5.3 K. We briefly discussed the comparison of cocktail effect of T c among BCC HEA superconductors.

New high-entropy alloy superconductor Hf21Nb25Ti15V15Zr24

We have developed embedded thin-film micro-strip-line-based sensor chips working in terahertz regime. In the chip, terahertz waves are generated and detected with femtosecond optical pulses accessed from the back side of the chips. Moreover, the spectroscopic sensitivity can be freely adjusted by changing the thickness of a polyimide cover layer. They make it easy to measure polar liquids. The measurement of water is demonstrated with spectral range from 30GHzto1–1.5THz, where the upper limit depends on the thickness of the cover layer.

/pdf/micro-strip-line-based-sensing-chips-for-characterization-of-4i198vnbbc.pdf

Jiro Kitagawa

Papers

Possible quadrupolar ordering in a Kondo-lattice compound Ce3Pd20Ge6

Terahertz wave emission and detection using photoconductive antennas made on low-temperature-grown InGaAs with 1.56μm pulse excitation

Detection of terahertz waves using low-temperature-grown InGaAs with 1.56 μm pulse excitation

New high-entropy alloy superconductor Hf21Nb25Ti15V15Zr24

Micro-strip-line-based sensing chips for characterization of polar liquids in terahertz regime