Variational method is used to investigate, at zero temperature, the deformed- Fermi-surfaces mechanism for solving the problem of superconducting pairing of two species of fermions (i.e., spin-up and -down) of mismatched Fermi surfaces due to the existence of a uniform exchange or Zeeman field. After analyzing the depairing regions in the whole three-dimensional parameter space, we obtain a trial groundstate wave-function as a function of the three variational parameters, one of which is the gap function. Then within the frame work of the weak-coupling BCS theory, the expectation value of the Hamiltonian of a conductor under an exchange or Zeeman field is derived, from which a gap equation is derived by differentiation. The influence of deformed Fermi surfaces on the chemical potential is then calculated. Computer programing is finally used to solve the gap equation, and find the minimum-energy state with respect to the remaining two variational parameters (δµ and z). These two parameters are better than the original parameters used in the trial Hamiltonian when compared with the FF state. And we also found if we keep the total number of electrons fixed, the system prefers an unchanged chemical potential and the ground state energy of the deformed-Fermi-surfaces state, which is found to be an angle dependent case of Sarma’s solution III, is no better than that of the unpolarized BCS state.

Variational method is used to investigate, at zero temperature, the deformed- Fermi-surfaces mechanism for solving the problem of superconducting pairing of two species of fermions (i.e., spin-up and -down) of mismatched Fermi surfaces due to the existence of a uniform exchange or Zeeman field. After analyzing the depairing regions in the whole three-dimensional parameter space, we obtain a trial groundstate wave-function as a function of the three variational parameters, one of which is the gap function. Then within the frame work of the weak-coupling BCS theory, the expectation value of the Hamiltonian of a conductor under an exchange or Zeeman field is derived, from which a gap equation is derived by differentiation. The influence of deformed Fermi surfaces on the chemical potential is then calculated. Computer programing is finally used to solve the gap equation, and find the minimum-energy state with respect to the remaining two variational parameters (δμ and z). These two parameters are better than the original parameters used in the trial Hamiltonian when compared with the FF state. And we also found if we keep the total number of electrons fixed, the system prefers an unchanged chemical potential and the ground state energy of the deformed-Fermi-surfaces state, which is found to be an angle dependent case of Sarma's solution III, is no better than that of the unpolarized BCS state.

Superconducting nanowire single photon detectors (SNSPDs) can detect single photons or low levels of infrared light in applications that require high speed and low timing jitter, such as integrated circuit analysis. Most applications also require a high device detection efficiency (DDE), but the DDE of SNSPDs is limited by many factors. A good optical design with an integrated optical cavity and dielectric layers can increase the absorptance of 1550-nm light in the active area to over 90%. Therefore, optical modeling using the transfer matrix method was used to guide the design and fabrication of high-efficiency detectors with a measured DDE of over 70%. In addition, finite element analysis was used to simulate the effect of adding different types of optical antennas to SNSPD designs to increase their active area without compromising their speed, and the fabrication of antennas integrated with nanowires achieved sub-10 nm gaps between features. Thin films of niobium nitride, the starting material of the SNSPDs, were investigated using several techniques for thin film characterization, including x-ray diffraction, Auger electron spectroscopy and x-ray photoelectron spectroscopy. Optical setups based on reflectometry and transmittometry were built to determine the film thickness more accurately than deposition time for optical modeling and to provide feedback on the deposition conditions. The optical setups are able to provide reproducible and precise thickness measurements to within 0.1 nm. / Engineering and Applied Sciences

重費米子系統CeCoIn5同時具有超導態(TC= 2.3 K)與反鐵磁態(TN= 3.8 K)的特性，此一系統很適宜研究量子臨界點(QCP)問題亦即利用摻雜使超導溫度消失或壓抑至0度。研究指出藉由摻雜錫(Sn)取代部分銦(In)改變CeCoIn5的電子結構使其合成CeCoIn5-xSnx會使得超導溫度有所改變，其Tc隨著摻雜濃度x的提升呈現線性關係下降；若摻雜Ru部份取代Co合成CeCo1-yRuyIn5 晶體，隨著Ru濃度y的上升其超導溫度亦會呈線性關係下降。在這篇論文中為了研究相轉變溫度與摻雜濃度的關係，我們決定同時結合上述兩者的摻雜條件，同時摻雜Ru(電洞摻雜)與Sn(電子摻雜)，希望可以在避免大幅改變其電子結構的狀態下去觀察超導溫度如何被壓抑以觀察在QCP附近狀態改變的機制。 我們以銦做為溶劑，使用助溶劑法(indium-flux method)成長CeCo1-yRuyIn5-xSnx 晶體。並且使用X-ray粉末繞射儀與SEM做初步的結構與成分分析，判斷是否有CeCoIn5的相位並從中挑出單晶。接著分別對單晶的樣品進行磁性、電性、低溫比熱的量測，磁性的量測以檢視樣品是否有超導行為。從電阻、低溫比熱的結果觀察到雙摻雜樣品其超導溫度TC有明顯的降低，在不同的摻雜情形下，無參雜CeCoIn5晶體之Tc為2.3 K，CeCo0.8Ru0.2In5晶體之TC為2.2 K，而雙摻雜Ru與Sn之CeCo0.8Ru0.2In4.9Sn0.1之晶體TC為1.53 K，比較無摻雜及單摻雜之樣品，其TC皆已大幅下降，惟無法降到0度。顯然研究CeCo1-yRuyIn5-xSnx 晶體量子臨界點現象單單摻雜是不夠的，或許改變其他參數，如加壓或磁場可達到此一目的。 / Heavy fermion system CeCoIn5 exhibits both superconducting and antiferromagnetic states with TC= 2.3 K and TN= 3.8 K respectively. It is a good candidate for the study of quantum critical point (QCP), i.e., the superconducting temperature can be suppressed to zero by doping or external applied pressure, etc. Based on the previous studies, the Sn doped CeCoIn5-xSnx would change the electronic structure and lower its superconducting temperature； Ru doped CeCo1-yRuyIn5 would also linearly decreases its superconducting temperature as well. In this thesis, we tried to double-dope Ru (hole doping) and Sn (electron doping) into CeCoIn5 system and observe its physical properties as the suppressed superconducting temperature approachs to zero by maintaining its number of valance electrons. All CeCoIn5 single crystals are synthesized by indium-flux method, and their single-phase crystal structures are confirmed by X-ray powder diffraction analysis. The magnetic susceptibility data measured by the Superconducting Quantum Interference Device Magnetometer (SQUID) shows the superconducting behavior and the quality of these single crystals. Moreover, the electrical resistivity and the specific heat measurements indicate the decreasing of the superconducting temperature. The un-doped sample CeCoIn5, TC is 2.3 K, whereas the superconducting temperature of Ru-doped sample (CeCo0.8Ru0.2In5) is dereeased toward 2.2 K. However, for the double-doped sample of CeCo0.8Ru0.2In4.9Sn0.1, its superconducting temperature is decreased to 1.53 K, which is greatly decreased as compared with the un-doped or single doped specimens. Obviously to suppress the superconducting temperature to zero, other parameters such as external pressure or applied magnetic field etc. are required.

This dissertation describes experimental studies of how a spatially alternating magnetic field can effectively pin the magnetic flux in a superconducting thin film (Pb 82 Bi18), thereby enhancing the superconductivity. The spatially alternating magnetic field was provided by a periodic array of nano-sized magnetic structures: 300 nm spacing triangular array of cobalt rods with 100 nm diameter and 300 nm height. The superconducting film deposited on top of the magnetic structures, or an embedded Ferromagnet- Superconductor Hybrids (FSH), showed enhanced critical current and critical magnetic field. The embedded FSH also showed the field matching effect, the field compensation effect, and hysteresis. This dissertation also explains how to fabricate and characterize magnetic nano- structures. Electron beam lithography and electroplating method were used to fabricate the magnetic nanostructures. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the structures of the magnetic rods. Magnetic force microscopy (MFM) was used to study their magnetic properties.

No description available.

No description available.

In searching for novel optimal superconductors, three strategic routes based on theoretical and experimental knowledge from the known high-Tc superconductors are followed. CaFe4As3 is a newly discovered 3D compound, with Fe2+ in tetrahedral coordination, similar to that in the parent compounds of the known superconductors. The thermodynamic and transport properties reveal a spin density wave (SDW) transition at TN = 88 K, and an incommensurate-to-commensurate SDW transition at T2=26.4 K. A large electronic specific heat coefficient γ=0.02 J/molK^2 and an unusually high Kadowaki-Woods (KW) ratio A/γ^2=55×10E−5 μΩcm mol^2K^2/mJ^2 point to strong electron correlations. While the commensurate SDW state below T2 is suppressed in Co-doped CaFe4As3, neither doping with P, Yb, Co and Cu, nor application of hydrostatic pressures up to 5 GPa, is able to fully suppress the robust incommensurate SDW order in this system. The new layered compound SrMnBi2 has been studied as a promising candidate for high Tc superconductivity as suggested by theoretical calculations. We found that SrMnBi2 is structurally similar to, but more two dimensional than the known Fe superconductors. Two phase transitions at T1=292 K and T2=252 K have been observed. A large electronic specific heat coefficient γ=36.5 mJ/molK^2 and a KW ratio of 9.38×10E−5 μΩcm mol^2K^2/mJ^2 indicate enhanced electron correlations. DFT calculations have revealed metallic Sr-Bi layers in SrMnBi2, as well as Dirac-cone like features in the band structure. Doping experiments on the Mott insulator Sr2F2Fe2OS2 have been carried out to search for superconductivity at the localized-to-itinerant moment crossover. Increasing amounts of T=Mn in Sr2F2(Fe1−xTx)2OS2 suppress the long range magnetic ordering at x≈0.2, and the subsequent increase in x results in a spin glass behavior for 0.2≤x≤0.5, and possibly a new magnetic order for x≥0.5. By contrast, Co-doping increases the AFM transition from TN=106 K for x=0 up to 124 K for x=0.3. The excitation gap determined from the electrical resistivity is minimized but remains finite around x=0.5 for T=Mn. In addition, a study has been done on a rare binary type I superconductor YbSb2. Besides the superconducting transition at Tc=1.30 K, a possible second superconducting phase is observed below Tc(2)=0.41 K. From thermodynamic and transport measurements, there is strong, unambiguous evidence for the type I nature of the superconductivity in YbSb2.

In searching for novel optimal superconductors, three strategic routes based on theoretical and experimental knowledge from the known high-Tc superconductors are followed. CaFe4As3 is a newly discovered 3D compound, with Fe2+ in tetrahedral coordination, similar to that in the parent compounds of the known superconductors. The thermodynamic and transport properties reveal a spin density wave (SDW) transition at TN = 88 K, and an incommensurate-to-commensurate SDW transition at T2=26.4 K. A large electronic specific heat coefficient γ=0.02 J/molK^2 and an unusually high Kadowaki-Woods (KW) ratio A/γ^2=55×10E−5 μΩcm mol^2K^2/mJ^2 point to strong electron correlations. While the commensurate SDW state below T2 is suppressed in Co-doped CaFe4As3, neither doping with P, Yb, Co and Cu, nor application of hydrostatic pressures up to 5 GPa, is able to fully suppress the robust incommensurate SDW order in this system. The new layered compound SrMnBi2 has been studied as a promising candidate for high Tc superconductivity as suggested by theoretical calculations. We found that SrMnBi2 is structurally similar to, but more two dimensional than the known Fe superconductors. Two phase transitions at T1=292 K and T2=252 K have been observed. A large electronic specific heat coefficient γ=36.5 mJ/molK^2 and a KW ratio of 9.38×10E−5 μΩcm mol^2K^2/mJ^2 indicate enhanced electron correlations. DFT calculations have revealed metallic Sr-Bi layers in SrMnBi2, as well as Dirac-cone like features in the band structure. Doping experiments on the Mott insulator Sr2F2Fe2OS2 have been carried out to search for superconductivity at the localized-to-itinerant moment crossover. Increasing amounts of T=Mn in Sr2F2(Fe1−xTx)2OS2 suppress the long range magnetic ordering at x≈0.2, and the subsequent increase in x results in a spin glass behavior for 0.2≤x≤0.5, and possibly a new magnetic order for x≥0.5. By contrast, Co-doping increases the AFM transition from TN=106 K for x=0 up to 124 K for x=0.3. The excitation gap determined from the electrical resistivity is minimized but remains finite around x=0.5 for T=Mn. In addition, a study has been done on a rare binary type I superconductor YbSb2. Besides the superconducting transition at Tc=1.30 K, a possible second superconducting phase is observed below Tc(2)=0.41 K. From thermodynamic and transport measurements, there is strong, unambiguous evidence for the type I nature of the superconductivity in YbSb2.

In searching for novel optimal superconductors, three strategic routes based on theoretical and experimental knowledge from the known high-Tc superconductors are followed. CaFe4As3 is a newly discovered 3D compound, with Fe2+ in tetrahedral coordination, similar to that in the parent compounds of the known superconductors. The thermodynamic and transport properties reveal a spin density wave (SDW) transition at TN = 88 K, and an incommensurate-to-commensurate SDW transition at T2=26.4 K. A large electronic specific heat coefficient γ=0.02 J/molK^2 and an unusually high Kadowaki-Woods (KW) ratio A/γ^2=55×10E−5 μΩcm mol^2K^2/mJ^2 point to strong electron correlations. While the commensurate SDW state below T2 is suppressed in Co-doped CaFe4As3, neither doping with P, Yb, Co and Cu, nor application of hydrostatic pressures up to 5 GPa, is able to fully suppress the robust incommensurate SDW order in this system. The new layered compound SrMnBi2 has been studied as a promising candidate for high Tc superconductivity as suggested by theoretical calculations. We found that SrMnBi2 is structurally similar to, but more two dimensional than the known Fe superconductors. Two phase transitions at T1=292 K and T2=252 K have been observed. A large electronic specific heat coefficient γ=36.5 mJ/molK^2 and a KW ratio of 9.38×10E−5 μΩcm mol^2K^2/mJ^2 indicate enhanced electron correlations. DFT calculations have revealed metallic Sr-Bi layers in SrMnBi2, as well as Dirac-cone like features in the band structure. Doping experiments on the Mott insulator Sr2F2Fe2OS2 have been carried out to search for superconductivity at the localized-to-itinerant moment crossover. Increasing amounts of T=Mn in Sr2F2(Fe1−xTx)2OS2 suppress the long range magnetic ordering at x≈0.2, and the subsequent increase in x results in a spin glass behavior for 0.2≤x≤0.5, and possibly a new magnetic order for x≥0.5. By contrast, Co-doping increases the AFM transition from TN=106 K for x=0 up to 124 K for x=0.3. The excitation gap determined from the electrical resistivity is minimized but remains finite around x=0.5 for T=Mn. In addition, a study has been done on a rare binary type I superconductor YbSb2. Besides the superconducting transition at Tc=1.30 K, a possible second superconducting phase is observed below Tc(2)=0.41 K. From thermodynamic and transport measurements, there is strong, unambiguous evidence for the type I nature of the superconductivity in YbSb2.