A Revisit of Superconductivity in 4Hb-TaS2−2xSe2x Single Crystals

The charge-density wave (CDW) is a ground state with broken translational symmetry of metals, brought by electron-phonon interactions. It was recognized that highly anisotropic band structures are important in leading to these ground state. And the ground states are coherent superposition of electron-hole pairs, and result in a periodic space variation of charge density as name. The CDW ground state is also accompanied by the opening of energy gap at the Fermi surface. Since the energy gap forms within the former conduction band, a 1D metal would be expected to become insulating in CDW state. Rare-earth transition-metal ternary silicides with three-dimensional crystallographic structures, such as the R5T4Si10 and R2T3Si5 types, have been shown to exhibit CDW phase transitions with remarkable anomalies observable in the thermal and electrical transport measurements. In this thesis, I describe the experimental results of probing CDW formation in two different compounds of 3-D materials: Dy5Ir4Si10 and Lu2Ir3Si5. The CDW study in Dy5Ir4Si10 The tetragonal rare-earth transition-metal silicide system, R5T4Si10, where R is Dy, Ho, Er, Tm, and Lu, and T = Ir and Rh, with seemingly three-dimensional crystallographic structure, has been shown to exhibit fascinating charge density wave (CDW) phase transitions, a phenomenon found largely in low-dimensional systems. In this study we report the investigations of CDW in Dy5Ir4Si10 at different temperatures using transmission electron microscopy (TEM) techniques including electron diffraction and dark-field imaging. Superlattice diffraction spots along c-axis were observed in the electron diffraction pattern when the sample was cooled below the CDW transition temperature (TCDW ~ 200K), indicating the presence of incommensurate CDW state with the modulation wave vector of . CDW become commensurate with further cooling below ~ 160K. Configurations of CDW dislocations convincingly show that the CDW phase transition is accompanied by a concomitant cell-doubling crystallographic structural phase transition. Furthermore, symmetry breakdown along c-axis observed by convergent beam electron diffraction (CBED) gives rise to two different type of CDW domains. Detailed characteristics of this unusual behavior will also be discussed. The CDW study in Lu2Ir3Si5 We report the investigation of charge density wave (CDW) in Lu2Ir3Si5 by electron diffraction and dark-field imaging using superlattice diffraction spots. The CDW state is confirmed by the presence of superlattice reflections. Most interestingly, the CDW state at low temperatures is found to be electronically phase-separated with the coexistence of CDW domains and low-temperature normal phase domains. Upon change of temperatures, unlike other typical incommensurate CDW systems in which commensurability varies with temperatures, we find that commensurability remains unchanged in the present case and the predominant change is in the redistribution of the area ratio of the two coexisted phases, which is clearly revealed in the dark-field images obtained from the CDW superlattice reflections. The electronic phase separation in the CDW state of Lu2Ir3Si5 is unprecedented in CDW systems, and its temperature dependence is also anomalous.

Polycrystalline TiSe2 and its Iron intercalated samples have been successfully grown by solid state synthesis method. The electrical transport property of pure sample is semiconducting in the entire temperature range with a clear anomaly depicting the onset of charge density wave (CDW) at 200K. Temperature that corresponds to CDW peak and CDW transition onset (TCDW-onset & TCDW-peak) decreases linearly with concentration of Iron (Fe). The 7.5% Fe intercalated sample shows a crossover from semiconductor to metallic nature at 145 K with linear contribution at high temperature side. Below 145K the resistance was fitted with -lnT contribution, which may be associated with Kondo like interactions due to the guest Fe moments with the conduction electrons of the host. However, the robustness of the resistance variation in magnetic fields point towards other mechanisms as well.

At low temperatures, the electronic structure of metals becomes unstable, and instabilities often develop correlated electronic states, decreasing the number of free carriers. One of these correlated states is a charge density wave state. It usually arises under the interaction of free carriers with phonons, and it is manifested as a superstructure over the original crystal lattice. The charge density wave state presents distinguishable experimental fingerprints in many physical properties of the crystal, spontaneously breaking its translational symmetry. The physics of the charge density wave is well established in the case of a one-dimensional metal; however, there are lot of open issues, when describing the systems of higher dimensions. The charge density wave state could potentially be utilized in low-energy electronic devices. Transition metal dichalcogenides are quasi two-dimensional materials, that exhibit a correlated electronic behavior, such as the superconductivity and the charge density wave. Due to their simple crystalline structures, nearly perfect crystals can be grown, and the electronic correlations can be studied in the absence of any disorder. An example of this class of materials is 2H-NbSe2, that undergoes both the charge density wave (T_CDW=33.5 K) and superconducting (T_SC=7.2 K) transitions. The first part of this work is focused on the single crystal growth, electronic transport characterization, and results of ultrafast transient reflectivity measurements of 2H-NbSe2 single crystals. Coherent lattice dynamics and characteristic relaxation times of photoexcited charge carriers have been extracted from the broadband transient optical spectroscopy. A wide temporal profile gives opportunity to track multiple ultrafast phenomena within the same experiment from 50 fs to few nanoseconds. The physical mechanisms leading to the charge density wave condensation are being discussed here. The second part of this work is focused on a studying of the temperature hysteresis in electrical transport properties in freshly grown undoped and doped 1T-TiSe₂ single crystals. This transition metal dichalcogenide undergoes a chiral charge density wave at low temperatures (T_CDW=202 K). An irreversible behavior of electrical properties upon the thermocycling indicates an existence of metastable states in the charge density wave phase, and it is commonly attributed to additional phase transitions. The absence of strong experimental evidence of these transitions is collaborated with an opened dispute on the nature of charge density wave state in this material. Physical origins of this phenomenon, including a complex charge density wave domain structure of 1T-TiSe₂, are being discussed in the second part of the work.

The structural characterization and electrical transport measurements at ambient and applied pressures of the compounds of the LaAg1−xAuxSb2 family are presented. Up to two charge density wave (CDW) transitions could be detected upon cooling from room temperature and an equivalence of the effects of chemical and physical pressure on the CDW ordering temperatures was observed with the unit cell volume being a salient structural parameter. As such LaAg1−xAuxSb2 is a rare example of a non-cubic system that exhibits good agreement between the effects of applied, physical, pressure and changes in unit cell volume from steric changes induced by isovalent substitution. Additionally, for LaAg0.54Au0.46Sb2 anomalies in low temperature electrical transport were observed in the pressure range where the lower charge density wave is completely suppressed.

A model-based method to perform odometry using an array of magnetometers that sense variations in a local magnetic field is presented. The method requires no prior knowledge of the magnetic field, nor does it compile any map of it. Assuming that the local variations in the magnetic field can be described by a curl and divergence free polynomial model, a maximum likelihood estimator is derived. To gain insight into the array design criteria and the achievable estimation performance, the identifiability conditions of the estimation problem are analyzed and the Cramer-Rao bound for the one-dimensional case is derived. The analysis shows that with a second-order model it is sufficient to have six magnetometer triads in a plane to obtain local identifiability. Further, the Cramer-Rao bound shows that the estimation error is inversely proportional to the ratio between the rate of change of the magnetic field and the noise variance, as well as the length scale of the array. The performance of the proposed estimator is evaluated using real-world data. The results show that, when there are sufficient variations in the magnetic field, the estimation error is of the order of a few percent of the displacement. The method also outperforms current state-of-the-art method for magnetic odometry.

Variations in the heliospheric magnetic field occur on most scales including those lying intermediate between the gyro‐radii of galactic cosmic rays and heliocentric radial distance. It is demonstrated that a correlation exists between these intermediate‐scale variations in the magnetic field and the variations in the cosmic ray distribution function that result from the field variations. This correlation will affect the average transport of galactic cosmic rays by significantly altering the patterns of gradient and curvature drifts in the heliosphere. During the current solar cycle, the altered drift patterns can lead to larger radial gradients of the galactic cosmic rays and significantly smaller latitude gradients than is expected only from drifts in the mean magnetic field.

Charge density wave (CDW) order is a key property of high-T_{c} cuprates, but its boundaries in the phase diagram and potential connections to other phases remain controversial. We report nuclear magnetic resonance (NMR) measurements in the prototypical cuprate YBa_{2}Cu_{3}O_{y} demonstrating that short-range static CDW order remains robust at optimal doping (p=0.165), exhibiting a strength and temperature dependence in the normal state similar to those observed at p≃0.11 in the underdoped regime. For an overdoped sample with p=0.184, we detect no static CDW down to T=T_{c}, though weak CDW order plausibly emerges below T_{c}. More broadly, we argue that both quenched disorder and competition with superconductivity influence the apparent boundary of the CDW phase, likely causing an underestimation of its intrinsic extent in doping. These findings challenge the view that the CDW phase boundary lies below p^{*}≃0.19, widely regarded as the critical doping where the pseudogap phase ends in YBa_{2}Cu_{3}O_{y}.

비파괴검사는 탐지물체에 물리적 손상을 가하지 않고 내부 정보를 파악할 수 있어 다양한 분야에서 이용되고 있다. 그 중 전자기를 이용한 물체의 형상추정방법의 경우 역 유한요소법을 이용하여야 하지만 이는 비선형성이 강하고 수치계산이 복잡하고 측정 센서의 개수가 미지수의 개수보다 훨씬 적어 정확한 결과를 얻는데 어려움이 있다. 본 논문에서는 탐지물체에 의한 자기장 변화 신호만을 이용하여, 물체의 시스템 내 각 센서별 위치에서 물체와 등가면적의 원의 비교를 통해 비교적 간편하게 자성물체의 부피를 판정하고 형상추정을 위한 다양한 보정과정을 거쳐 탐지물체의 형상판정이 가능한 알고리즘을 제안하고 검증하였다. We suggest the algorithm that it detects volume and shape according with a variation of magnetic field in non-contact electromagnetic measurement system. It is possible to assess an object shape through a variation of magnetic field. The basic idea is compared a length difference with a variation of magnetic field in a detected object and a circle which modeled equivalent area. And the shape is detected to many calibration process that it is similar to signal pattern between a length difference and a variation of magnetic field in object and equivalent circle. This is the shape detection algorithm that use only the variation of magnetic field. In this paper, it has application to the shape detection algorithm about the object as hexagon, pentagon, rectangle, trigon. we can detect the object shape easily because the shape detection algorithm is only used to the variation of magnetic field.

We report on single crystal growth, single crystal x-ray diffraction, physical properties and density functional theory (DFT) electronic structure as well as Fermi surface calculations for two ternary carbides, LuCoC2 and LuNiC2. Electrical resistivity measurements reveal for LuNiC2 a charge density wave (CDW) transition at T_{CDW}~ 450 K and, for T > T_{CDW}, a significant anisotropy of the electrical resistivity, which is lowest along the orthorhombic a-axis. The analysis of x-ray superstructure reflections suggest a commensurate CDW state with a Peierls-type distortion of the Ni atom periodicity along the orthorhombic a-axis. DFT calculations based on the CDW modulated monoclinic structure model of LuNiC2 as compared to results of the orthorhombic parent-type reveal the formation of a partial CDW gap at the Fermi level which reduces the electronic density of states from N(E_{F})= 1.03 states/eV f.u. without CDW to N(E_{F})= 0.46 states/eV f.u. in the CDW state. The corresponding bare DFT Sommerfeld value of the latter, gamma_{DFT}^{CDW}= 0.90 mJ/molK^2, reaches reasonable agreement with the experimental value gamma= 0.83(5) mJ/mol\,K^2 of LuNiC2. LuCoC2 displays a simple metallic behavior with neither CDW ordering nor superconductivity above 0.4 K. Its experimental Sommerfeld coefficient, gamma= 5.9 (1) mJ/molK^2, is in realistic correspondence with the calculated, bare Sommerfeld coefficient, gamma_{DFT}= 3.82 mJ/molK^2, of orthorhombic LuCoC2.

Kagome metals AV_{3}Sb_{5} (A=K, Rb, and Cs) exhibit a characteristic superconducting ground state coexisting with a charge density wave (CDW), whereas the mechanisms of the superconductivity and CDW have yet to be clarified. Here we report a systematic angle-resolved photoemission spectroscopy (ARPES) study of Cs(V_{1-x}Nb_{x})_{3}Sb_{5} as a function of Nb content x, where isovalent Nb substitution causes an enhancement of superconducting transition temperature (T_{c}) and the reduction of CDW temperature (T_{CDW}). We found that the Nb substitution shifts the Sb-derived electron band at the Γ point downward and simultaneously moves the V-derived band around the M point upward to lift up the saddle point (SP) away from the Fermi level, leading to the reduction of the CDW-gap magnitude and T_{CDW}. This indicates a primary role of the SP density of states to stabilize the CDW. The present result also suggests that the enhancement of superconductivity by Nb substitution is caused by the cooperation between the expansion of the Sb-derived electron pocket and the recovery of the V-derived density of states at the Fermi level.

The presence of different electronic orders other than superconductivity populating the phase diagram of cuprates suggests that they might be the key to disclose the mysteries of this class of materials. In particular charge order in the form of charge density waves (CDW), i.e., the incommensurate modulation of electron density in the CuO$_2$ planes, is ubiquitous across different families and presents a clear interplay with superconductivity. Until recently, CDW had been found to be confined inside a rather small region of the phase diagram, below the pseudogap temperature and the optimal doping. This occurrence might shed doubts on the possibility that such "low temperature phenomenon" actually rules the properties of cuprates either in the normal or in the superconducting states. However, recent resonant X-ray scattering (RXS) experiments are overturning this paradigm. It results that very short-ranged charge modulations permeate a much wider region of the phase diagram, coexisting with CDW at lower temperatures and persisting up to temperatures well above the pseudogap opening. Here we review the characteristics of these high temperature charge modulations, which are present in several cuprate families, with similarities and differences. A particular emphasis is put on their dynamical character and on their coupling to lattice and magnetic excitations, properties that can be determined with high resolution resonant inelastic x-ray scattering (RIXS).

Single crystals of ${\mathrm{HfTe}}_{3}$ were successfully grown using a chemical transport reaction in an extremely narrow temperature range. Here, we report a comparative study of polycrystalline and single-crystal samples. The electrical resistivity $\ensuremath{\rho}(T)$ measured on polycrystalline samples shows a broad hump and clear drop at 80 and 1.7 K, which correspond to the formation of the charge density wave (CDW) and superconducting (SC) transition, respectively. For the single crystals, $\ensuremath{\rho}(T)$ shows a sharp change at ${T}_{\mathrm{CDW}}=93$ K, and the superconductivity is absent, in contrast to the polycrystalline samples. With the current flowing along the $a$ and $b$ directions, the coincidence of the linear temperature dependence of $\ensuremath{\rho}(T)/\ensuremath{\rho}(300\phantom{\rule{4pt}{0ex}}\mathrm{K})$ above ${T}_{\mathrm{CDW}}$ strongly implies that the electron-electron scattering mechanism dominates the transport properties in a quasi-one-dimensional chain. Furthermore, a metal-semiconductor-like transition is confirmed below ${T}_{\mathrm{CDW}}$ in ${\ensuremath{\rho}}_{c}$. The drop observed at 4.3 K in ${\ensuremath{\rho}}_{b}(T)$ for the single crystal with more defects (small residual resistivity ratio, large ${\ensuremath{\rho}}_{0}$, and weak drop) provides direct evidence of a disorder-related SC fluctuation in the CDW system. With temperature decreasing, the carrier density exhibits a similar and rapid decrease below ${T}_{\mathrm{CDW}}$ for flowing current in both the $a$ and $b$ directions, whereas an obvious enhancement of carrier mobility appears as $I\ensuremath{\parallel}b$. An analysis of x-ray photoelectron spectroscopy spectra suggests that the mixed-valence states of Hf and Te could be related to the CDW formation in the multichain system of ${\mathrm{HfTe}}_{3}$.

In this thesis I investigate the relationship between the charge density wave (CDW) phase and superconductivity in the T-x phase diagram of Cu_xTiSe₂. I find that the incommensurate (IC)-CDW is related to the superconducting phase due to the fact that the former effectively isolates the CDW subsystem degrees of freedom. This increases the symmetry of the electronic populations within the IC-CDW band structure and leave them susceptible to internal instabilities, which in turn give rise to the superconducting phase. Because the correlated properties of these solid-state phases of matter are highly dependent on the crystalline quality of our samples, I also detail the growth of pristine single crystals and utilize several characterization techniques to aid in this purpose. In this portion of the thesis the single crystals are deliberately injected with heat and monitored to deduce the formation of defects through selenium migration. I also confirm the existence of chiral symmetry breaking in the bulk commensurate (C)-CDW phase in TiSe₂ brought about by the cooperation of phonon and exciton degrees of freedom, and also observe chiral character in fluctuations above T_[CDW]. These thermal fluctuations were observed up to 80 K above T_[CDW] via optical signatures of the folded Se-4p band and Raman signatures of the soft L_1^- phonon mode. The suppression of the excitonic degree of freedom with Cu intercalation brings about a quantum phase transition into the IC-CDW at x=0.04. Large quantum fluctuations of the folded Se-4p electronic band were observed at the quantum phase transition where measurements of the phonon system show the onset of incommensuration in the CDW super-lattice. Optical measurements demonstrate a large decoupling of the electron-phonon degrees of freedom within the electronic band structure of the IC-CDW subsystem.

Cu modified ZnO nanoflakes: An efficient visible light-driven photocatalyst and a promising photoanode for dye sensitized solar cell (DSSC)

It is shown that the nature of radiation-induced point defects and dopant interactions can cause a shift in the optimum base doping concentration for terrestrial and space solar cells. The base doping concentration has been optimized for high-efficiency Si, GaAs, and InP solar cells before and after electron irradiation. A combination of detailed carrier lifetime calculations and cell modeling is used to show that the optimum doping concentration for irradiated cells increases for InP cells, decreases for Si cells, and remains essentially unchanged for GaAs cells compared to their counterpart terrestrial cells. The optimum base doping for Si cells decreases from 8.94*10/sup 16/ cm/sup -3/ to approximately 6.6*10/sup 14/ cm/sup -1/ after 1-MeV electron irradiation. In the case of GaAs, the optimum base doping concentration remains at approximately 2*10/sup 17/ cm/sup -3/ for both irradiated and unirradiated cells. The InP base doping needs to be increased in the range of (2-6)*10/sup 17/ cm/sup -3/ from 2*10/sup 17/ cm/sup -3/ for radiation fluences in the range of 10/sup 15/ to 10/sup 16/ cm/sup -2/. >