Manipulation of Superconductivity and Charge-Density Wave Formation in the 2H\textit{H}-TaS2_{2} System

At present, the aim of superconductivity research is still expected to provide directions and a theoretical framework for the search for new superconductors, even high-temperature superconductors. Given the difficulty of discovering new superconductors, superconductivity research has focused on the alterations of existing of superconductors and the phases coexisting with superconductivity, such as a charge-density wave (CDW) state. The relationship between superconductivity and CDW has been an important research topic for the detailed understanding of superconductivity. In 2H polytypes of transition metal dichalcogenides (TMDs), the natural CDW and superconductivity coexist without any magnetic order, which provides an ideal platform to further explore the interplay between superconductivity and the CDW state. This thesis presents a systematic study of the 2H-TaS2 system with the introduction of external atoms and disorder to investigate the CDW state, superconductivity, and their interplay. In the lithium-intercalated 2H-TaS2 system, the intercalated lithium can tip the delicate balance between superconductivity and the CDW state, revealing the underlying interactions that give rise to them. The formation of superconductivity and the CDW state has been investigated by electrical and thermal transport properties, in which the CDW formation temperature is continuously suppressed, and the transition temperature of superconductivity (Tc) increases with increasing lithium intercalation, indicating that superconductivity and CDW compete with each other in lithium-intercalated 2H-TaS2. Furthermore, the electronic contribution to the specific heat and Hall resistivity data further demonstrate that the CDW weakens with lithium intercalation to indirectly increase the charge carrier density and boost superconductivity. In the hydrated 2H-Lix(H2O)yTaS2 system, the intercalated lithium can absorb water into the interlayer of 2H-LixTaS2. The amount of water is related to the content of the interlayer lithium and remains constant when the lithium content is within a certain range, accompanied by the complete suppression of the CDW state. The superconductivity has been studied by electrical and thermal transport and magnetic properties, showing a dome-shaped dependence on the lithium content x. The Debye temperatures ΘD, the electron-phonon coupling λep and the electron density of states at the Fermi level (DOS(EF)), estimated from the heat capacity data, show a close relationship with the superconductivity, indicating that the lithium intercalation tunes Tc by changing the DOS(EF) in 2H-Lix(H2O)yTaS2, but the simultaneous changes of λep and ΘD may also play a certain role. In the disordered 2H-TaS2-x system, the level of disorder can be well controlled by the number of structural defects induced by sulphur-vacancies, which influence the evolution of the long- range CDW and superconductivity. Measurements of complementary magnetization, electronic and thermal transport properties show that the long-range CDW is continuously suppressed, leading to strange-metal behaviour with linear resistivity at the endpoint of the long-range CDW, which is accompanied by the emergence of a short-range CDW phase. The superconductivity shows at first a two-step-like behaviour but reaches a maximum at the endpoint of long-range CDW with a single homogeneous phase, suggesting an interplay between superconductivity and CDW order. Moreover, the results suggest that the observed strange-metal behaviour, which could arise from the short-range charge density fluctuations, is a signature of quantum criticality with Planckian dissipation

Competing spin-glass and spin-fluctuation states in NdxPr4-xNi3O8

Neodymium nickelates have attracted research interest due to their strongly correlated behaviour and remarkable magnetic properties. More importantly, superconductivity has recently been confirmed in thin-film samples of Sr-doped NdNiO2, bringing the layered rare earth nickel oxides into the research spotlight. In this report, we present results on a series of NdNiO2 analogues, NdxPr4-xNi3O8 (x = 0.1, 0.25, 1, 2, and 4) obtained by topotactic reduction, in which we observe systematic changes in the magnetic behaviour. As the Nd3+ content increases, the initially large spin-freezing region with magnetic frustration becomes smaller and gradually shifts to low temperatures, while the magnetic response gradually increases. The muon-spin spectroscopy measurements on Nd4Ni3O8 show that this phenomenon is likely due to the enhancement of spin fluctuations in NdxPr4-xNi3O8, which weakens the spin frustration behaviour for high Nd3+ contents and at low temperatures. These spin fluctuations can be caused by both Nd and Ni ions and could be one of the factors determining the occurrence of possible superconductivity.Comment: 21 pages 10 figure

Competing spin-glass and spin-fluctuation states in Ndx_xPr4−x_{4-x}Ni3_3O8_8

Neodymium nickelates have attracted research interest due to their strongly correlated behavior and remarkable magnetic properties. More importantly, superconductivity has recently been confirmed in thin-film samples of Sr-doped NdNiO$_2$, bringing the layered rare earth nickel oxides into the research spotlight. In this report, we present results on a series of NdNiO$_2$ analogs, Nd$_x$Pr$_{4−x}$Ni$_3$O$_8$ (x=0.1, 0.25, 1, 2, and 4) obtained by topotactic reduction, in which we observe systematic changes in the magnetic behavior. As the Nd$^{3+}$ content increases, the initially large spin-freezing region with magnetic frustration becomes smaller and gradually shifts to low temperatures, while the magnetic response gradually increases. The muon-spin spectroscopy measurements on Nd$_4$Ni$_3$O$_8$ show that this phenomenon is likely due to the enhancement of spin fluctuations in Nd$_x$Pr$_{4-x}$Ni$_3$O$_8$, which weakens the spin frustration behavior for high Nd$^{3+}$ contents and at low temperatures. These spin fluctuations can be caused by both Nd and Ni ions and could be one of the factors determining the occurrence of possible superconductivity

Two-gap to single-gap superconducting transition on a honeycomb lattice in Ca1−xSrxAlSi

We report on the structural and microscopic superconducting properties of the Ca1−xSrxAlSi solid solution. Specifically, we have realized the continuous solid solution, which for all members, other than x=0 (CaAlSi), crystallizes in the AlB2-type structure. For CaAlSi, we present an improved structural model where all Al/Si layers are buckled, leading to a 6-folded structure along the crystallographic c direction. We, furthermore, find indications for the structural instability in the parent compound CaAlSi to enhance the superconductivity across the solid solution. Our investigation of the magnetic penetration depths by means of muon-spin rotation experiments reveals that CaAlSi is a two-gap superconductor, that SrAlSi is a single-gap superconductor, and that there is a continuous transition from one electronic state to the other across the solid solution. Hence, we show that the Ca1−xSrxAlSi solid solution is a highly tunable two-gap to single-gap superconducting system on a honeycomb lattice, where the superconductivity is strongly connected to a structural instability, i.e., the buckling of the Al/Si layers

Spin Transfer Driven Magnetization Dynamics in Spin Valves and Magnetic Tunnel Junctions

This thesis describes experimental studies of magnetization dynamics in both spin valves (SVs) and magnetic tunnel junctions (MTJs) subject to spin-polarized currents. A spin-polarized electrical current can transfer its angular momentum to a ferromagnet through a spin-transfer torque (STT), resulting in intriguing magnetization dynamics such as the reversal of the magnetization direction, precession and relaxation. The ferromagnetic systems investigated were nanopillars, tens to hundreds of nanometers in cross section and a few nanometers in thickness, which were further integrated into SV or MTJ structures. The magnetization switching and relaxation studies were performed on all-perpendicularly magnetized SVs. The switching probabilities were investigated for different pulse conditions at room temperature, where thermal fluctuations can play an important role. The pulse duration was varied over 10 orders of magnitude, from the fundamental timescales of magnetization precessional dynamics, 50 ps, to 1 s. Three switching regimes were found at different timescales. In the short-time regime, the switching probability was mainly determined by the angular momentum transfer between the current and the magnetization. In the long-time regime, the switching becomes thermal activation over an effective energy barrier modified by the STT. In the crossover regime, both spin-transfer and thermal effects are important. The magnetization relaxation was studied by a two-pulse correlation method, where the relaxation time is measured by the interval between the two pulses. The thermal effects were shown to be important even at nanosecond time scales. The switching and precession of magnetization were also studied in structures where a perpendicular spin polarizing layer is employed with an in-plane magnetized MTJ. When subject to pulses, the initial STT from the polarizer to the free layer is perpendicular to the free layer plane. For a large enough STT, this tilts the free layer magnetization out of the plane to create a large demagnetization field, typically at tens or hundreds of millitesla. This demagnetization field then becomes the dominant magnetic field acting on the free layer, leading to the precession of its magnetization. This magnetization precession was observed through real-time device resistance measurements, where precessions with hundreds of picoseconds are found from single current pulse stimuli

Evaluation of Sustainable Use of Water Resources in the Beijing-Tianjin-Hebei Region Based on S-Type Functions and Set Pair Analysis

Beijing-Tianjin-Hebei is an area with insufficient per capita water resources. This study evaluates the current status and development trend of sustainable use of water resources in the region, and identifies specific factors influencing sustainable development so as to provide a theoretical basis and data support for the management of water resources in the Beijing-Tianjin-Hebei region. Applying the compound system of social, economic and ecological environment, this study established an evaluation index system. The evaluation index data is obtained through the relevant calculations based on the National Economic and Social Development Announcement, the Water Resources Bulletin and the National Bureau of Statistics data. The index weights are calculated using a combination of rough set and fuzzy theory. The obtained weights were added to the set pair analysis method to acquire evaluation results. Based on the traditional set pair analysis method, this study proposes a new set pair analysis method (Method 4) using S-type functions (Formula (11)) instead of the addition weighted synthesis method (Formula (10)) in the calculation of weighted connections. In order to verify the feasibility of this method, the Spearman correlation coefficient method was used to calculate the correlation coefficient between evaluation results of Method 4 and evaluation results of other traditional methods. In addition, the set pair exponential potential is adopted to determine the impact indicators of the sustainable utilization of water resources in Beijing-Tianjin-Hebei in this study. The results show that in the Beijing-Tianjin-Hebei region, the overall level of sustainable use of water resources has been gradually raised in the past 12 years. The results of the set pair analysis method that cites S-type functions have obtained higher Spearman correlation coefficients than traditional methods. The values of the correlation coefficients are 0.9954, 0.9910, and 0.9928 respectively in Beijing, Tianjin and Hebei. Moreover, according to the results of set pair exponential potential, the indicators in the region are quasi-inverse potential or strong inverse potential, including per capita water resources and the ecological environment water use rate. Thus, a dense population with scarce water resources, and a lack of ecological water are the common problems that Beijing, Tianjin and Hebei have to face

Superconductivity and charge density wave formation in lithium-intercalated 2H−TaS2

We systematically investigated the superconducting properties and the interplay between charge density waves (CDWs) and superconductivity (SC) in lithium-intercalated 2H−TaS2. By gradually increasing the lithium content x, the CDW formation temperature is continuously suppressed, and the onset temperature of SC is increased with a maximum transition temperature Tc=3.5K for x=0.096. The bulk nature of SC is confirmed by a superconducting shielding fraction of the order of unity for this composition. The electronic contribution to the specific heat and Hall resistivity data demonstrates that the CDW weakens with lithium intercalation, thereby indirectly increasing the carrier density and boosting SC. While the sign of the charge carriers in undoped 2H−TaS2 changes from electronlike to hole type near the CDW formation temperature ∼75K, the lithium-intercalated LixTaS2 shows predominantly hole-type carriers in the CDW phase even for very low lithium content

Evaluation of Sustainable Use of Water Resources in Beijing Based on Rough Set and Fuzzy Theory

Water resources are not only the essence of human life, but also an important prerequisite which ensures the economic and social development of a country or city. Due to chronically global overexploitation, water crises have started to emerge. China, for example, is experiencing acute water scarcity. Even its capital, Beijing, faces enormous challenges in the sustainable use of water resources. Combining the compound system of social, economic, and ecological environments with the current situation in Beijing, this study established an evaluation index system. To determine the weight, the rough set theory was adopted in the index layer, fuzzy theory was employed in the rule layer and set pair analysis method, and attribute recognition theory was used as evaluation methods. In order to verify the accuracy of the results, the weights of indices were taken into the evaluation methods mentioned above after being calculated by using the analytic hierarchy process (AHP) and entropy weight method, respectively. The matter-element model was used to evaluate the system and the Spearman correlation coefficient method aimed to compare all the results of the evaluation. The results indicated that water use efficiency and water management have been increasingly improved in Beijing on a yearly basis. Additionally, the results of the Spearman correlation coefficient method showed that the method of weight determination based on rough set and fuzzy theory is feasible in the evaluation of sustainable use of water resources