Iron based pnictide and chalcogenide superconductors studied by muon spin spectroscopy

In the present thesis the superconducting properties of the Iron-based Ba_{1-x}Rb_{x}Fe_{2}As_{2} arsenides, and A_{x}Fe_{2-y}Se_{2} (A = Cs, Rb, K) chalcogenides are investigated by means of Muon Spin Rotation Spectroscopy. The temperature and pressure dependence of the magnetic penetration depth is obtained form muSR experiments and analyzed to probe the superconducting gap-symmetries for each samples. The Ba_{1-x}Rb_{x}Fe_{2}As_{2} system is described within the multi-gap s+s-wave scenario and results are discussed in the light of the suppression of inter-band processes upon hole doping. Due to the lowered upper critical field Bc2 and reduced Tc, a large section of B-T-p phase diagram is studied for the hole-overdoped x=1 case. By applying hydrostatic pressure, the RbFe_{2}As_{2} system exhibits a classical BCS superconducting characteristics. The A_{x}Fe_{2-y}Se_{2} chalcogenide represents a system containing magnetically ordered and superconducting phases simultaneously. In all investigated chalcogenide samples, about 90% of the total volume show the strong antiferromagnetic phase and 10% exhibit a paramagnetic behavior. Magnetization measurements reveal a 100% Meissner effect, while muSR clearly indicates that the paramagnetic phase is a perfect superconductor. Up to now, there is no clear evidence whether the antiferromagnetic phase is also superconducting. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed. Moreover, a new hydrostatic double-wall pressure cell is developed and produced, satisfying the demands of muSR experiments. The designs and characteristics of the new pressure cell are reviewed in the present thesis

Pressure driven magnetic order in Sr 1-x Ca x Co 2 P 2

The magnetic phase diagram of Sr1-xCaxCo2P2 as a function of hydrostatic pressure and temperature is investigated by means of high pressure muon spin rotation, relaxation and resonance (μ+SR). The weak pressure dependence for the x≠ 1 compounds suggests that the rich phase diagram of Sr1-xCaxCo2P2 as a function of x at ambient pressure may not solely be attributed to chemical pressure effects. The x= 1 compound on the other hand reveals a high pressure dependence, where the long range magnetic order is fully suppressed at pc 2≈ 9.8 kbar, which seem to be a first order transition. In addition, an intermediate phase consisting of magnetic domains is formed above pc 1≈ 8 kbar where they co-exist with a magnetically disordered state. These domains are likely to be ferromagnetic islands (FMI) and consist of an high- (FMI-1) and low-temperature (FMI-2) region, respectively, separated by a phase boundary at Ti≈ 20 K. This kind of co-existence is unusual and is originating from a coupling between lattice and magnetic degrees of freedoms

Iron based pnictide and chalcogenide superconductors studied by muon spin spectroscopy

In the present thesis the superconducting properties of the Iron-based Ba_{1-x}Rb_{x}Fe_{2}As_{2} arsenides, and A_{x}Fe_{2-y}Se_{2} (A = Cs, Rb, K) chalcogenides are investigated by means of Muon Spin Rotation Spectroscopy. The temperature and pressure dependence of the magnetic penetration depth is obtained form muSR experiments and analyzed to probe the superconducting gap-symmetries for each samples. The Ba_{1-x}Rb_{x}Fe_{2}As_{2} system is described within the multi-gap s+s-wave scenario and results are discussed in the light of the suppression of inter-band processes upon hole doping. Due to the lowered upper critical field Bc2 and reduced Tc, a large section of B-T-p phase diagram is studied for the hole-overdoped x=1 case. By applying hydrostatic pressure, the RbFe_{2}As_{2} system exhibits a classical BCS superconducting characteristics. The A_{x}Fe_{2-y}Se_{2} chalcogenide represents a system containing magnetically ordered and superconducting phases simultaneously. In all investigated chalcogenide samples, about 90% of the total volume show the strong antiferromagnetic phase and 10% exhibit a paramagnetic behavior. Magnetization measurements reveal a 100% Meissner effect, while muSR clearly indicates that the paramagnetic phase is a perfect superconductor. Up to now, there is no clear evidence whether the antiferromagnetic phase is also superconducting. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed. Moreover, a new hydrostatic double-wall pressure cell is developed and produced, satisfying the demands of muSR experiments. The designs and characteristics of the new pressure cell are reviewed in the present thesis

Iron based pnictide and chalcogenide superconductors studied by muon spin spectroscopy

In the present thesis the superconducting properties of the Iron-based Ba_{1-x}Rb_{x}Fe_{2}As_{2} arsenides, and A_{x}Fe_{2-y}Se_{2} (A = Cs, Rb, K) chalcogenides are investigated by means of Muon Spin Rotation Spectroscopy. The temperature and pressure dependence of the magnetic penetration depth is obtained form muSR experiments and analyzed to probe the superconducting gap-symmetries for each samples. The Ba_{1-x}Rb_{x}Fe_{2}As_{2} system is described within the multi-gap s+s-wave scenario and results are discussed in the light of the suppression of inter-band processes upon hole doping. Due to the lowered upper critical field Bc2 and reduced Tc, a large section of B-T-p phase diagram is studied for the hole-overdoped x=1 case. By applying hydrostatic pressure, the RbFe_{2}As_{2} system exhibits a classical BCS superconducting characteristics. The A_{x}Fe_{2-y}Se_{2} chalcogenide represents a system containing magnetically ordered and superconducting phases simultaneously. In all investigated chalcogenide samples, about 90% of the total volume show the strong antiferromagnetic phase and 10% exhibit a paramagnetic behavior. Magnetization measurements reveal a 100% Meissner effect, while muSR clearly indicates that the paramagnetic phase is a perfect superconductor. Up to now, there is no clear evidence whether the antiferromagnetic phase is also superconducting. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed. Moreover, a new hydrostatic double-wall pressure cell is developed and produced, satisfying the demands of muSR experiments. The designs and characteristics of the new pressure cell are reviewed in the present thesis

μ+SR Study of K2Cr8O16 Under Hydrostatic Pressure

In this study, the magnetic ground state of the hollandite type material K2Cr8O16 was tuned by externally applied pressure and investigated using µ+SR method in Zero-field (ZF) and weak-transversefield (wTF) configurations. As a result, the obtained magnetic transition temperature for the measuredpressures differs notably from magnetization measurements. Moreover, both wTF and ZF data reveala transition between two different magnetically ordered states at low temperatures for higher pressures. Further theoretical and experimental studies are currently being planned in order to elucidatethe detailed nature of the magnetically ordered phase. QC 20211130</p

Magnetic phase diagram of K2Cr8O16 clarified by high-pressure muon spin spectroscopy

The K2Cr8O16 compound belongs to a series of quasi-1D compounds with intriguing magnetic properties that are stabilized through a high-pressure synthesis technique. In this study, a muon spin rotation, relaxation and resonance (mu+SR) technique is used to investigate the pressure dependent magnetic properties up to 25 kbar. mu+SR allows for measurements in true zero applied field and hereby access the true intrinsic material properties. As a result, a refined temperature/pressure phase diagram is presented revealing a novel low temperature/high pressure (p(C1) = 21 kbar) transition from a ferromagnetic insulating to a high-pressure antiferromagnetic insulator. Finally, the current study also indicates the possible presence of a quantum critical point at p(C2) similar to 33 kbar where the magnetic order in K2Cr8O16 is expected to be fully suppressed even at T = 0 K

Magnetism in semiconducting molybdenum dichalcogenides

Transition metal dichalcogenides (TMDs) are interesting for understanding the fundamental physics of two-dimensional (2D) materials as well as for applications to many emerging technologies, including spin electronics. Here, we report the discovery of long-range magnetic order below TM = 40 and 100 K in bulk semiconducting TMDs 2H-MoTe2 and 2H-MoSe2, respectively, by means of muon spin rotation (μSR), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The μSR measurements show the presence of large and homogeneous internal magnetic fields at low temperatures in both compounds indicative of long-range magnetic order. DFT calculations show that this magnetism is promoted by the presence of defects in the crystal. The STM measurements show that the vast majority of defects in these materials are metal vacancies and chalcogen-metal antisites, which are randomly distributed in the lattice at the subpercent level. DFT indicates that the antisite defects are magnetic with a magnetic moment in the range of 0.9 to 2.8 μB. Further, we find that the magnetic order stabilized in 2H-MoTe2 and 2H-MoSe2 is highly sensitive to hydrostatic pressure. These observations establish 2H-MoTe2 and 2H-MoSe2 as a new class of magnetic semiconductors and open a path to studying the interplay of 2D physics and magnetism in these interesting semiconductors