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The electronic nematic phase—in which electronic degrees of freedom lower the crystal rotational symmetry—is commonly observed in high-temperature superconductors. However, understanding the role of nematicity and nematic fluctuations in Cooper pairing is often made more complicated by the coexistence of other orders, particularly long-range magnetic order. Here we report the enhancement of superconductivity in a model electronic nematic system that is not magnetic, and show that the enhancement is directly born out of strong nematic fluctuations associated with a quantum phase transition. We present measurements of the resistance as a function of strain in Ba 1− x Sr x Ni 2 As 2 to show that strontium substitution promotes an electronically driven nematic order in this system. In addition, the complete suppression of that order to absolute zero temperature leads to an enhancement of the pairing strength, as evidenced by a sixfold increase in the superconducting transition temperature. The direct relation between enhanced pairing and nematic fluctuations in this model system, as well as the interplay with a unidirectional charge-density-wave order comparable to that found in the cuprates, offers a means to investigate the role of nematicity in strengthening superconductivity. Transport measurements show that nematic fluctuations near a phase transition increase the temperature at which superconductivity occurs by a factor of nearly six. This happens in a non-magnetic nickel-based compound.

Compounds with 3d- and 4f -electrons can often be tuned to manifest new physics and evolve into new ground states with multiple parameters: pressure, magnetic field, and chemical substitution. In this work chemical substitution and magnetic field were used to tune correlated states coming from 3 d- and 4f-electrons. The first part of this thesis summarizes the study of Lifshitz transitions in K- and TM- (TM=Co, Rh, Ru, and Mn) substituted BaFe2As2 single crystals by thermoelectric power (TEP) measurements. The second part of this thesis presents two studies of tuning the low-temperature states of Ce-based materials. The first of these is a comprehensive study of transport and thermodynamic properties of CeZn11 and LaZn 11 single crystals as well as the search for a possible field-induced quantum critical point in CeZn11. CeZn11 orders antiferromagnetically below ~ 2 K. The zero-field resistivity and thermoelectric power data show features characteristic of a Ce-based intermetallic with crystal-electric-field splitting and possible Kondo-lattice effects. The constructed T - H phase diagram for the magnetic field applied along the easy [110] direction shows that the magnetic field required to suppress TN below 0.4 K is in the range of 45-47.5 kOe. A linear behavior of the rho(T) data, H||[110], was observed only for H = 45 kOe for 0.46 K ≤T ≤ 1.96 K followed by the Landau-Fermi-liquid regime for a limited range of fields, 47.5 kOe ≤ H ≤60 kOe. From the analysis of the data, it appears that CeZn11 is a local moment compound with little or no electronic correlations arising from the Ce 4f-shell. Given the very high quality of the single crystals, quantum oscillations are found for both CeZn11 and LaZn11. In order to study a system with clearer Kondo-like features, the effects of La dilution of the Kondo lattice CeCu2Ge2 were studied as well. CeCu2Ge2 orders antiferromagnetically below TN ~ 4 K with the Kondo temperature TK in the range of 4-6 K. The study of (Ce1-xLax)Cu2Ge 2 system indicated that with La-substitution TN is suppressed in an almost linear fashion and moves below 0.36 K, the base temperature of the measurements, for x > 0.8. Remarkably, in addition to robust antiferromagnetism, the system also shows low temperature coherent scattering below Tcoh up to ~ 0.9 of La, indicating a small percolation limit ~ 9% of Ce that separates a coherent state from a single-ion Kondo impurity state. T coh as a function of magnetic field was found to have different functional dependencies in coherent and single-ion regimes. Remarkably, (Tcoh)2was found to be linearly proportional to TN. The Kondo temperature was found to slowly change in a non-linear fashion from ~ 4 K to ~ 1 K upon La substitution. For Ce concentrations, y = 1 - x, in the range of 0.01 ≤ y ≤ 0.08, Tmin in the resistivity data is proportional to y 1/5 as expected for the single-ion Kondo impurity. The jump in the magnetic specific heat deltaCm at TN as a function of TK/TN for (Ce1-xLax)Cu2Ge 2 system follows the theoretical prediction based on the molecular field calculation for the S =1/2 resonant level model. (Abstract shortened by UMI.)

Magnetic penetration depth, \\(\\lambda_{m}\\), was measured as a function of temperature and magnetic field in single crystals of low carrier density superconductor YPtBi by using a tunnel-diode oscillator technique. Measurements in zero DC magnetic field yield London penetration depth, \\(\\lambda_{L}\\left(T\\right)\\), but in the applied field the signal includes the Campbell penetration depth, \\(\\lambda_{C}\\left(T\\right)\\), which is the characteristic length of the attenuation of small excitation field, \\(H_{ac}\\), into the Abrikosov vortex lattice due to its elasticity. Whereas the magnetic field dependent \\(\\lambda_C\\) exhibit \\(\\lambda_{C}\\sim B^{p}\\) with \\(p=1/2\\) in most of the conventional and unconventional superconductors, we found that \\(p\\approx 0.23\\ll1/2\\) in YPtBi due to rapid suppression of the pinning strength. From the measured \\(\\lambda_{C}(T,H)\\), the critical current density is \\(j_{c}\\approx40\\,\\mathrm{A}/\\mathrm{cm^{2}}\\) at 75 mK. This is orders of magnitude lower than that of conventional superconductors of comparable \\(T_{c}\\). Since the pinning centers (lattice defects) and vortex structure are not expected to be much different in YPtBi, this observation is direct evidence of the low density of the Cooper pairs because \\(j_{c}\\propto n_s\\).

Most of the half-Heusler RPtBi compounds (R=rare earth) host various surface states due to spin-orbit coupling driven topological band structure. While recent ARPES measurements ubiquitously reported the existence of surface states in RPtBi, their evidence by other experimental techniques remains elusive. Here we report the angle-dependent magnetic field response of electrical transport properties of YPtBi in both the normal and superconducting states. The angle dependence of both magnetoresistance and the superconducting upper critical field breaks the rotational symmetry of the cubic crystal structure, and the angle between the applied magnetic field and the measurement plane of a plate-like sample prevails. Furthermore, the measured upper critical field is notably higher than the bulk response for an in-plane magnetic field configuration, suggesting the presence of quasi-2D superconductivity. Our work suggests the transport properties cannot be explained solely by the bulk carrier response, requiring robust normal and superconducting surface states to flourish in YPtBi.

Transition metal-pnictide compounds have received attention for their tendency to combine magnetism and unconventional superconductivity. Binary CoAs lies on the border of paramagnetism and the more complex behavior seen in isostructural CrAs, MnP, FeAs, and FeP. Here we report the properties of CoAs single crystals grown with two distinct techniques along with density functional theory calculations of its electronic structure and magnetic ground state. While all indications are that CoAs is paramagnetic, both experiment and theory suggest proximity to a ferromagnetic instability. Quantum oscillations are seen in torque measurements up to 31.5~T, and support the calculated paramagnetic Fermiology.

The proposed high-spin superconductivity in the half-Heusler compounds changes the landscape of superconductivity research. While superconducting instability is possible only in systems with quantum mechanically coherent quasiparticles, it has not been verified for any proposed high-spin Fermi surfaces. Here we report an observation of anomalous Shubnikov-de Haas effect in half-Heusler YPtBi, which is compatible with a coherent \\(j=3/2\\) Fermi surface. The quantum oscillation (QO) signal in cubic YPtBi manifests extreme anisotropy upon rotation of the magnetic field from [100] to [110] crystallographic direction where the QO signal drastically vanishes near [110]. This radical anisotropy for a cubic system cannot be explained by trivial scenarios for QO involving effective mass or impurity scattering, but it is naturally explained by the warping feature of the \\(j=3/2\\) Fermi surface YPtBi. Our results prove the high-spin nature of the quasiparticle in the half-Heusler compounds, which makes the realization of the unprecedented high-spin superconductivity more plausible.

Quantum oscillations in the binary antiferromagnetic metal FeAs are presented and compared to theoretical predictions for the electronic band structure in the anomalous spin density wave state of this material. Demonstrating a new method for growing single crystals out of Bi flux, we utilize the highest quality FeAs to perform torque magnetometry experiments up to 35 T, using rotations of field angle in two planes to provide evidence for one electron and one hole band in the magnetically ordered state. The resulting picture agrees with previous experimental evidence for multiple carriers at low temperatures, but the exact Fermi surface shape differs from predictions, suggesting that correlations play a role in deviation from ab initio theory and cause up to a four-fold enhancement in the effective carrier mass.

The electronic nematic phase, wherein electronic degrees of freedom lower the crystal rotational symmetry, is a common motif across a number of high-temperature superconductors. However, understanding the role and influence of nematicity and nematic fluctuations in Cooper pairing is often complicated by the coexistence of other orders, particularly long-range magnetic order. Here we report the enhancement of superconductivity in a model electronic nematic system absent of magnetism, and show that the enhancement is directly born out of strong nematic fluctuations emanating from a tuned quantum phase transition. We use elastoresistance measurements of the Ba\\(_{1-x}\\)Sr\\(_{x}\\)Ni\\(_2\\)As\\(_2\\) substitution series to show that strontium substitution promotes an electronically driven \\(B_{1g}\\) nematic order in this system, and that the complete suppression of that order to absolute zero temperature evokes a dramatic enhancement of the pairing strength, as evidenced by a sixfold increase in the superconducting transition temperature. The direct relation between enhanced pairing and nematic fluctuations in this model system, as well as the interplay with a unidirectional charge density wave order comparable to that found in the cuprates, offers a means to elucidate the role of nematicity in boosting superconductivity.

We study the temperature dependence of electrical resistivity for currents directed along all crystallographic axes of the spin-triplet superconductor UTe\\(_{2}\\). We focus particularly on an accurate determination of the resistivity along the \\(c\\)-axis (\\(\\rho_c\\)) by using a generalized Montgomery technique that allows extraction of crystallographic resistivity components from a single sample. In contrast to expectations from the observed highly anisotropic band structure, our measurement of the absolute values of resistivities in all current directions reveals a surprisingly nearly isotropic transport behavior at temperatures above Kondo coherence, with \\(\\rho_c \\sim \\rho_b \\sim 2\\rho_a\\), that evolves to reveal qualitatively distinct behaviors on cooling. The temperature dependence of \\(\\rho_c\\) exhibits a peak at a temperature much lower than the onset of Kondo coherence observed in \\(\\rho_a\\) and \\(\\rho_b\\), consistent with features in magnetotransport and magnetization that point to a magnetic origin. A comparison to the temperature-dependent evolution of the scattering rate observed in angle-resolved photoemission spectroscopy experiments provides important insights into the underlying electronic structure necessary for building a microscopic model of superconductivity in UTe\\(_{2}\\).

In all known fermionic superfluids, Cooper pairs are composed of spin-1/2 quasi-particles that pair to form either spin-singlet or spin-triplet bound states. The \"spin\" of a Bloch electron, however, is fixed by the symmetries of the crystal and the atomic orbitals from which it is derived, and in some cases can behave as if it were a spin-3/2 particle. The superconducting state of such a system allows pairing beyond spin-triplet, with higher spin quasi-particles combining to form quintet or septet pairs. Here, we report evidence of unconventional superconductivity emerging from a spin-3/2 quasiparticle electronic structure in the half-Heusler semimetal YPtBi, a low-carrier density noncentrosymmetric cubic material with a high symmetry that preserves the \\(p\\)-like \\(j=3/2\\) manifold in the Bi-based \\(\\Gamma_8\\) band in the presence of strong spin-orbit coupling. With a striking linear temperature dependence of the London penetration depth, the existence of line nodes in the superconducting order parameter \\(\\Delta\\) is directly explained by a mixed-parity Cooper pairing model with high total angular momentum, consistent with a high-spin fermionic superfluid state. We propose a \\(\\mathbf{k\\cdot p}\\) model of the \\(j=3/2\\) fermions to explain how a dominant \\(J\\)=3 septet pairing state is the simplest solution that naturally produces nodes in the mixed even-odd parity gap. Together with the underlying topologically non-trivial band structure, the unconventional pairing in this system represents a truly novel form of superfluidity that has strong potential for leading the development of a new generation of topological superconductors.