Experimental studies of noncentrosymmetric superconductors

Noncentrosymmetric (NCS) superconductors lack a centre of inversion symmetry. This along with antisymmetric spin orbit coupling can result in an admixture of singlet and triplet pairing in the superconducting state. This admixture can give rise to several unconventional superconducting behaviours such as exotic superconducting gap functions, upper critical fields that violate the Pauli limiting field and time-reversal symmetry breaking among others. In this thesis, the NCS superconductors Re6Zr, La7Pd3, La7Ir3, TaRh2B2 and NbRh2B2 have been studied using a combination of low-temperature magnetisation, heat capacity, resistivity and muon spin spectroscopy techniques (μSR). High quality single crystal and polycrystalline samples have been prepared by a variety of synthesis techniques. Polycrystalline samples of Re6Zr and La7Pd3 were prepared by arc melting. Polycrystalline samples of NbRh2B2 and TaRh2B2 have been synthesised by solid state reaction. Single crystal samples of La7Ir3 have been prepared using the Czochralski process. The superconducting and normal-state properties of the cubic NCS superconductor Re6Zr with Tc = 6:75(5) K have been investigated. The properties of Re6Zr are observed to be dominated by the effects of disorder with electrical resistivity measurements indicating poor metallic behaviour. Heat capacity measurements of Re6Zr indicate that the superconducting gap is isotropic and s-wave in nature with enhanced electron-phonon coupling. La7Pd3 and La7Ir3 are hexagonal NCS superconductors with Tc of 1:46(5) and 2:41(5) K respectively. Magnetisation, resistivity, heat capacity and transverse- field μSR measurements of polycrystalline La7Pd3 reveal that it is a fully gapped s-wave superconductor. However, zero-field μSR measurements indicate that timereversal symmetry is broken in the superconducting state. La7Ir3 single crystals have been synthesised and characterised to look for evidence of anisotropy in the superuid density to explain the observation of time-reversal symmetry breaking in polycrystalline samples. TaRh2B2 and NbRh2B2 are isostructural chiral NCS superconductors with a Tc of 6:05(5) and 7:58(5) K respectively. Both compounds have upper critical fields that violate the Pauli limiting field which can not be modelled by the Werthamer- Helfand-Hohenberg model. Evidence for multigap superconductivity has been found in heat capacity and μSR measurements performed on TaRh2B2 and NbRh2B2

Heparin-stabilised iron oxide for MR applications : a relaxometric study

Superparamagnetic nanoparticles have strong potential in biomedicine and have seen application as clinical magnetic resonance imaging (MRI) contrast agents, though their popularity has plummeted in recent years, due to low efficacy and safety concerns, including haemagglutination. Using an in situ procedure, we have prepared colloids of magnetite nanoparticles, exploiting the clinically approved anti-coagulant, heparin, as a templating stabiliser. These colloids, stable over several days, produce exceptionally strong MRI contrast capabilities particularly at low fields, as demonstrated by relaxometric investigations using nuclear magnetic resonance dispersion (NMRD) techniques and single field r1 and r2 relaxation measurements. This behaviour is due to interparticle interactions, enhanced by the templating effect of heparin, resulting in strong magnetic anisotropic behaviour which closely maps particle size. The nanocomposites have also reliably prevented protein-adsorption triggered thrombosis typical of non-stabilised nanoparticles, showing great potential for in vivo MRI diagnostics

Superconductivity and the upper critical field in the chiral noncentrosymmetric superconductor NbRh2B2

NbRh2B2 crystallises in a chiral noncentrosymmetric structure and exhibits bulk type-II superconductivity below 7.46(5) K. Here we show that the temperature dependence of the upper critical field deviates from the behaviour expected for both Werthamer-Helfand-Hohenberg and the Ginzburg-Landau models and that μ0Hc2 (0) ~ 18 T exceeds the Pauli paramagnetic limit, μ0HP = 13.9 T. We explore the reasons for this enhancement. Transverse-field muon spectroscopy measurements suggest that the superconducting gap is either s-wave or (s + s)-wave, a the pressure dependence of Tc reveals the superconducting gap is primarily s- wave in character. The magnetic penetration depth lambda(0) = 595(5) nm. Heat capacity measurements reveal the presence of a multigap (s + s)-wave superconducting order parameter and moderate electron-phonon coupling

Direct evidence from high-field magnetotransport for a dramatic change of quasiparticle character in van der Waals ferromagnet Fe_3−⁢GeTe₂

Magnetometry and magnetoresistance (MR) data taken on the van der Waals ferromagnet Fe3−⁢GeTe2 (FGT) reveal three distinct contributions to the MR: a linear negative component, a contribution from closed Fermi-surface orbits, and an enhancement proportional to the square of the applied magnetic field which is linked to a noncoplanar spin arrangement. Contrary to earlier studies on FGT, by accounting for the field dependence of the anomalous Hall effect, we find that the ordinary Hall coefficient decreases markedly below 80 K, indicating a significant change in character of the electrons and holes on the Fermi-surface at this temperature. The resulting altered ground state eventually causes the Hall coefficient to reverse sign at 35 K. Our Hall data support the proposal that Kondo-lattice behavior develops in this -electron material below 80 K. Additional evidence comes from the negative linear component of the MR, which arises from electron-magnon scattering with an atypical temperature dependence attributable to the onset of Kondo screening

Evidence for the coexistence of time-reversal symmetry breaking and Bardeen-Cooper-Schrieffer-like superconductivity in La7Pd3

Time-reversal symmetry breaking (TRSB) with a Bardeen-Cooper-Schrieffer (BCS) -like superconductivity occurs in a small, but growing number of noncentrosymmetric (NCS) materials, although the mechanism is poorly understood. We present heat capacity, magnetization, resistivity, and muon spin resonance/relaxation (μSR) measurements on polycrystalline samples of NCS La7Pd3. Transverse-field μSR and heat capacity data show La7Pd3 is a type-II superconductor with a BCS-like gap structure, while zero-field μSR results provide evidence of TRSB. We discuss the implications of these results for both the La7X3 (where X = Ni, Pd, Rh, Ir) group of superconductors and other CS and NCS superconductors for which TRSB has been observed

Superconductivity in monocrystalline YNiSi3 and LuNiSi3

We report the discovery of bulk superconductivity in the ternary intermetallics YNiSi3 and LuNiSi3. High-quality single crystals were grown via the Sn-flux method and studied using magnetization, specific-heat, and resistivity measurements at low temperatures. The critical temperatures obtained from these different techniques are in very good agreement and yield Tc=1.36(3)K and Tc=1.61(2)K for YNiSi3 and LuNiSi3, respectively. Magnetization measurements indicate that both compounds are among the rare cases where type-I superconductivity occurs in a ternary intermetallic, however, the jump in the specific heat at the transition is lower than the value expected from BCS theory (ΔCel/γnTc=1.43) in both materials and is equal to 1.14(9) and 0.71(5) for the Y and Lu compounds, respectively. Resistivity measurements exhibit sharp transitions but with critical fields μ0Hc(0) (≈0.05T for YNiSi3 and ≈0.08T for LuNiSi3) considerably higher than those obtained from the magnetization and specific heat (≈0.01T). First-principles density functional theory calculated electronic structure shows that these compounds have highly anisotropic and complex Fermi surfaces with one electronic and two holelike branches. One hole branch and the electron branch have a large cylindrical topology connecting the first Brillouin-zone boundaries, the former being built up by the hybridization of Y(Lu) d, Ni d, and Si p states, and the latter being built up by Ni d and Si p states. The calculated phononic structures indicate that the coupling of the Y(Lu), Ni d, and Si p electrons in the low-lying optical phonon branches is responsible for the formation of Cooper pairs and the observed superconducting state. Therefore, these compounds can be classified as anisotropic three-dimensional metals with multiband superconducting ground states in the weak-coupling regime

Giant topological and planar Hall effect in Cr1/3NbS2

Cr1/3NbS2 is a transition metal dichalcogenide that has been of significant interest due to its ability to host a magnetic chiral soliton lattice. Conventional and planar Hall measurements provide valuable insight into the detection of exotic spin structures in chiral magnets. We show that the presence of a giant planar Hall effect (PHE) can be attributed to a tilted soliton lattice in Cr1/3NbS2. Our detailed angular-dependent study shows the PHE and anisotropic magnetoresistance are intrinsically linked in complex noncoplanar magnets. From the conventional Hall signal we show the presence of a giant unconventional, likely topological Hall component that is the fingerprint of noncoplanar spin textures

Quantum muon diffusion and the preservation of time-reversal symmetry in the superconducting state of type-I rhenium

Elemental rhenium exhibiting type-II superconductivity has been previously reported to break time-reversal symmetry in the superconducting state. We have investigated an arc-melted sample of rhenium exhibiting type-I superconductivity. Low-temperature zero-field muon-spin relaxation measurements indicate that time-reversal symmetry is preserved in the superconducting state. Muon diffusion is observed, which is due to quantum mechanical tunneling between interstitial sites. The normal state behavior is characterized by the conduction electrons screening the muons and thermal broadening, and is typical for a metal. Energy asymmetries between muon trapping sites and the superconducting energy gap also characterize the superconducting state behavior

Magnetic structure investigation of the intercalated transition metal dichalcogenide V1/3NbS2

We investigate the temperature evolution of the magnetic structure of V1/3NbS2 using neutron diffraction techniques. We find that V1/3NbS2 has two propagation vectors: k0 = (0, 0, 0) and k1 = (0, 0, 1/3 ). The k0 vector can be associated with an antiferromagnetic ordering of in-plane moments with a refined value of 0.90(5)μB, and k1 can be associated with moments along the c axis in an up-down-down configuration with refined values of 1.21(12)μB and 0.61(6)μB. Both k0 and k1 magnetic components couple with an out-of-plane ferromagnetic moment consistent with magnetization data. Furthermore, single-crystal neutron diffraction shows evidence of diffuse magnetic scattering between the (010) and (01±1/3 ) Bragg peaks. We also characterize the field and temperature evolution of the magnetic structure in V1/3NbS2 by magnetic susceptibility and heat capacity measurements. The dc susceptibility measurements give an antiferromagnetic transition temperature of TN = 50 K, and the field scans reveal that the moment does not saturate at magnetic fields up to 100 kOe

Effects of Fe deficiency and Co substitution in polycrystalline and single crystals of Fe3GeTe2

Fe3GeTe2 is a two-dimensional van der Waals material with a ferromagnetic ground state and a maximum transition temperature Tc ∼ 225 K. However, when Fe3GeTe2 is synthesized, lower values of Tc are often reported. This is attributed to a deficiency in the Fe at the 2c site in the crystal structure. Here, we investigate the effect of Fe deficiency and the substitution of Co for Fe on the magnetic properties of this system. We have synthesized both polycrystalline material and single crystals by chemical vapor transport and the flux method, with the largest crystals obtained using the flux method. Cobalt substitution at the Fe site is found to significantly reduce the magnetic transition temperature. Crystals of Fe3GeTe2 grown by chemical vapor transport with ∼ 8% excess Fe in the starting materials display an optimum Fe content and magnetic transition temperature