arXiv subjects

Darren C. Peets

Publications and source records attributed to Darren C. Peets.

29 records · Page 2Linked to original sources

arXiv2019

Type-I superconductivity in Al$_6$Re

While the pure elements tend to exhibit Type-I rather than Type-II superconductivity, nearly all compound superconductors are Type-II, with only a few known exceptions. We report single crystal growth and physical characterization of the rhenium aluminide Al$_6$Re, which we conclude is a Type-I superconductor based on magnetization, ac-susceptibility, and specific-heat measurements. This detection of superconductivity, despite the strong similarity of Al$_6$Re to a family of W and Mo aluminides that do not superconduct, suggests that these aluminides are an ideal testbed for identifying the relative importance of valence electron count and inversion symmetry in determining whether a material will superconduct.

cond-mat.supr-con↗

arXiv2018

Kinetic stabilization of 1D surface states near twin boundaries in noncentrosymmetric BiPd

The search for one-dimensional (1D) topologically-protected electronic states has become an important research goal for condensed matter physics owing to their potential use in spintronic devices or as a building block for topologically non-trivial electronic states. Using low temperature scanning tunneling microscopy, we demonstrate the formation of 1D electronic states at twin boundaries at the surface of the noncentrosymmetric material BiPd. These twin boundaries are topological defects which separate regions with antiparallel orientations of the crystallographic \textit{b} axis. We demonstrate that the formation of the 1D electronic states can be rationalized by a change in effective mass of two-dimensional surface states across the twin boundary. Our work therefore reveals a novel route towards designing 1D electronic states with strong spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2018

Physical properties of noncentrosymmetric tungsten and molybdenum aluminides

A lack of spatial inversion symmetry gives rise to a variety of unconventional physics, from noncollinear order and Skyrmion lattice phases in magnetic materials to topologically-protected surface states in certain band insulators, to mixed-parity pairing states in superconductors. The search for exotic physics in such materials is largely limited by a lack of candidate materials, and often by difficulty in obtaining crystals. Here, we report the single crystal growth and physical properties of the noncentrosymmetric tungsten aluminide cage compounds Al$_4$W and Al$_5$W, alongside related molybdenum aluminides in which spin-orbit coupling should be significantly weaker. All compounds are nonmagnetic metals. Their high conductivities suggest the opportunity to find superconductivity at lower temperatures, while the limits we can place on their transition temperatures suggest that any superconductivity may be expected to exhibit significant parity mixing.

cond-mat.mtrl-sci↗

arXiv2018

Correct Brillouin zone and electronic structure of BiPd

A promising route to the realization of Majorana fermions is in non-centrosymmetric superconductors, in which spin-orbit-coupling lifts the spin degeneracy of both bulk and surface bands. A detailed assessment of the electronic structure is critical to evaluate their suitability for this through establishing the topological properties of the electronic structure. This requires correct identification of the time-reversal-invariant momenta. One such material is BiPd, a recently rediscovered non-centrosymmetric superconductor which can be grown in large, high-quality single crystals and has been studied by several groups using angular resolved photoemission to establish its surface electronic structure. Many of the published electronic structure studies on this material are based on a reciprocal unit cell which is not the actual Brillouin zone of the material. We show here the consequences of this for the electronic structures and show how the inferred topological nature of the material is affected.

cond-mat.mtrl-sci↗

arXiv2017

Properties of spin 1/2 triangular lattice antiferromagnets: CuRE2Ge2O8 (RE=Y, La)

We found new two-dimensional (2D) quantum (S=1/2) antiferromagnetic systems: CuRE2Ge2O8 (RE=Y and La). According to our analysis of high-resolution X-ray and neutron diffraction experiments, the Cu-network of CuRE2Ge2O8 (RE=Y and La) exhibits a 2D triangular lattice linked via weak bonds along the perpendicular b-axis. Our bulk characterizations from 0.08 to 400 K show that they undergo a long-range order at 0.51(1) and 1.09(4) K for the Y and La systems, respectively. Interestingly, they also exhibit field induced phase transitions. For theoretical understanding, we carried out the density functional theory (DFT) band calculations to find that they are typical charge-transfer-type insulators with a gap of Eg = 2 eV. Taken together, our observations make CuRE2Ge2O8 (RE=Y and La) additional examples of low-dimensional quantum spin triangular antiferromagnets with the low-temperature magnetic ordering.

cond-mat.mtrl-sci↗

arXiv2017

Magnetic Transitions in the Chiral Armchair-Kagome System Mn$_2$Sb$_2$O$_7$

The competition between interactions in frustrated magnets allows a wide variety of new ground states, often exhibiting emergent physics and unique excitations. Expanding the suite of lattices available for study enhances our chances of finding exotic physics. Mn$_2$Sb$_2$O$_7$ forms in a chiral, kagome-based structure in which a fourth member is added to the kagome-plane triangles to form an armchair unit and link adjacent kagome planes. This structural motif may be viewed as intermediate between the triangles of the kagome network and the tetrahedra in the pyrochlore lattice. Mn$_2$Sb$_2$O$_7$ exhibits two distinct magnetic phase transitions, at 11.1 and 14.2K, at least one of which has a weak ferromagnetic component. The magnetic propagation vector does not change through the lower transition, suggesting a metamagnetic transition or a transition involving a multi-component order parameter. Although previously reported in the $P3_121$ space group, Mn$_2$Sb$_2$O$_7$ actually crystallizes in $P2$, which allows ferroelectricity, and we show clear evidence of magnetoelectric coupling indicative of multiferroic order. The quasi-two-dimensional `armchair-kagome' lattice presents a promising platform for probing chiral magnetism and the effect of dimensionality in highly frustrated systems.

cond-mat.str-el↗

arXiv2016

The $3d$-Electron Heisenberg Pyrochlore Mn$_2$Sb$_2$O$_7$

In frustrated magnetic systems, geometric constraints or the competition amongst interactions introduce extremely high degeneracy and prevent the system from readily selecting a low-temperature ground state. The most frustrated known spin arrangement is on the pyrochlore lattice, but nearly all magnetic pyrochlores have unquenched orbital angular momentum, constraining the spin directions through spin-orbit coupling. Pyrochlore Mn$_2$Sb$_2$O$_7$ is an extremely rare Heisenberg pyrochlore system, with directionally-unconstrained spins and low chemical disorder. We show that it undergoes a spin-glass transition at 5.5K, which is suppressed by disorder arising from Mn vacancies, indicating this ground state to be a direct consequence of the spins' interactions. The striking similarities to $3d$ transition metal pyrochlores with unquenched angular momentum suggests that the low spin-orbit coupling in the $3d$ block makes Heisenberg pyrochlores far more accessible than previously imagined.

cond-mat.str-el↗

arXiv2016

Upper critical field of the noncentrosymmetric superconductor BiPd

The superconducting parameters and upper critical field of the noncentrosymmetric superconductor BiPd have proven contentious. This material is of particular interest because it is a singular example of a $4f$-electron-free noncentrosymmetric superconductor of which crystals may be grown and cleaved, enabling surface-sensitive spectroscopies. Here, using bulk probes augmented by tunnelling data on defects, we establish that the lower of the previously reported upper critical fields corresponds to the bulk transition. The material behaves as a nearly-weak-coupled BCS s-wave superconductor, and we report its superconducting parameters as drawn from the bulk upper critical field. Possible reasons behind the order-of-magnitude discrepancy in the reported upper critical fields are discussed.

cond-mat.supr-con↗

arXiv2015

Signatures of New $d$-Wave Vortex Physics in Overdoped Tl$_2$Ba$_2$CuO$_{6+x}$ Revealed by TF-$μ^+$SR

The spontaneous expulsion of applied magnetic field, the Meissner effect, is a defining feature of superconductors; in Type-II superconductors above the lower critical field, this screening takes the form of a lattice of magnetic flux vortices. Using implanted spin-1/2 positive muons, one can measure the vortex lattice field distribution through the spin precession and deduce key parameters of the superconducting ground state, and thereby fundamental properties of the superconducting pairing. Muon spin rotation/relaxation ($μ$SR) experiments have indeed revealed much interesting physics in the underdoped cuprates, where superconductivity is closely related to, or coexistent with, disordered or fluctuating magnetic and charge excitations. Such complications should be absent in overdoped cuprates, which are believed to exhibit conventional Fermi liquid behaviour. These first transverse field (TF)-$μ^+$SR experiments on heavily-overdoped single crystals reveal a superfluid density exhibiting a clear inflection point near 0.5$T_c$, with a striking doping-independent scaling. This reflects hitherto unrecognized physics intrinsic to $d$-wave vortices, evidently generic to the cuprates, and may offer fundamentally new insights into their still-mysterious superconductivity.

cond-mat.supr-con↗

arXiv2015

Dirac Surface States and Nature of Superconductivity in Noncentrosymmetric BiPd

In non-magnetic bulk materials, inversion symmetry protects the spin degeneracy. If the bulk crystal structure lacks a centre of inversion, however, spin-orbit interactions lift the spin degeneracy, leading to a Rashba metal whose Fermi surfaces exhibit an intricate spin texture. In superconducting Rashba metals a pairing wavefunction constructed from these complex spin structures will generally contain both singlet and triplet character. Here we examine the possible triplet components of the order parameter in noncentrosymmetric BiPd, combining for the first time in a noncentrosymmetric superconductor macroscopic characterization, atomic-scale ultra-low-temperature scanning tunnelling spectroscopy, and relativistic first-principles calculations. While the superconducting state of BiPd appears topologically trivial, consistent with Bardeen-Cooper-Schrieffer theory with an order parameter governed by a single isotropic s-wave gap, we show that the material exhibits Dirac-cone surface states with a helical spin polarization.

cond-mat.supr-con↗

arXiv2015

High-resolution structure studies and magnetoelectric coupling of relaxor multiferroic Pb(Fe$_{0.5}$Nb$_{0.5}$)O$_3$

Pb(Fe$_{0.5}$Nb$_{0.5}$)O$_3$ (PFN), one of the few relaxor multiferroic systems, has a $G$-type antiferromagnetic transition at $T_N$ = 143 K and a ferroelectric transition at $T_C$ = 385 K. By using high-resolution neutron-diffraction experiments and a total scattering technique, we paint a comprehensive picture of the long- and short-range structures of PFN: (i) a clear sign of short-range structural correlation above $T_C$, (ii) no sign of the negative thermal expansion behavior reported in a previous study, and (iii) clearest evidence thus far of magnetoelectric coupling below $T_N$. We conclude that at the heart of the unusual relaxor multiferroic behavior lies the disorder between Fe$^{3+}$ and Nb$^{5+}$ atoms. We argue that this disorder gives rise to short-range structural correlations arising from O disorder in addition to Pb displacement.

cond-mat.mtrl-sci↗