arXiv subjects

J. Paul Attfield

Publications and source records attributed to J. Paul Attfield.

11 recordsLinked to original sources

arXiv2026

Quantum sensing of nanoscale electronic phase segregation

Doping of transition metal oxides such as CaFe$_3$O$_5$ offers a controlled way to tune the interplay of charge, spin, and lattice degrees of freedom, yet local-probe studies remain difficult because strong correlations and dynamic charge-spin fluctuations obscure fine spectroscopic features in powder samples. Here, we employ quantum magnetometry based on nitrogen-vacancy (NV) centers in nanodiamonds impressed into an Mn-doped CaFe$_3$O$_5$ powder pellet to probe static and dynamic magnetic fields at the nanoscale across the weak ferromagnetic transition. The splitting and broadening of the optically detected magnetic resonance (ODMR) spectra exhibit an order-parameter-like increase by ~ 15 MHz upon cooling below the critical temperature, T$_{\rm c}$. Concomitantly, the spin-lattice relaxation rate, 1/T$_1$, exhibits a pronounced, divergence-like enhancement at T$_{\rm c}$, increasing by about one order of magnitude from its high-temperature value. Moreover, detailed lineshape fits of ODMR spectra together with the stretched-exponential NV magnetization recovery curves corroborate the proposed electronic phase segregation in charge-ordered and charge-averaged phases at the nanometric scales. The presented study demonstrates the viability of using nanodiamonds as a platform for nanoscale magnetic probing of strongly correlated matter, including phenomena such as electronic phase separation.

cond-mat.mes-hall↗

arXiv2021

Slow oxidation of magnetite nanoparticles elucidates the limits of the Verwey transition

Magnetite (Fe3O4) is of fundamental importance as the original magnetic material and also for the Verwey transition near T_V = 125 K, below which a complex lattice distortion and electron orders occur. The Verwey transition is suppressed by strain or chemical doping effects giving rise to well-documented first and second-order regimes, but the origin of the order change is unclear. Here, we show that slow oxidation of monodisperse Fe3O4 nanoparticles leads to an intriguing variation of the Verwey transition that elucidates the doping effects. Exposure to various fixed oxygen pressures at ambient temperature leads to an initial drop to TV minima as low as 70 K after 45-75 days, followed by recovery to a constant value of 95 K after 160 days that persists in all experiments for aging times up to 1070 days. A physical model based on both doping and doping-gradient effects accounts quantitatively for this evolution and demonstrates that the persistent 95 K value corresponds to the lower limit for homogenously doped magnetite and hence for the first order regime. In comparison, further suppression down to 70 K results from inhomogeneous strains that characterize the second-order region. This work demonstrates that slow reactions of nanoparticles can give exquisite control and separation of homogenous and inhomogeneous doping or strain effects on an nm scale and offers opportunities for similar insights into complex electronic and magnetic phase transitions in other materials.

cond-mat.mtrl-sci↗

arXiv2020

V 3$d$ charge and orbital states in V$_2$OPO$_4$ probed by x-ray absorption spectroscopy

V 3$d$ charge and orbital states in V$_2$OPO$_4$ have been investigated by means of x-ray absorption spectroscopy (XAS). The electronic structure of V$_2$OPO$_4$ is very unique in that the charge transfer between V$^{2+}$ and V$^{3+}$ in face sharing VO$_6$ chains provides negative thermal expansion as reported by Pachoud {\it et al.} [J. Am. Chem. Soc. {\bf 140}, 636 (2018).] The near edge region of O 1$s$ XAS exhibits the three features which can be assigned to transitions to O 2$p$ mixed into the unoccupied V 3$d$ $t_{2g}$ and $e_{g}$ orbitals of V$^{2+}$ and V$^{3+}$. The V 2$p$ XAS line shape can be reproduced by multiplet calculations for a mixed valence state with V$^{2+}$ and V$^{3+}$. The polarization dependence of the O 1$s$ and V 2$p$ XAS spectra indicates V 3$d$ orbital order in which $xy$ and $yz$ (or $zx$) orbitals are occupied at the V$^{3+}$ site in the face sharing chains. The occupied $xy$ orbital is essential for the antiferromagnetic coupling between the V$^{2+}$ and V$^{3+}$ sites along the chains while the occupied $yz$ (or $zx$) orbital provides the antiferromagnetic coupling between the V$^{2+}$ and V$^{3+}$ sites between the chains.

cond-mat.str-el↗

arXiv2019

Coexistence of metallic and nonmetallic properties in the pyrochlore Lu$_2$Rh$_2$O$_7$

Transition metal oxides of the $4d$ and $5d$ block have recently become the targets of materials discovery, largely due to their strong spin-orbit coupling that can generate exotic magnetic and electronic states. Here we report the high pressure synthesis of Lu$_2$Rh$_2$O$_7$, a new cubic pyrochlore oxide based on $4d^5$ Rh$^{4+}$ and characterizations via thermodynamic, electrical transport, and muon spin relaxation measurements. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetic contribution, while heat capacity shows an enhanced Sommerfeld coefficient, $γ$ = 21.8(1) mJ/mol-Rh K$^2$. Muon spin relaxation measurements confirm that Lu$_2$Rh$_2$O$_7$ remains paramagnetic down to 2 K. Taken in combination, these three measurements suggest that Lu$_2$Rh$_2$O$_7$ is a correlated paramagnetic metal with a Wilson ratio of $R_W = 2.5$. However, electric transport measurements present a striking contradiction as the resistivity of Lu$_2$Rh$_2$O$_7$ is observed to monotonically increase with decreasing temperature, indicative of a nonmetallic state. Furthermore, although the magnitude of the resistivity is that of a semiconductor, the temperature dependence does not obey any conventional form. Thus, we propose that Lu$_2$Rh$_2$O$_7$ may belong to the same novel class of non-Fermi liquids as the nonmetallic metal FeCrAs.

cond-mat.str-el↗

arXiv2013

Breathing Pyrochlore Lattice Realized in A-Site Ordered Spinel Oxides LiGaCr4O8 and LiInCr4O8

A unique type of frustrated lattice is found in two A-site ordered spinel oxides, LiGaCr4O8 and LiInCr4O8. Because of the large size mismatch between Li+ and Ga3+/In3+ ions at the A site, the pyrochlore lattice, made up of Cr3+ ions carrying spin 3/2, becomes an alternating array of small and large tetrahedra, i.e., a "breathing" pyrochlore lattice. We introduce a parameter, the breathing factor Bf, which quantifies the degree of frustration in the pyrochlore lattice: Bf is defined as J'/J, where J' and J are nearest-neighbor magnetic interactions in the large and small tetrahedra, respectively. LiGaCr4O8 with Bf ~ 0.6 shows magnetic susceptibility similar to that of conventional Cr spinel oxides such as ZnCr2O4. In contrast, LiInCr4O8 with a small Bf ~ 0.1 exhibits a spin-gap behavior in its magnetic susceptibility, suggesting a proximity to an exotic singlet ground state. Magnetic long-range order occurs at 13.8 and 15.9 K for LiGaCr4O8 and LiInCr4O8, respectively, in both cases likely owing to the coupling to structural distortions.

cond-mat.mtrl-sci↗

arXiv2012

Charge order at the frontier between the molecular and solid states in Ba3NaRu2O9

We show that the valence electrons of Ba3NaRu2O9, which has a quasi-molecular structure, completely crystallize below 210 K. Using an extended Hubbard model, we show that the charge ordering instability results from long-range Coulomb interactions. However, orbital ordering, metal-metal bonding and formation of a partial spin gap enforce the magnitude of the charge separation. The striped charge order and frustrated hcp lattice of Ru2O9 dimers lead to competition with a quasi-degenerate charge-melted phase under photo-excitation at low temperature. Our results establish a broad class of simple metal oxides as models for emergent phenomena at the border between the molecular and solid states.

cond-mat.str-el↗

arXiv2012

Electronic Orders in the Verwey Structure of Magnetite

Electronic structure calculations of the Verwey ground state of magnetite, Fe3O4, using density functional theory with treatment of on-site Coulomb interactions (DFT+U scheme) are reported. These calculations use the recently-published experimental crystal structure coordinates for magnetite in the monoclinic space group Cc. The computed density distribution for minority spin electron states close to the Fermi level demonstrates that charge order and Fe2+-orbital order are present at the B-type lattice sites to a first-approximation. However, Fe2+/Fe3+ charge differences are diminished through weak bonding interactions of the Fe2+-states to specific pairs of neighboring iron sites that create linear, three-B-atom trimeron units that may be regarded as 'orbital molecules'. Trimerons are ordered evenly along most Fe atom chains in the Verwey structure, but more complex interactions are observed within one chain.

cond-mat.str-el↗

arXiv2010

Chemistry and High Temperature Superconductivity

Seven distinct families of superconductors with critical temperatures at ambient pressure that equal or surpass the historic 23 K limit for Nb3Ge have been discovered in the last 25 years. Each family is reviewed briefly and their common chemical features are discussed. High temperature superconductors are distinguished by having a high (\geq 50%) content of nonmetallic elements and fall into two broad classes. 'Metal-nonmetal' superconductors require a specific combination of elements such as Cu-O and Fe-As which give rise to the highest known Tc's, probably through a magnetic pairing mechanism. 'Nonmetal-bonded' materials contain covalently-bonded nonmetal anion networks and are BCS-like superconductors. Fitting an extreme value function to the distribution of Tc values for the known high-Tc families suggests that the probability of a newly discovered superconductor family having maximum Tc > 100 K is ~0.1-1%, decreasing to ~0.02-0.2% for room temperature superconductivity.

cond-mat.supr-con↗

arXiv2009

Investigation of Superconducting Gap Structure in TbFeAsO$_{0.9}$F$_{0.1}$ using Point Contact Andreev Reflection

Bulk samples of TbFeAsO$_{0.9}$F$_{0.1}$ (T$_{c}$(on) = 50K) were measured by point contact Andreev reflection spectroscopy. The spectra show unambiguous evidence for multiple gap-like features plus the presence of high bias shoulders. By measuring the spectra as a function of temperature with both gold and superconducting niobium tips, we establish that the gap-like features are associated with superconducting order parameter in this material. We discuss whether the well defined zero bias conductance peak that we observe infrequently is associated with a nodal superconducting order parameter.

cond-mat.supr-con↗

arXiv2008

High pressure synthesis of late rare earth RFeAs(O,F) superconductors; R = Tb and Dy

New TbFeAs(O,F) and DyFeAs(O,F) superconductors with critical temperatures Tc= 46 and 45 K and very high critical fields over 100 T have been prepared at 1100- 1150C and 10-12 GPa, demonstrating that high pressure may be used to synthesise late rare earth derivatives of the recently reported RFeAs(O,F) (R = La - Nd, Sm, Gd) high temperature superconductors.

cond-mat.supr-con↗

arXiv2008

Metamagnetism and 1/3 Plateau in the Spin Chain Compound CoV2O6

The pseudo-one dimensional brannerite type compound CoV2O6 has been studied by magnetisation and heat capacity measurements in the temperature range 1.8 < T < 300 K and in applied fields of up to 9 T. Our measurements show an unusual balance of exchange interactions (CW= 2.8(2) K) and a single antiferromagnetic transition at TN = 7 K. M(H) isotherms recorded below 5 K show a metamagnetic transition and a 1/3 magnetisation plateau. From 5-7 K, only the metamagnetic transition is observed. For T < TN < ~ 25 K, our heat capacity and magnetisation measurements show evidence for strong low dimensional ferromagnetic fluctuations. We propose a simple phase diagram, discuss the principle features and emphasise the importance of Ising anisotropy.

cond-mat.str-el↗