arXiv subjects

Thomas J. Hicken

Publications and source records attributed to Thomas J. Hicken.

27 records · Page 2Linked to original sources

arXiv2025

Pressure-enhanced splitting of density wave transitions in La$_3$Ni$_2$O$_{7-δ}$

The observation of superconductivity in La$_3$Ni$_2$O$_{7-δ}$ under pressure, following the suppression of a high-temperature density wave state, has attracted considerable attention. The nature of this density wave order was not clearly identified. Here, we probe the magnetic response of the zero-pressure phase of La$_3$Ni$_2$O$_{7-δ}$ as hydrostatic pressure is applied and find that the apparent single density wave transition at zero applied pressure splits into two. The comparison of our muon-spin rotation and relaxation experiments with dipole-field numerical analysis reveals the magnetic structure's compatibility with a stripe-type arrangement of Ni moments, characterized by alternating lines of magnetic moments and nonmagnetic stripes at ambient pressure. When pressure is applied, the magnetic ordering temperature increases, while the unidentified density wave transition temperature falls. Our findings reveal that the ground state of the La$_3$Ni$_2$O$_{7-δ}$ system is characterized by the coexistence of two distinct orders -- a magnetically ordered spin density wave and a lower-temperature ordering that is most likely a charge density wave -- with a notable pressure-enhanced separation between them.

cond-mat.supr-con↗

arXiv2025

Pressure Effect on the Spin Density Wave Transition in La$_2$PrNi$_2$O$_{6.96}$

High-pressure studies reveal a stark contrast between the superconducting properties of double-layer Ruddlesden-Popper (RP) nickelates La$_2$PrNi$_2$O$_7$ and La$_3$Ni$_2$O$_7$. While La$_2$PrNi$_2$O$_7$ exhibits bulk superconductivity, La$_3$Ni$_2$O$_7$ displays filamentary behavior, suggesting that superconductivity is confined to phase interfaces rather than the bulk. Since magnetism emerges near the superconducting phase, understanding its differences in La$_3$Ni$_2$O$_7$ and La$_2$PrNi$_2$O$_7$ is essential for clarifying their underlying electronic and magnetic properties. In this work we study the magnetic responce of La$_2$PrNi$_2$O$_{6.96}$ under pressures up to 2.3 GPa using the muon-spin rotation/relaxation ($μ$SR) technique. The application of external pressure increases the Néel temperature $T_{\rm N}$ from approximately 161 K at ambient pressure ($p=0$) to about 170 K at $p=2.3$ GPa. The temperature dependence of the internal magnetic field $B_{\rm int}(T)$ (i.e., the magnetic order parameter) follows the power-law relation $B_{\rm int} = B_{\rm int}(0) \left(1 - \left[T/T_{\rm N}\right]^α\right)^β$, with consistent exponent values of $α\simeq 1.95$ and $β\simeq 0.35$ across different pressures. The value of the ordered moments at the Ni sites, which is proportional to $B_{\rm int}$, remain unaffected by pressure. Our findings suggest that the magnetic properties of double-layer RP nickelate La$_3$Ni$_2$O$_7$ are broadly unaffected by Pr to La substitution.

cond-mat.supr-con↗

arXiv2025

Unraveling Spin Density Wave Order in Layered Nickelates $\mathrm{La_3Ni_2O_7}$ and $\mathrm{La_2PrNi_2O_7}$ via Neutron Diffraction

The discovery of pressure-induced superconductivity in two- and three-layer Ruddlesden-Popper nickelates has generated significant interest in these materials as a platform for unconventional superconductivity. While their ground state exhibits magnetism, a direct determination of their magnetic structure remains elusive. Understanding this aspect is crucial, as magnetism may play a role in the pairing mechanism of superconductivity in these compounds. We resolve the magnetic structures of the bilayer (2222) polymorphs of La3Ni2O7 and La2PrNi2O7 using neutron powder diffraction (NPD) and muon-spin rotation/relaxation (muSR). Magnetic neutron scattering appears below approximately 150 K in both compounds and is observed at the (qx, 1/2, 0) position, with qx = 0 and 1/2 for La3Ni2O7 and qx = 0 for La2PrNi2O7. Within a single layer, alternating low (0.05 - 0.075 muB) and high (0.66 muB) magnetic moment stripes form. These layers stack antiferromagnetically along the c-direction to form bilayers. The presence of two propagation vectors (qx = 0 and 1/2) in undoped La3Ni2O7 suggests the coexistence of two magnetic stacking polymorphs within a single crystallographic phase. The muSR spectra further confirm these magnetic structures. Our findings provide a detailed understanding of the magnetic ground state in bilayer nickelates, offering insights into possible precursor states that may influence the emergence of superconductivity in these materials.

cond-mat.supr-con↗

arXiv2024

Uncovering the Hidden Ferroaxial Density Wave as the Origin of the Axial Higgs Mode in RTe$_3$

The recent discovery of an axial amplitude (Higgs) mode in the long-studied charge density wave (CDW) systems GdTe$_3$ and LaTe$_3$ suggests a heretofore unidentified hidden order. A theoretical study proposed that the axial Higgs results from a hidden ferroaxial component of the CDW, which could arise from non-trivial orbital texture. Here, we report extensive experimental studies on ErTe$_3$ and HoTe$_3$ that possess a high-temperature CDW similar to other RTe$_3$ (R = rare earth), along with an additional low-temperature CDW with an orthogonal ordering vector. Combining Raman spectroscopy with large-angle convergent beam electron diffraction (LACBED), rotational anisotropy second-harmonic generation (RA-SHG), and muon-spin relaxation ($μ$SR), we provide unambiguous evidence that the high-temperature CDW breaks translation, rotation, and all vertical and diagonal mirror symmetries, but not time-reversal or inversion. In contrast, the low-temperature CDW only additionally breaks translation symmetry. Simultaneously, Raman scattering shows the high-temperature CDW produces an axial Higgs mode while the low-temperature mode is scalar. The weak monoclinic structural distortion and clear axial response in Raman and SHG are consistent with a ferroaxial phase in RTe$_3$ driven by coupled orbital and charge orders. Thus, our study provides a new standard for uncovering unconventional orders and confirms the power of Higgs modes to reveal them.

cond-mat.str-el↗

arXiv2024

Coexistence of local magnetism and superconductivity in the heavy-fermion CeRh$_2$As$_2$ revealed by $μ$SR studies

The superconducting (SC) state ($T_\mathrm{c}$ = 0.3 K) of the heavy-fermion compound CeRh$_2$As$_2$, which undergoes an unusual field-induced transition to another high-field SC state, emerges from an unknown ordered state below $T_\mathrm{o}$ = 0.55 K. While an electronic multipolar order of itinerant Ce-4$f$ states was proposed to account for the $T_\mathrm{o}$ phase, the exact order parameter has not been known to date. Here, we report on muon spin relaxation ($μ$SR) studies of the magnetic and SC properties in CeRh$_2$As$_2$ single crystals at low temperatures. We reveal a magnetic origin of the $T_\mathrm{o}$ order by identifying a spontaneous internal field below $T_\mathrm{o}$ = 0.55 K. Furthermore, we find evidence of a microscopic coexistence of local magnetism with bulk superconductivity. Our findings open the possibility that the $T_\mathrm{o}$ phase involves both dipole and higher order Ce-4$f$ moment degrees of freedom and accounts for the unusual non-Fermi liquid behavior.

cond-mat.str-el↗

arXiv2021

Probing the magnetic polaron state in the ferromagnetic semiconductor HgCr$_2$Se$_4$ with resistance fluctuation and muon-spin spectroscopy measurements

Combined resistance noise and muon-spin relaxation ($μ$SR) measurements of the ferromagnetic semiconductor HgCr$_2$Se$_4$ suggest a degree of magnetoelectric coupling and provide evidence for the existence of isolated magnetic polarons. These form at elevated temperatures and undergo a percolation transition with a drastic enhancement of the low-frequency 1/$f$-type charge fluctuations at the insulator-to-metal transition at $\sim 95 - 98$ K in the vicinity of the magnetic ordering temperature $T_C \sim 105 - 107$ K. Upon approaching the percolation threshold from above, the strikingly unusual dynamics of a distinct two-level fluctuator superimposed on the $1/f$ noise can be described by a slowing down of the dynamics of a nanoscale magnetic cluster, a magnetic polaron, when taking into account an effective radius of the polaron depending on the spin correlation length. Coinciding temperature scales found in $μ$SR and noise measurements suggest changes in the magnetic dynamics over a wide range of frequencies and are consistent with the existence of large polarized and domain-wall-like regions at low temperatures, that result from the freezing of spin dynamics at the magnetic polaron percolation transition.

cond-mat.str-el↗

arXiv2021

Magnetic ground-state of the one-dimensional ferromagnetic chain compounds $M$(NCS)$_2$(thiourea)$_2$; $M$ = Ni, Co

The magnetic properties of the two isostructural molecule-based magnets, Ni(NCS)$_{2}$(thiourea)$_{2}$, $S$ = 1, [thiourea = SC(NH$_2$)$_2$] and Co(NCS)$_{2}$(thiourea)$_{2}$, $S$ = 3/2, are characterised using several techniques in order to rationalise their relationship with structural parameters and ascertain magnetic changes caused by substitution of the spin. Zero-field heat capacity and muon-spin relaxation measurements reveal low-temperature long-range ordering in both compounds, in addition to Ising-like ($D < 0$) single-ion anisotropy ($D_{\rm{Co}} \sim$ -100 K, $D_{\rm{Ni}} \sim$ -10 K). Crystal and electronic structure, combined with DC-field magnetometry, affirm highly quasi-one-dimensional behaviour, with ferromagnetic intrachain exchange interactions $J_{\rm{Co}}\approx+4$ K and $J_{\rm{Ni}}\sim+100$ K and weak antiferromagnetic interchain exchange, on the order of $J'$ $\sim-0.1$ K. Electron charge and spin-density mapping reveals through-space exchange as a mechanism to explain the large discrepancy in $J$-values despite, from a structural perspective, the highly similar exchange pathways in both materials. Both species can be compared to the similar compounds $M$Cl$_2$(thiourea)$_4$, $M$ = Ni(II) (DTN) and Co(II) (DTC), where DTN is know to harbour two magnetic field-induced quantum critical points. Direct comparison of DTN and DTC with the compounds studied here shows that substituting the halide Cl$^-$ ion, for the NCS$^-$ ion, results in a dramatic change in both the structural and magnetic properties.

cond-mat.str-el↗

arXiv2019

A Near-Ideal Molecule-Based Haldane Spin-Chain

The molecular coordination complex NiI$_2$(3,5-lut)$_4$ [where (3,5-lut) $=$ (3,5-lutidine) $=$ (C$_7$H$_9$N)] has been synthesized and characterized by several techniques including synchrotron X-ray diffraction, ESR, SQUID magnetometry, pulsed-field magnetization, inelastic neutron scattering and muon spin relaxation. Templated by the configuration of 3,5-lut ligands the molecules pack in-registry with the Ni--I$\cdots$I--Ni chains aligned along the $c$--axis. This arrangement leads to through-space I$\cdots$I magnetic coupling which is directly measured for the first time in this work. The net result is a near-ideal realization of the $S = 1$ Haldane chain with $J = 17.5~\rm{K}$ and energy gaps of $Δ^{\parallel} = 5.3~{\rm K}$ $Δ^{\perp} =7.7~{\rm K}$, split by the easy-axis single-ion anisotropy $D=-1.2~{\rm K}$. The ratio $D/J = -0.07$ affords one of the most isotropic Haldane systems yet discovered, while the ratio $Δ_0/J = 0.40(1)$ (where $Δ_0$ is the average gap size) is close to its ideal theoretical value, suggesting a very high degree of magnetic isolation of the spin chains in this material. The Haldane gap is closed by orientation-dependent critical fields $μ_0H_{\rm c}^{\parallel} = 5.3~\rm{T}$ and $μ_0H_{\rm c}^{\perp} = 4.3~\rm{T}$, which are readily accessible experimentally and permit investigations across the entirety of the Haldane phase, with the fully polarized state occurring at $μ_0 H_{\rm s}^{\parallel}=46.0~\rm{T}$ and $μ_0 H_{\rm s}^{\perp}=50.7~\rm{T}$. The results are explicable within the so-called fermion model, in contrast to other reported easy-axis Haldane systems. Zero-field magnetic order is absent down to $20~{\rm mK}$ and emergent end-chain effects are observed in the gapped state, as evidenced by detailed low-temperature measurements.

cond-mat.str-el↗

arXiv2018

Magnetic phases of skyrmion-hosting GaV$_4$S$_{8-y}$Se$_{y}$ ($y = 0, 2, 4, 8$) probed with muon spectroscopy

We present the results of a muon-spin spectroscopy investigation of GaV$_4$S$_{8-y}$Se$_{y}$ with $y=0, 2, 4$ and 8. Zero-field measurements suggest that GaV$_{4}$Se$_{8}$ and GaV$_{4}$S$_{8}$ have distinct magnetic ground states, with the latter material showing an anomalous temperature-dependence of the local magnetic field. It is not possible to evolve the magnetic state continuously between these two systems, with the intermediate $y=2$ and $4$ materials showing glassy magnetic behaviour at low temperature. The skyrmion lattice (SkL) phase is evident in the $y=0$ and 8 materials through an enhanced response of the muon-spin relaxation to the emergent dynamics that accompany the SkL. For our polycrystalline samples of GaV$_4$Se$_{8}$, this enhanced dynamic response is confined to a smaller region of the magnetic field-temperature phase diagram than the previous reports of the SkL in single crystals.

cond-mat.mtrl-sci↗