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P. C. Canfield

Publications and source records attributed to P. C. Canfield.

At least 37 records · Page 2Linked to original sources
arXiv2023

Measurements of nematic susceptibility with phase sensitive nuclear magnetic resonance in pulsed strain fields

We present nuclear magnetic resonance data in BaFe$_2$As$_2$ in the presence of pulsed strain fields that are interleaved in time with the radiofrequency excitation pulses. In this approach, the precessing nuclear magnetization acquires a phase shift that is proportional to the strain and pulse time. The sensitivity in this approach is limited by the homogeneous decoherence time, $T_2$, rather than the inhomogeneous linewidth. We measure the nematic susceptibility as a function of temperature, and demonstrate three orders of magnitude improvement in sensitivity. This approach will enable studies of the strain response in a broad range of materials that previously were inaccessible due to inhomogeneous broadening.

cond-mat.supr-con↗
arXiv2023

Formation of a simple cubic antiferromagnet through charge ordering in a double Dirac material

The appearance of spontaneous charge order in chemical systems is often associated with the emergence of novel, and useful, properties. Here we show through single crystal diffraction that the Eu ions in the mixed valent metal EuPd$_3$S$_4$ undergo long-range charge ordering at $T_{\mathrm{CO}} = 340 \mathrm{~K}$ resulting in simple cubic lattices of Eu$^{2+}$ ($J = 7/2$) and Eu$^{3+}$ ($J = 0$) ions. As only one of the two sublattices has a non-magnetic ground state, the charge order results in the emergence of remarkably simple G-type antiferromagnetic order at $T_{\mathrm{N}} = 2.85(6) \mathrm{~K}$, observed in magnetization, specific heat, and neutron diffraction. Application of a $0.3 \mathrm{~T}$ field is sufficient to induce a spin flop transition to a magnetically polarized, but still charge ordered, state. Density functional theory calculations show that this charge order also modifies the electronic degeneracies present in the material: without charge order, EuPd$_3$S$_4$ is an example of a double Dirac material containing 8-fold degenerate electronic states, greater than the maximum degeneracy of six possible in molecular systems. The symmetry reduction from charge order transmutes 8-fold double Dirac states into 4-fold Dirac states, a degeneracy that can be preserved even in the presence of the magnetic order. Our results show not only how charge order can be used to produce interesting magnetic lattices, but also how it can be used to engineer controlled degeneracies in electronic states.

cond-mat.str-el↗
arXiv2023

Short-range magnetic correlations in quasicrystalline i-Tb-Cd

We report on elastic and inelastic neutron scattering from single-grain isotopically-enriched samples to elucidate the local magnetic correlations between Tb$^{3+}$ moments in quasicrystalline i-Tb-Cd. The inelastic neutron scattering measurements of the CEF excitations demonstrated that the Tb$^{3+}$ moments are directed primarily along the local five-fold axes of the Tsai-type cluster as was found for the TbCd6 approximant phase. Based on the inelastic measurements, we consider of a simple Ising-type model for the moment configurations on a single Tb$^{3+}$ icosahedron and enumerate the lowest energy moment configurations. We then calculate the diffuse scattering from these configurations and compare with the experimental magnetic diffuse scattering measurements to identify the most likely single cluster moment configurations and find reasonable agreement between the broad features observed in our scattering simulations. We further consider the role of higher-order (longer range) intercluster correlations for the magnetic scattering.

cond-mat.str-el↗
arXiv2023

Microscopic characterization of the magnetic properties of the itinerant antiferromagnet La2Ni7 by 139La NMR/NQR measurements

139La nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been performed to investigate the magnetic properties of the itinerant magnet La2Ni7 which shows a series of antiferromagnetic (AFM) phase transitions at $T_{N1}$=61 K, $T_{N2}$=56 K, and $T_{N3}$=42 K under zero magnetic field. Two distinct La NMR signals were observed due to the two crystallographically inequivalent La sites in La2Ni7 (La1 and La2 in the La2Ni4 and the LaNi5 sub-units of the La2Ni7 unit cell, respectively). From the 139La NQR spectrum in the AFM state below $T_{N3}$, the AFM state was revealed to be a commensurate state where Ni ordered moments align along the crystalline c axis. Owing to the two different La sites, we were able to estimate the average values of the Ni ordered moments ($\sim$0.09-0.10 $μ_{B}$/Ni and $\sim$0.17$μ_{B}$/Ni around La1 and La2, respectively) from 139La NMR spectrum measurements in the AFM state below $T_{N3}$, suggesting a non-uniform distribution of the Ni-ordered moments in the AFM state. In contrast, a more uniform distribution of the Ni-ordered moments in the saturated paramagnetic state induced by the application of high magnetic fields is observed. The temperature dependence of the sublattice magnetization measured by the internal field at the La2 site in the AFM state was reproduced by a local moment model better than the self-consistent renormalization (SCR) theory for weak itinerant antiferromagnets. Given the small Ni-ordered moments in the magnetically ordered state, our results suggest that La2Ni7 has characteristics of both itinerant and localized natures in its magnetism. With this in mind, it is noteworthy that the temperature dependence of nuclear spin-relaxation rates in the paramagnetic state above $T_{N1}$ measured at zero magnetic field can be explained qualitatively by both the SCR theory and the local-moment model.

cond-mat.str-el↗
arXiv2023

Spin-density-wave order controlled by uniaxial stress in CeAuSb$_2$

The tetragonal heavy-fermion compound CeAuSb$_2$ (space group $P4/nmm$) exhibits incommensurate spin density wave (SDW) order below $T_{N}\approx6.5~K$ with the propagation vector $\mathbf{q}_A = (δ_A,δ_A,1/2)$. The application of uniaxial stress along the [010] direction induces a sudden change in the resistivity ratio $ρ_a/ρ_b$ at a compressive strain of $ε\approx -0.5$\%. Here we use neutron scattering to show that the uniaxial stress induces a first-order transition to a SDW state with a different propagation vector $(0,δ_B,1/2)$ with $δ_B=0.25$. The magnetic structure of the new (B) phase consists of Ce layers with ordered moments alternating with layers with zero moment stacked along the $c$-axis. The ordered layers have an up-up-down-down configuration along the $b$-axis. This is an unusual situation in which the loss of spatial inversion is driven by the magnetic order. We argue that the change in SDW wavevector leads to Fermi surface reconstruction and a concomitant change in the transport properties.

cond-mat.str-el↗
arXiv2023

Strong enhancement of magnetic ordering temperature and structural/valence transitions in EuPd3S4 under high pressure

We present a comprehensive study of the mixed valent compound, EuPd3S4, by electrical transport, X-ray diffraction, time-domain 151Eu synchrotron Mössbauer spectroscopy, and X-ray absorption spectroscopy measurements under high pressure. The electrical transport measurements show that the antiferromagnetic ordering temperature, TN, increases rapidly from 2.8 K at ambient pressure to 23.5 K at ~19 GPa and plateaus between ~19 and ~29 GPa after which no anomaly associated with TN is detected. A pressure-induced first order structural transition from cubic to tetragonal is observed, with a rather broad coexistence region (~20 GPa to ~32 GPa) that corresponds to the TN plateau. Mössbauer spectroscopy measurements show a clear valence transition from approximately 50:50 Eu2+:Eu3+ to fully Eu3+ at ~28 GPa, consistent with the vanishing of the magnetic order at the same pressure. X-ray absorption data show a transition to a fully trivalent state at a similar pressure. Our results show that pressure first greatly enhances TN, most likely via enhanced hybridization between the Eu 4f states and the conduction band, and then, second, causes a structural phase transition that coincides with the conversion of the europium to a fully trivalent state.

cond-mat.str-el↗
arXiv2023

Magnetic ground state and perturbations of the distorted kagome Ising metal TmAgGe

We present the magnetic orders and excitations of the distorted kagome intermetallic magnet TmAgGe. Using neutron single crystal diffraction we identify the propagation vectors $\bf{k}$ = (1/2 0 0) and $\bf{k}$ = (0 0 0) and determine the magnetic structures of the zero-field and magnetic field-induced phases for $H$ along the $a$ and [-1 1 0] crystal directions. We determine the experimental magnetic field-temperature ($H$, $T$)-phase diagram and reproduce it by Monte-Carlo simulations of an effective spin exchange Hamiltonian for one distorted kagome layer. Our model includes a strong axial single-ion anisotropy and significantly smaller exchange couplings which span up to the third-nearest neighbours within the layer. Single crystal inelastic neutron scattering (INS) measurements reveal an almost flat, only weakly dispersive mode around 7 meV that we use alongside bulk magnetization data to deduce the crystal-electric field (CEF) scheme for the Tm$^{3+}$ ions. Random phase approximation (RPA) calculations based on the determined CEF wave functions of the two lowest quasi-doublets enable an estimation of the interlayer coupling that is compatible with the experimental INS spectra. No evidence for low-energy spin waves associated to the magnetic order was found, which is consistent with the strongly Ising nature of the ground state.

cond-mat.str-el↗
arXiv2023

Directional effects of antiferromagnetic ordering on the electronic structure in NdSb

The recent discovery of unconventional surface state pairs, which give rise to Fermi arcs and spin textures, in antiferromagnetically ordered NdBi raised the interest in rare-earth monopnictides. Several scenarios of antiferromagnetic order have been suggested to explain the origin of these states with some of them being consistent with the presence of non-trivial topologies. In this study, we use angle-resolved photoemission spectroscopy (ARPES) and density-functional-theory (DFT) calculations to investigate the electronic structure of NdSb. We found the presence of distinct domains that have different electronic structure at the surface. These domains correspond to different orientations of magnetic moments in the AFM state with respect to the surface. We demonstrated remarkable agreement between DFT calculations and ARPES that capture all essential changes in the band structure caused by transition to a magnetically ordered state.

cond-mat.str-el↗
arXiv2023

Unconventional Surface State Pairs in a High-Symmetry Lattice with Anti-ferromagnetic Band-folding

Many complex magnetic structures in a high-symmetry lattice can arise from a superposition of well-defined magnetic wave vectors. These "multi-q" structures have garnered much attention because of interesting real-space spin textures such as skyrmions. However, the role multi-q structures play in the topology of electronic bands in momentum space has remained rather elusive. Here we show that the type-I anti-ferromagnetic 1q, 2q and 3q structures in an face-centered cubic sublattice with band inversion, such as NdBi, can induce unconventional surface state pairs inside the band-folding hybridization bulk gap. Our density functional theory calculations match well with the recent experimental observation of unconventional surface states with hole Fermi arc-like features and electron pockets below the Neel temperature. We further show that these multi-q structures have Dirac and Weyl nodes. Our work reveals the special role that band-folding from anti-ferromagnetism and multi-q structures can play in developing new types of surface states.

cond-mat.mtrl-sci↗
arXiv2023

Interrelationships between nematicity, antiferromagnetic spin fluctuations and superconductivity: Role of hotspots in FeSe$_{1-x}$S$_{x}$ revealed by high pressure $^{77}$Se NMR study

The sulfur-substituted FeSe, FeSe$_{1-x}$S$_{x} $, is one of the unique systems that provides an independent tunability of nematicity, antiferromagnetism and superconductivity under pressure ($p$). Recently Rana et al. [Phys. Rev. B 101, 180503(R) (2020)] reported, from $^{77}$Se nuclear magnetic resonance (NMR) measurements on FeSe$_{0.91}$S$_{0.09}$ under pressure, that there exists a clear role of nematicity on the relationship between antiferromagnetic (AFM) spin fluctuations and superconducting transition temperature ($T_{\rm c}$) where the AFM spin fluctuations are more effective in enhancing $T_{\rm c}$ in the absence of nematicity than with nematicity. Motivated by the work, we carried out $^{77}$Se NMR measurements on FeSe$_{1-x}$S$_{x}$ with $x$= 0.15 and 0.29 under pressure up to 2.10 GPa to investigate the relationship in a wide range of $x$ in the FeSe$_{1-x}$S$_x$ system. Based on the new results together with the previously reported data for $x$=0 [P. Wiecki et al., Phys. Rev. B 96, 180502(R) (2017)] and 0.09 [K. Rana et al. Phys. Rev. B 101, 180503(R) (2020)], we established a $p$ - $x$ - temperature ($T$) phase diagram exhibiting the evolution of AFM spin fluctuations. From the systematic analysis of the NMR data, we found that the superconducting (SC) state in nematic state arises from a non Fermi liquid state with strong stripe-type AFM spin fluctuations while the SC state without nematicity comes from a Fermi liquid state with mild stripe-type AFM spin fluctuations. Furthermore, we show that the previously reported impact of nematicity on the relationship between AFM fluctuations and superconductivity holds throughout the wide range of $x$ from $x$ = 0 to 0.29 in FeSe$_{1-x}$S$_{x}$ under pressure. We discuss the origin of the role of nematicity in terms of the different numbers of hotspots on Fermi surfaces with and without nematicity.

cond-mat.supr-con↗
arXiv2023

Orbital character of the spin-reorientation transition in TbMn$_6$Sn$_6$

Ferromagnetic (FM) order in a two-dimensional kagome layer is predicted to generate a topological Chern insulator without an applied magnetic field. The Chern gap is largest when spin moments point perpendicular to the kagome layer, enabling the capability to switch topological transport properties, such as the quantum anomalous Hall effect, by controlling the spin orientation. In TbMn$_{6}$Sn$_{6}$, the uniaxial magnetic anisotropy of the Tb$^{3+}$ ion is effective at generating the Chern state within the FM Mn kagome layers while a spin-reorientation (SR) transition to easy-plane order above $T_{SR}=310$ K provides a mechanism for switching. Here, we use inelastic neutron scattering to provide key insights into the fundamental nature of the SR transition. The observation of two Tb excitations, which are split by the magnetic anisotropy energy, indicates an effective two-state orbital character for the Tb ion, with a uniaxial ground state and an isotropic excited state. The simultaneous observation of both modes below $T_{SR}$ confirms that orbital fluctuations are slow on magnetic and electronic time scales $<$ ps and act as a spatially-random orbital alloy. A thermally-driven critical concentration of isotropic Tb ions triggers the SR transition.

cond-mat.str-el↗
arXiv2023

Unusual coercivity and zero field stabilization of fully saturated magnetization in single crystals of LaCrGe$_3$

LaCrGe$_3$ is an itinerant, metallic ferromagnet with a Curie temperature ($T_C$) of $\sim$ 86 K. Whereas LaCrGe$_3$ has been studied extensively as a function of pressure as an example of avoided ferromagnetic quantum criticality, questions about its ambient pressure ordered state remain; specifically, whether there is a change in the nature of the ferromagnetically ordered state below $T_C$ $\sim$ 86 K. We present anisotropic $M$($H$) isotherms, coupled with anisotropic AC susceptibility data, and demonstrate that LaCrGe$_3$ has a remarkable, low temperature coercivity associated with exceptionally sharp, complete magnetization reversals to and from fully polarized states. This coercivity is temperature dependent, it drops to zero in the 40 - 55 K region and reappears in the 70 - 85 K regions. At low temperatures LaCrGe$_3$ has magnetization loops and behavior that has previously associated with micromagnetic/nanocrystalline materials, not bulk, macroscopic samples.

cond-mat.str-el↗
arXiv2023

Superconductivity and magnetic and transport properties of single-crystalline CaK(Fe$_{1-x}$Cr$_{x}$)$_{4}$As$_{4}$

Members of the CaK(Fe$_{1-x}$Cr$_{x}$)$_{4}$As$_{4}$ series have been synthesized by high-temperature solution growth in single crystalline form and characterized by X-ray diffraction, elemental analysis, magnetic and transport measurements. The effects of Cr substitution on the superconducting and magnetic ground states of CaKFe$_4$As$_4$ ($T_c$ = 35 K) have been studied. These measurements show that the superconducting transition temperature decreases monotonically and is finally suppressed below 1.8 K as $x$ is increased from 0 to 0.038. The magnetic transition temperature increases in a roughly linear manner as Cr substitution increases. A temperature-composition (\textit{T}-\textit{x}) phase diagram is constructed, revealing a half-dome of superconductivity with the magnetic transition temperature, $T^*$, appearing near 22~K for $x$ $\sim$ 0.017 and rising slowly up to 60~K for $x$ $\sim$ 0.077. The $T$-$x$ phase diagrams for CaK(Fe$_{1-x}$$T$$_{x}$)$_4$As$_4$ for $T$ = Cr and Mn are essentially the same despite the nominally different band filling; this is in marked contrast to $T$ = Co and Ni series for which the $T$-$x$ diagrams scale by a factor of two, consistent with the different changes in band filling Co and Ni would produce when replacing Fe. Superconductivity of CaK(Fe$_{1-x}$Cr$_{x}$)$_{4}$As$_{4}$ is also studied as a function of magnetic field. A clear change in $H^\prime_{c2}$($T$)/$T_c$, where $H^\prime_{c2}$($T$) is d$H_{c2}$($T$)/d$T$, at $x$ $\sim$ 0.012 is observed and probably is related to change of the Fermi surface due to magnetic order. Coherence length and the London penetration depths are also calculated based on $H_{c1}$ and $H_{c2}$ data. Coherence lengths as the function of $x$ also shows changes near $x$ = 0.012, again consistent with Fermi surfaces changes associated with the magnetic ordering seen for higher $x$-values.

cond-mat.supr-con↗
arXiv2023

Chiral and flat-band magnetic cluster excitations in a ferromagnetic kagome metal

TbMn6Sn6 is a metallic ferrimagnet that displays signatures of band topology arising from a combination of uniaxial ferromagnetism and spin-orbit coupling within its Mn kagome layers. Whereas the low energy magnetic excitations can be described as collective spin waves using a local moment Heisenberg model, sharply defined optical and flat-band collective magnon modes are not observed. In their place, we find overdamped chiral and flat-band spin correlations that are localized to hexagonal plaquettes within the kagome layer.

cond-mat.str-el↗
arXiv2023

Antiferromagnetic order and its interplay with superconductivity in CaK(Fe$_{1-x}$Mn$_x$)$_4$As$_4$

The magnetic order for several compositions of CaK(Fe$_{1-x}$Mn$_x$)$_4$As$_4$ has been studied by nuclear magnetic resonance (NMR), Mössbauer spectroscopy, and neutron diffraction. Our observations for the Mn-doped 1144 compound are consistent with the hedgehog spin vortex crystal (hSVC) order which has previously been found for Ni-doped $\text{Ca}\text{K}\text{Fe}_4\text{As}_4$. The hSVC state is characterized by the stripe-type propagation vectors $(π\,0)$ and $(0\,π)$ just as in the doped 122 compounds. The hSVC state preserves tetragonal symmetry at the Fe site, and only this SVC motif with simple AFM stacking along $\textbf{c}$ is consistent with all our observations using NMR, Mössbauer spectroscopy, and neutron diffraction. We find that the hSVC state in the Mn-doped 1144 compound coexists with superconductivity (SC), and by combining the neutron scattering and Mössbauer spectroscopy data we can infer a quantum phase transition, hidden under the superconducting dome, associated with the suppression of the AFM transition temperature ($T_\text{N}$) to zero for $x\approx0.01$. In addition, unlike several 122 compounds and Ni-doped 1144, the ordered magnetic moment is not observed to decrease at temperatures below the superconducting transition temperature ($T_\text{c}$).

cond-mat.str-el↗
arXiv2022

Canted Antiferromagnetic phases in the layered candidate Weyl material EuMnSb$_2$

EuMnSb$_2$ is a candidate topological material which can be tuned towards a Weyl semimetal, but there are differing reports for its antiferromagnetic (AFM) phases. The coupling of bands dominated by pure Sb layers hosting topological fermions to Mn and Eu magnetic states provides a potential path to tune the topological properties. We present a detailed analysis of the magnetic structure on three AFM phases based on single-crystal neutron diffraction, magnetization, and heat capacity data as well as polycrystalline $^{151}$Eu Mössbauer data. The Mn magnetic sublattice orders into a C-type AFM structure below $323(1)$~K with the ordered Mn magnetic moment $μ_{\text{Mn}}$ lying perpendicular to the layers. AFM ordering of the Eu sublattice occurs below $23(1)$~K with the ordered Eu magnetic moment $μ_{\text{Eu}}$ canted away from the layer normal and $μ_{\text{Mn}}$ retaining its higher-temperature order. $μ_{\text{Eu}}$ is ferromagnetically aligned within each Eu layer but exhibits a complicated AFM layer stacking. Both of these higher-temperature phases are described by magnetic space group (MSG) $Pn^{\prime}m^{\prime}a^{\prime}$ with the chemical and magnetic unit cells having the same dimensions. Cooling below $=9(1)$~K reveals a third AFM phase where $μ_{\text{Mn}}$ remains unchanged but $μ_{\text{Eu}}$ develops an additional in-plane canting. This phase has MSG $P11\frac{2_1}{a^{\prime}}$. We additionally find evidence of short-range magnetic correlations associated with the Eu between $12~\text{K} \lesssim T \lesssim 30~\text{K}$. Using the determined magnetic structures, we postulate the signs of nearest-neighbor intralayer and interlayer exchange constants and the magnetic anisotropy within a general Heisenberg-model. We then discuss implications of the various AFM states in EuMnSb$_2$ and its topological properties.

cond-mat.str-el↗
arXiv2022

Low temperature competing magnetic energy scales in the topological ferrimagnet TbMn6Sn6

TbMn6Sn6 is a metallic ferrimagnet displaying signatures of both topological electrons and topological magnons arising from ferromagnetism and spin-orbit coupling within its Mn kagome layers. Inelastic neutron scattering measurements find strong ferromagnetic (FM) interactions within the Mn kagome layer and reveal a magnetic bandwidth of ~230 meV. The low-energy magnetic excitations are characterized by strong FM Mn-Mn and antiferromagnetic (AFM) Mn-Tb interlayer magnetic couplings. We observe weaker, competing long-range FM and AFM Mn-Mn interlayer interactions similar to those driving helical magnetism in the YMn6Sn6 system. Combined with density-functional theory calculations, we find that competing Mn-Mn interlayer magnetic interactions occur in all RMn6Sn6 compounds with R= Y, Gd-Lu, resulting in magnetic instabilities and tunability when Mn-R interactions are weak. In the case of TbMn6Sn6, strong AFM Mn-Tb coupling ensures a highly stable three-dimensional ferrimagnetic network.

cond-mat.str-el↗
arXiv2022

Rare-earth monopnoctides -- family of antiferromagnets hosting magnetic Fermi arcs

Since the discovery of topological insulators a lot of research effort has been devoted to magnetic topological materials, in which non-trivial spin properties can be controlled by magnetic fields, culminating in a wealth of fundamental phenomena and possible applications. The main focus was on ferromagnetic materials that can host Weyl fermions and therefore spin textured Fermi arcs. The recent discovery of Fermi arcs and new magnetic bands splitting in antiferromagnet (AFM) NdBi has opened up new avenues for exploration. Here we show that these uncharted effects are not restricted to this specific compound, but rather emerge in CeBi, NdBi, and NdSb when they undergo paramagnetic to AFM transition. Our data show that the Fermi arcs in NdSb have 2-fold symmetry, leading to strong anisotropy that may enhance effects of spin textures on transport properties. Our findings thus demonstrate that the RBi and RSb series are materials that host magnetic Fermi arcs and may be a potential platform for modern spintronics.

cond-mat.other↗