arXiv subjects

Paul C. Canfield

Publications and source records attributed to Paul C. Canfield.

At least 91 records · Page 5Linked to original sources

arXiv2022

Spin-polarized imaging of strongly interacting fermions in the ferrimagnetic state of Weyl candidate CeBi

CeBi has an intricate magnetic phase diagram whose fully-polarized state has recently been suggested as a Weyl semimetal, though the role of $f$ states in promoting strong interactions has remained elusive. Here we focus on the less-studied, but also time-reversal symmetry-breaking ferrimagnetic phase of CeBi, where our density functional theory (DFT) calculations predict additional Weyl nodes near the Fermi level $E_\mathrm{F}$. We use spin-polarized scanning tunneling microscopy and spectroscopy to image the surface ferrimagnetic order on the itinerant Bi $p$ states, indicating their orbital hybridization with localized Ce $f$ states. We observe suppression of this spin-polarized signature at $E_\mathrm{F}$, coincident with a Fano line shape in the conductance spectra, suggesting the Bi $p$ states partially Kondo screen the $f$ magnetic moments, and this $p-f$ hybridization causes strong Fermi-level band renormalization. The $p$ band flattening is supported by our quasiparticle interference (QPI) measurements, which also show band splitting in agreement with DFT, painting a consistent picture of a strongly interacting magnetic Weyl semimetal.

cond-mat.str-el↗

arXiv2022

Emergence of Fermi arcs and novel magnetic splitting in an antiferromagnet

The Fermi arcs are signatures of exotic states in solids because they defy conventional concept of Fermi surfaces as closed contours in momentum space. Fermi arcs were first discovered in cuprates, and caused by the pseudogap. Weyl semimetals provided another way to generate Fermi arcs by breaking either the time reversal symmetry (TRS) or inversion symmetry of a 3D Dirac semimetal, which can result in a Weyl semimetal with pairs of Weyl nodes that have opposite chirality. The bulk-boundary correspondence associated with the Chern number leads to the emergence of Fermi arcs on the boundary. Here, we present experimental evidence that pairs of magnetically split hole- and electron-like Fermi arcs emerge below the Neel temperature, in the antiferromagnetic (AFM) state of cubic NdBi due to a novel band splitting effect. Whereas TRS is broken by the AFM order, both inversion and nonsymmorphic TRS are preserved in the bulk, precluding the possibility of a Weyl semimetal. The observed magnetic splitting is highly unusual, as it creates bands of opposing curvature, that changes with temperature and follows the antiferromagnetic order parameter. This is completely different from previously reported cases of magnetic splittings such as traditional Zeeman and Rashba, where the curvature of the bands is preserved. Therefore, our finding represents a new Fermionic state created by new type of magnetic band splitting in the presence of a long-range AFM order that are not readily explained by existing theoretical ideas.

cond-mat.str-el↗

arXiv2022

Ubiquitous enhancement of nematic fluctuations across the phase diagram of iron based superconductors probed by the Nernst effect

The role of nematic fluctuations for unconventional superconductivity has been subject of intense discussions for many years. In iron-based superconductors, the most established probe for electronic-nematic fluctuations, i.e. the elastoresistivity seems to imply that superconductivity is reinforced by electronic-nematic fluctuations, since the elastoresistivity amplitude peaks at or close to optimal $T_C$. However, on the over-doped side of the superconducting dome, the diminishing elastoresistivity suggests a negligible importance in the mechanism of superconductivity. Here we introduce the Nernst coefficient as a genuine probe for electronic nematic fluctuations, and we show that the amplitude of the Nernst coefficient tracks the superconducting dome of two prototype families of iron-based superconductors, namely Rh-doped $BaFe_{2}As_{2}$ and Co-doped $LaFeAsO$. Our data thus provide fresh evidence that in these systems nematic fluctuations foster the superconductivity throughout the phase diagram.

cond-mat.supr-con↗

arXiv2022

Hydrostatic pressure effect on Co-based honeycomb magnet BaCo2(AsO4)2

The honeycomb antiferromagnet BaCo2(AsO4)2, in which small in-plane magnetic fields (H1 = 0.26 T and H2 = 0.52 T at T = 1.8 K < TN = 5.4 K) induce two magnetic phase transitions, has attracted attention as a possible candidate material for the realization of Kitaev physics based on the 3d element Co2+. Here, we report on the change of the transition temperature TN and the critical fields H1 and H2 of BaCo2(AsO4)2 with hydrostatic pressure up to ~ 20 kbar, as determined from magnetization and specific heat measurements. Within this pressure range, a marginal increase of the magnetic ordering temperature is observed. At the same time, the critical fields are changed significantly (up to ~ 25-35 %). Specifically, we find that H1 is increased with hydrostatic pressure, i.e., the antiferromagnetic state is stabilized with hydrostatic pressure, whereas H2, which was previously associated with a transition into a proposed Kitaev spin liquid state, decreases with increasing pressure. These results put constraints on the magnetic models that are used to describe the low-temperature magnetic properties of BaCo2(AsO4)2.

cond-mat.str-el↗

arXiv2021

Magnetization and magneto-transport measurements on CeBi single crystals

We report the synthesis of CeBi single crystals out of Bi self flux and a systematic study of the magnetic and transport properties with varying temperature and applied magnetic fields. From these $R(T,H)$ and $M(T,H)$ data we could assemble the field-temperature ($H-T$) phase diagram for CeBi and visualize the three dimensional $M-T-H$ surface. In the phase diagram, we identify regions with well defined magnetization values, and identify a new phase region. The magnetoresistance (MR) in the low temperature regime shows, above $6~$T a power-law, non-saturated behavior with large MR ($\sim 3\times10^5 \%$ at $2~$K and $13.95~$T), along with Shubnikov-de Haas oscillations. With increasing temperatures, MR decreases, and then becomes negative for $T\gtrsim 10~$K. This crossover in MR seems to be unrelated to any specific magnetic or metamagnetic transitions, but rather is associated with changing from a low-temperature normal metal regime with little or no scattering from the Ce$^{3+}$ moments and an anomalously large MR, to an increased scattering from local Ce moments and a negative MR as temperature increases.

cond-mat.str-el↗

arXiv2021

A Low Temperature Structural Transition in Canfieldite, Ag$_8$SnS$_6$, Single Crystals

Canfieldite, Ag$_8$SnS$_6$, is a semiconducting mineral notable for its high ionic conductivity, photosensitivity, and low thermal conductivity. We report the solution growth of large single crystals of Ag$_8$SnS$_6$ of mass up to 1 g from a ternary Ag-Sn-S melt. On cooling from high temperature, Ag$_8$SnS$_6$ undergoes a known cubic (F-43m) to orthorhombic (Pna2$_1$) phase transition at $\approx$ 460 K. By studying the magnetization and thermal expansion between 5-300 K, we discover a second structural transition at $\approx$ 120 K. Single crystal X-ray diffraction reveals the low temperature phase adopts a different orthorhombic structure with space group Pmn2$_1$ (a = 7.6629(5) Å, b = 7.5396(5) Å, c = 10.6300(5) Å, Z = 2 at 90 K) that is isostructural to the room temperature forms of the related Se-based compounds Ag$_8$SnSe$_6$ and Ag$_8$GeSe$_6$. The 120 K transition is first-order and has a large thermal hysteresis. Based on magnetization and thermal expansion data, the room temperature polymorph can be kinetically arrested into a metastable state by rapidly cooling to temperatures below 40 K. We lastly compare the room and low temperature forms of Ag$_8$SnS$_6$ with its argyrodite analogues, Ag$_8$TQ$_6$ (T = Si, Ge, Sn; Q = S, Se), and identify a trend relating the preferred structures to the unit cell volume, suggesting smaller phase volume favors the Pna2$_1$ arrangement. We support this picture by showing that the transition to the Pmn2$_1$ phase is avoided in Ge alloyed Ag$_8$Sn$_{1-x}$Ge$_x$S$_6$ samples as well as pure Ag$_8$GeS$_6$.

cond-mat.mtrl-sci↗

arXiv2021

Small moment antiferromagnetic ordering in single crystalline La2Ni7

Single crystals of La2Ni7 have been grown out of a binary, La-Ni melt. Temperature dependent, zero magnetic field, specific heat, electrical resistivity, and low field magnetization measurements indicate that there is a series of antiferromagnetic phase transitions at T1 = 61.0 \pm 0.2 K, T2 = 56.5 \pm 0.2 K and T3 = 42.2 \pm 0.2 K. The three specific heat anomalies found at these temperatures qualitatively have very small entropy changes associated with them and the anisotropic M(H) data saturate at ~ 0.12 μB/Ni; both observations strongly suggesting the AFM order is associated with very small, itinerant, moments. Anisotropic, H||c and H{\perp}c, ρ(H) and M(H) isotherms as well as constant field, ρ(T) and M(T) sweeps manifest signatures of multiple phase lines and result in H-T phase diagrams that are clearly anisotropic. Analysis of M(T) and M(H) data allow for the identification of the two lower temperature magnetically ordered states as antiferromagnetically ordered, with the moments aligned along the crystallographic c-axis, and the higher temperature, T2 < T < T1, state as having a finite ferromagnetic component. In addition, the metamagnetic transition at low temperatures, for H applied along the crystallographic c-axis (H||c) appears to be a near classic example of a spin-flop transition, resulting in a field stabilized antiferromagnetic state with the moments ordered perpendicular to the c-axis. Although the small moment ordering, and existence of multiple phase transitions in field and temperature, suggesting an energetic proximity of these states, could foretell a degree of pressure sensitivity, our measurements of R(T) for applied pressures up to 2.0 GPa indicate that there is very little pressure dependence of T1, T2 and T3.

cond-mat.str-el↗

arXiv2021

Construction of $A$-$B$ hetero-layer intermetallic crystals: case studies of the 1144-phase TM-phosphides \textit{AB}(TM)$_4$P$_4$ (TM=Fe, Ru, Co, Ni

The discovery of the 1144-phase, e.g. CaKFe$_4$As$_4$, creates opportunities to build novel intermetallics with alternative stacking of two parent compounds. Here we formalize the idea by defining a class of bulk crystalline solids with $A$-$B$ stacking (including 1144-phases and beyond), which is a generalization of hetero-structures from few-layer or thin-film semi-conductors to bulk intermetallics. Theoretically, four families of phosphides \textit{AB}(TM)$_4$P$_4$ (TM=Fe, Ru, Co, Ni) are investigated by first-principles calculations, wherein configurational, vibrational and electronic degrees of freedom are considered. It predicts a variety of stable 1144-phases (especially Ru- and Fe-phosphides). Stability rules are found and structural/electronic properties are discussed. Experimentally, we synthesize high-purity CaKRu$_4$P$_4$ as a proof of principle example. The synthetic method is simple and easily applied. Moreover, it alludes to a strategy to explore complex multi-component compounds, facilitated by a phase diagram coordinated by collective descriptors.

cond-mat.mtrl-sci↗

arXiv2021

Pseudoelasticity of SrNi$_2$P$_2$ micropillar via Double Lattice Collapse and Expansion

The maximum recoverable strain of most crystalline solids is less than 1% because plastic deformation or fracture usually occurs at a small strain. In this work, we show that a SrNi$_2$P$_2$ micropillar exhibits pseudoelasticity with a large maximum recoverable strain of ~14% under uniaxial compression via unique reversible structural transformation, double lattice collapse-expansion that is repeatable under cyclic loading. Its high yield strength (~3.8$\pm$0.5 GPa) and large maximum recoverable strain bring out the ultrahigh modulus of resilience (~146$\pm$19MJ/m$^3$) a few orders of magnitude higher than that of most engineering materials. The double lattice collapse-expansion mechanism shows stress-strain behaviors similar with that of conventional shape memory alloys, such as hysteresis and thermo-mechanical actuation, even though the structural changes involved are completely different. Our work suggests that the discovery of a new class of high performance ThCr$_2$Si$_2$-structured materials will open new research opportunities in the field of pseudoelasticity.

cond-mat.mtrl-sci↗

arXiv2021

Phase diagram of CeSb$_2$ from magnetostriction and magnetization measurements: Evidence for ferrimagnetic and antiferromagnetic states

Cerium diantimonide (CeSb$_2$) is one of a family of rare earth based magnetic materials that exhibit metamagnetism, enabling control of the magnetic ground state through an applied magnetic field. At low temperatures, CeSb$_2$ hosts a rich phase diagram with multiple magnetically ordered phases for many of which the order parameter is only poorly understood. In this paper, we report a study of its metamagnetic properties by Scanning Tunneling Microscopy (STM) and magnetization measurements. We use STM measurements to characterize the sample magnetostriction with sub-picometer resolution from magnetic field and temperature sweeps. This allows us to directly assess the bulk phase diagram as a function of field and temperature and relate spectroscopic features from tunneling spectroscopy to bulk phases. Our magnetostriction and magnetisation measurements indicate that the low temperature ground state at zero field is ferrimagnetic. Quasiparticle interference mapping shows evidence for a reconstruction of the electronic structure close to the Fermi energy upon entering the magnetically ordered phase.

cond-mat.str-el↗

arXiv2021

Pressure-induced ferromagnetism in the topological semimetal EuCd$_2$As$_2$

The antiferromagnet and semimetal EuCd$_2$As$_2$ has recently attracted a lot of attention due to a wealth of topological phases arising from the interplay of topology and magnetism. In particular, the presence of a single pair of Weyl points is predicted for a ferromagnetic configuration of Eu spins along the $c$-axis in EuCd$_2$As$_2$. In the search for such phases, we investigate here the effects of hydrostatic pressure in EuCd$_2$As$_2$. For that, we present specific heat, transport and $μ$SR measurements under hydrostatic pressure up to $\sim\,2.5\,$GPa, combined with {\it ab initio} density functional theory (DFT) calculations. Experimentally, we establish that the ground state of EuCd$_2$As$_2$ changes from in-plane antiferromagnetic (AFM$_{ab}$) to ferromagnetic at a critical pressure of $\,\approx\,$2\,GPa, which is likely characterized by the moments dominantly lying within the $ab$ plane (FM$_{ab}$). The AFM$_{ab}$-FM$_{ab}$ transition at such a relatively low pressure is supported by our DFT calculations. Furthermore, our experimental and theoretical results indicate that EuCd$_2$As$_2$ moves closer to the sought-for FM$_c$ state (moments $\parallel$ $c$) with increasing pressure further. We predict that a pressure of $\approx$\,23\,GPa will stabilize the FM$_c$ state, if Eu remains in a 2+ valence state. Thus, our work establishes hydrostatic pressure as a key tuning parameter that (i) allows for a continuous tuning between magnetic ground states in a single sample of EuCd$_2$As$_2$ and (ii) enables the exploration of the interplay between magnetism and topology and thereby motivates a series of future experiments on this magnetic Weyl semimetal.

cond-mat.str-el↗

arXiv2021

Topological Magnetic Hysteresis in Single Crystals of CeAgSb2 Ferromagnet

Closed-topology magnetic domains are usually observed in thin films and in an applied magnetic field. Here we report the observation of rectangular cross-section tubular ferromagnetic domains in thick single crystals of CeAgSb2 in zero applied field. Relatively low exchange energy, small net magnetic moment, and anisotropic in-plane crystal electric fields lower the domain wall energy and allow for the formation of the closed-topology patterns. Upon cycling the magnetic field, the domain structure irreversibly transforms into a dendritic open-topology pattern. This transition between closed and open topologies results in a "topological magnetic hysteresis" - the actual hysteresis in magnetization, not due to the imperfections and pinning, but due to the difference in the pattern morphology. Similar physics was suggested before in pure type-I superconductors and is believed to be a generic feature of other nonlinear multi-phase systems in the clean limit.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic order in van der Waals antiferromagnet CrPS4: anisotropic H-T phase diagrams and effects of pressure

Single crystalline samples of the van der Waals antiferromagnet CrPS4 were studied by measurements of specific heat and comprehensive anisotropic temperature- and magnetic field-dependent magnetization. In addition, measurements of the heat capacity and magnetization were performed under pressures of up to ~21 kbar and ~14 kbar respectively. At ambient pressure, two magnetic transitions are observed, second order from a paramagnetic to an antiferromagnetic state at TN ~ 37 K, and a first-order spin reorientation transition at T* ~ 34 K. Anisotropic H - T phase diagrams were constructed using the M(T,H) data. As pressure is increased, TN is weakly suppressed with dTN/dP ~ -0.1 K/kbar. T*, on the other hand, is suppressed quite rapidly, with dT*/dP ~ -2 K/kbar, extrapolating to a possible quantum phase transition at Pc ~ 15 kbar.

cond-mat.mtrl-sci↗

arXiv2021

Comment on "Unconventional enhancement of ferromagnetic interactions in Cd-doped GdFe2Zn20 single crystals studied by ESR and 57Fe Mossbauer spectroscopies"

In the recent publication, Phys. Rev. B 102, 144420 (2020), Cabrera-Baez et al. present a study of the effects of Cd-substitution for Zn in the ferromagnetic compound GdFe2Zn20. As part of this paper, they claim that for GdFe2Zn18.6Cd1.4 the effective moment of Gd is reduced by 25% and the saturated moment of Gd is reduced by over 40%. We regrew representative members of the GdFe2Zn(20-x)Cdx series and did not find any such reductions. In addition, we measured several crystals from the growth batch that was used by Cabrera-Baez et al. and did not see such reductions. Although there is a modest increase in TC with Cd substitution, there is no significant change in the Gd effective moment or the saturated moment associated with the low temperature ferromagnetic state.

cond-mat.mtrl-sci↗

arXiv2021

Prediction of Spin Polarized Fermi Arcs in Quasiparticle Interference of CeBi

We predict that CeBi in the ferromagnetic state is a Weyl semimetal. Our calculations within density functional theory show the existence of two pairs of Weyl nodes on the momentum path $(0, 0, k_z)$ at $15$ meV} above and $100$ meV below the Fermi level. Two corresponding Fermi arcs are obtained on surfaces of mirror-symmetric (010)-oriented slabs at $E=15$ meV and both arcs are interrupted into three segments due to hybridization with a set of trivial surface bands. By studying the spin texture of surface states, we find the two Fermi arcs are strongly spin-polarized but in opposite directions, which can be detected by spin-polarized ARPES measurements. Our theoretical study of quasiparticle interference (QPI) for a nonmagnetic impurity at the Bi site also reveals several features related to the Fermi arcs. Specifically, we predict that the spin polarization of the Fermi arcs leads to a bifurcation-shaped feature only in the spin-dependent QPI spectrum, serving as a fingerprint of the Weyl nodes.

cond-mat.str-el↗

arXiv2021

Multi-Atom Quasiparticle Scattering Interference for Superconductor Energy-Gap Symmetry Determination

Complete theoretical understanding of the most complex superconductors requires a detailed knowledge of the symmetry of the superconducting energy-gap $Δ_\mathbf{k}^α$, for all momenta $\mathbf{k}$ on the Fermi surface of every band $α$. While there are a variety of techniques for determining $|Δ_\mathbf{k}^α|$, no general method existed to measure the signed values of $Δ_\mathbf{k}^α$. Recently, however, a new technique based on phase-resolved visualization of superconducting quasiparticle interference (QPI) patterns centered on a single non-magnetic impurity atom, was introduced. In principle, energy-resolved and phase-resolved Fourier analysis of these images identifies wavevectors connecting all k-space regions where $Δ_\mathbf{k}^α$ has the same or opposite sign. But use of a single isolated impurity atom, from whose precise location the spatial phase of the scattering interference pattern must be measured is technically difficult. Here we introduce a generalization of this approach for use with multiple impurity atoms, and demonstrate its validity by comparing the $Δ_\mathbf{k}^α$ it generates to the $Δ_\mathbf{k}^α$ determined from single-atom scattering in FeSe where $s_{\pm}$ energy-gap symmetry is established. Finally, to exemplify utility, we use the multi-atom technique on LiFeAs and find scattering interference between the hole-like and electron-like pockets as predicted for $Δ_\mathbf{k}^α$ of opposite sign.

cond-mat.supr-con↗

arXiv2020

Extremely Weakly Interacting $ΔS_z = 0$ and $ΔS_z = 1$ Excitations and Evidence for Fractional Quantization in a Magnetization Plateau: CeSb

The plateau at 1/3 of the saturation magnetization, $M_s$, in the metamagnet CeSb is accompanied by a state of ferromagnetic layers of spins in an up-up-down sequence. We measured $M$ and the specific heat, $C$, in the plateau, spin wave analyses of which reveal two distinct branches of excitations. Those with $ΔS_z = 1$ as measured by $M$, coexist with a much larger population of $ΔS_z = 0$ excitations measured by $C$ but invisible to $M$. The large density of $ΔS_z = 0$ excitations, their energy gap, and their seeming lack of interaction with $ΔS_z = 1$ excitations suggest an analogy with astrophysical dark matter. Additionally, in the middle of the plateau three sharp jumps in $M(H)$ are seen, the size of which, $0.15 $%$M_s$, is consistent with fractional quantization of magnetization-per-site in the down-spin layers.

cond-mat.str-el↗

arXiv2020

Evolution of nematic fluctuations in CaK(Fe$_{1-x}$Ni$_{x}$)$_{4}$As$_{4}$ with spin-vortex crystal magnetic order

The CaK(Fe$_{1-x}$Ni$_{x}$)$_{4}$As$_{4}$ superconductors resemble the archetypal 122-type iron-based materials but have a crystal structure with distinctly lower symmetry. This family hosts one of the few examples of the so-called spin-vortex crystal magnetic order, a non-collinear magnetic configuration that preserves tetragonal symmetry, in contrast to the orthorhombic collinear stripe-type magnetic configuration common to the 122-type systems. Thus, nematic order is completely absent from its phase diagram. To investigate the evolution of nematic fluctuations in such a case, we present elastoresistance and elastic modulus measurements in CaK(Fe$_{1-x}$Ni$_{x}$)$_{4}$As$_{4}$ ($x = 0-0.05$) combined with phenomenological modeling and density functional theory. We find clear experimental signatures of considerable nematic fluctuations, including softening of the Young's modulus $Y_{[110]}$ and a Curie-Weiss type divergence of the B2g elastoresistance coefficient in CaK(Fe$_{0.951}$Ni$_{0.049}$)$_4$As$_4$. Overall, nematic fluctuations within this series bear strong similarities to the hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ series, including a substitution-induced sign change. Our theoretical analysis addresses the effect of the specific crystal symmetry of the 1144-type structure in determining its magnetic ground state and on the nematic fluctuations.

cond-mat.supr-con↗