arXiv subjects

Paul C. Canfield

Publications and source records attributed to Paul C. Canfield.

At least 19 recordsLinked to original sources

arXiv2026

Visualizing impurity-driven scattering phase textures in EuCd2As2

Understanding how disorder modifies electronic states in magnetic semiconductors is important for controlling spin-dependent transport and topological responses. Here we use scanning tunneling microscopy to visualize scattering phase textures in EuCd2As2. By isolating a single surface wavevector we reconstruct spatial phase maps of the local density of states and identify phase dislocations characterized by 2pi winding around impurity sites. These phase singularities emerge systematically within charge puddles generated by Eu interstitials and their positions evolve with bias voltage. We show that their spatial structure is consistent with interference between multiple scattering channels, including contributions from spin-orbit coupling. We provide a model which reproduces phase dislocations and relates the decay of the phase gradient to the relative strength of spin-orbit and scalar scattering. Our results establish a route to access the phase of electronic scattering in real space and study the role of local disorder and spin-orbit interactions in shaping electronic states in quantum materials.

cond-mat.mtrl-sci↗

arXiv2026

Bridging atomic and mesoscopic length scales with Replica Scanning Tunneling Microscopy: Visualizing the atomic lattice of UTe$_2$ and the atomic scale superconducting gap modulations of FeSe close to micron length scales

Scanning Tunneling Microscopy is a cornerstone technique for visualizing the electronic density of states with atomic resolution (typically below 0.1 nm). While the field of view of most STM setups extends up to a few microns, obtaining atomic resolution over these large areas is often impractical and excessively time-consuming. This is due to the need to acquire maps with a point number reaching $10^7$ or more with a full current or conductance vs voltage curve at each point. The standard procedure is to make large scale maps and then select small regions to zoom-in for high-resolution atomic scale analysis. However, this approach fails to address a question which is often critical: Does a specific atomic-scale modulation of the electronic density of states persist over much larger, mesoscopic length scales? Here we present a new method: Replica STM (R-STM), that overcomes this limitation, allowing the study of atomic-scale phenomena up to micron length scales. We obtained new large-area STM tunneling conductance maps in UTe$_2$ and FeSe, spanning areas over 200 nm in size. In these large scale maps we discovered periodic signals with wavelengths significantly exceeding interatomic distances. We show that these large-wavelength periodic signals are replicas of the underlying atomic-scale density of states modulations. R-STM leverages these replica signals to efficiently track atomic-scale features over large areas. We discuss the influence of phase slips, disorder and defects in the replicas. Our results suggest that atomic scale modulations of the superconducting density of states could persist over large length scales in FeSe. R-STM provides a new capability for STM to compare atomic scale with micrometer scale phenomena. The proof of principle of R-STM can be extended to any other scanning probe microscopy experiment where a periodic signal is traced as a function of position.

cond-mat.supr-con↗

arXiv2026

Robust two-dimensional surface superconductivity and vortex lattice in the Weyl semimetal $γ$-PtBi$_2$

The layered compound $γ$-PtBi$_2$ is a topological semimetal with Fermi arcs at the surface joining bulk Weyl points. Recent work has found signatures of surface superconductivity consisting of gap openings compatible with a critical temperature orders of magnitude larger than the bulk value. However, no superconducting vortices have been identified, raising questions about the robustness of the phase coherence. Here we use very low temperature Scanning Tunneling Microscopy (STM) and find robust superconductivity with T$_C=$2.9 K and H$_{C2}\approx$1.8 T linked to the Fermi arcs. We observe quantized superconducting vortices and the Josephson effect, demonstrating two-dimensional macroscopic quantum phase coherence.

cond-mat.supr-con↗

arXiv2026

Room-temperature shape-memory effect in Sr(Ni$_{1-x}$Cu$_x$)$_2$P$_2$

The compound SrNi$_2$P$_2$ can exhibit multiple crystal structures with no P-P pairs bonded (uncollapsed tetragonal, or ucT, state), with one-third of the P-P pairs bonded (one-third collapsed orthorhombic, or tcO, state), or with all P-P pairs bonded (collapsed tetragonal, or cT, state) across the Sr layers. The system can be tuned into its different states by changing temperature, mechanical stress, or chemical composition. Changes in bonding may manifest in changes of macroscopic properties of the material, such as its shape, electrical conductivity, or magnetism. In this work, we show that SrNi$_2$P$_2$ can be tuned among the three states by changing Cu substitution and temperature. We present temperature-dependent resistance and single-crystal x-ray diffraction results in Sr(Ni$_{1-x}$Cu$_x$)$_2$P$_2$ single-crystals that show that Cu substitution favors the P-P bonding, stabilizing the cT state at ambient pressure. We construct a $T-x$ phase diagram that shows how all of these transition temperatures increase with increasing Cu fraction, $x$. The transition between the tcO state and the cT state exhibits a very large thermal hysteresis, which can be tuned to temperatures close to room temperature. In particular, the properties of Sr(Ni$_{0.963}$Cu$_{0.037}$)$_2$P$_2$ may make it suitable for applications as a shape memory material at room temperature.

cond-mat.mtrl-sci↗

arXiv2026

Complex crystal structure prediction using ML-enhanced multi-minima iterative genetic algorithm

Current machine learning (ML) approaches for materials discovery rely heavily on known structural databases, limiting their ability to identify entirely novel structure types. In this work, we develop a multi-minima iterative genetic algorithm (MMIGA) that integrates an artificial-neural-network machine learning (ANN-ML) interatomic potential with an iterative, metadynamics-inspired penalty scheme. We demonstrate the robustness of this method on a complex ternary La-Co-Pb system, characterized by Co-Pb immiscibility and an intricate energy landscape. The ML-enhanced MMIGA successfully predicts the ground-state Pbam structure of the recently synthesized La4Co4Pb antagonistic-pair-phase, a novel structure missed by previous database-reliant ML predictions, while also identifying multiple metastable competing phases. Additionally, we challenged the MMIGA method to predict the structure of La5CoPb2 antagonistic-pair-phase, a new compound discovered during earlier attempts to synthesize the predicted phase La3CoPb. With only knowledge of the composition, our MMIGA approach successfully predicts the orthorhombic structure of La5CoPb2, producing an exact match with the structure independently determined by x-ray diffraction. By efficiently mapping both global minimum and relevant competing metastable states, this approach provides critical theoretical insights into phase selection for novel quantum and magnetic materials.

cond-mat.mtrl-sci↗

arXiv2026

Observation of Flat Bands in Type-II Weyl Semimetal TaRhTe$_{4}$

Flat bands have been theoretically predicted for decades but have only recently been realized in quantum materials such as magic-angle twisted bilayer graphene, kagome and Lieb lattices, and rare-earth metal compounds. To date, only twisted layered materials have enabled tuning of flat-band energies near the electronic chemical potential, thereby influencing transport and thermodynamic properties. Here, we report the presence of flat bands near the chemical potential in bulk TaRhTe$_{4}$, a noncentrosymmetric van-der Waals type-II Weyl semimetal. Flat bands are rarely observed in Weyl semimetals, particularly in nonmagnetic bulk systems, and the observed flat bands were not predicted by density functional theory calculations. TaRhTe$_{4}$ therefore provides a platform in which nontrivial topology coexists with flat bands near the Fermi level, as evidenced by our angle-resolved photoemission spectroscopy measurements.

cond-mat.mtrl-sci↗

arXiv2026

Notes on remanent magnetization measurements in superconductors and hard ferromagnets

Data on zero applied field measurements of remanent magnetization and magnetic relaxation in a BCS superconductor LuNi2B2C and several hard ferromagnets are presented and compared. Apparent similarities and differences, in particular in Thermoremanent Magnetization (TRM) - like, Isothermal Remanent Magnetization (IRM) - like, and remanent magnetization measurements with zigzag temperature sweep measurements are outlined. It is discussed how these results could be relevant for the magnetization measurements in diamond anvil cells.

cond-mat.mtrl-sci↗

arXiv2026

Physical properties of R$_2$Co$_6$Al$_{20-δ}$ (R = Gd-Tm, Y) single crystals

Rare-earth (R) based intermetallic compounds can often exhibit diverse physical properties and distinct magnetic anisotropies. A Notable example are the light rare earth members of the mono-clinic, R$_2$Co$_6$Al$_{19}$ series that are known to display a range of physical properties, from non-Fermi liquid behavior to antiferromagnetic (AFM) ordering, with properties that vary depending on R. In this work, we have extended this series to the heavy rare earths and systematically investigate the synthesis, crystal structure, and physical properties of single crystals of R$_2$Co$_6$Al$_{20-δ}$ for R = Gd - Tm and Y. Single crystal X-ray diffraction reveals that these materials adopt an orthorhombic Imma-type structure with delta varying non-monotonically across the heavy rare earths; ranging from 0.73 for Dy to 0.91 for Gd. Temperature-dependent specific heat, resistivity, and magnetization measurements demonstrate AFM ordering in all materials, with the Neel temperature (TN) ranging from 1.8 K for Ho to 11.8 K for Tb. Notably, Gd and Tb-based materials exhibit two distinct AFM transitions, separated by approximately 2 - 3 K. These findings establish the heavy rare-earth members of the R2Co6Al20-delta series as anisotropic antiferromagnets with strong crystal electric field effects and exchange anisotropy. The observed deviation from de Gennes scaling and the anisotropy crossover across the series highlight the important interplay between RKKY exchange and crystal electric field interactions in this orthorhombic system.

cond-mat.mtrl-sci↗

arXiv2026

Itinerant antiferromagnetism in the antagonistic pair compound Y$_4$Co$_3$Ag

Low dimensional crystallographic motifs have long been associated with desirable physical properties. The confinement of electrons to low dimensions is thought to enhance quantum fluctuations and may promote correlated phenomena. Here, using the antagonistic pair concept, we add Y to the immiscible Co-Ag pair to discover Y$_4$Co$_3$Ag. This compound adopts a monoclinic $I$2/$m$ structure consisting of Y channels that are filled by one-dimensional zigzag and hexagonal Co chains, which extend along the crystallographic $b$-axis with no nearest neighbor contacts between Co and Ag atoms. Transport, magnetic, and specific heat measurements reveal that Y$_4$Co$_3$Ag orders antiferromagnetically at $T_N=14.9$ K with an effective magnetic moment $μ_{\text{eff}}$ = 1.4 $μ_{\text{B}}$/Co. Specific heat measurements show only a small entropy loss on the order of $0.1\,R\ln2$ associated with magnetic order, and magnetization isotherms, in DC fields up to 70 kOe at 1.8 K and in pulsed fields up to 600 kOe at 500 mK, indicate a small ordered moment of less than 0.2 $μ_B$/Co. Taken together, our results imply the presence of small, itinerant moments and strong fluctuations in Y$_4$Co$_3$Ag, suggesting that Y$_4$Co$_3$Ag may be a promising candidate material to investigate itinerant magnetic interactions in a quasi-one dimensional system.

cond-mat.str-el↗

arXiv2026

Ultrafast Magneto-Pressure Spectroscopy and Control of Correlated Phases in a Trilayer Nickelate

Ultrafast spectroscopy under simultaneous high pressure and magnetic field provides a versatile approach for investigating pressure-driven electronic instabilities and correlated phases, and for probing potential bulk superconducting behavior under extreme conditions. However, such an experimental platform has yet to be implemented, standing as a roadblock to a fuller understanding of nonequilibrium superconductivity and vortex-controlled quasi-particle (QP) dynamics. Here, we bridge this capability gap by developing high pressure (up to 40 GPa), high magnetic field (up to 7 T), cryogenic (down to 5 K) femtosecond spectroscopy, and using it to probe magneto-pressure evolution of quasiparticle dynamics in the trilayer nickelate $\mathrm{Pr}_4\mathrm{Ni}_3\mathrm{O}_{10}$. We observe pronounced critical slowing down of QP relaxation at the charge-density-wave transition, which collapses under applied pressure. At higher pressures, the relaxation instead lengthens at low temperature, consistent with incipient superconducting correlations. However, the negligibel magnetic-field-dependence up to 7~T and absence of vortex-induced pre-bottleneck dynamics--robust signatures observed in our controlled bulk superconducting samples--indicates that any superconducting state under the present pressure conditions is likely non-bulk, filamentary, or strongly inhomogeneous. The magneto-pressure ultrafast capability opens a new avenue for resolving outstanding questions surrounding pressure-induced superconductivity and intertwined orders in correlated quantum materials.

cond-mat.supr-con↗

arXiv2026

Growth-Controlled Twinning and Magnetic Anisotropy in CeSb$_2$

Cerium diantimonide (CeSb$_2$) is a layered heavy-fermion Kondo lattice material that hosts complex magnetism and pressure-induced superconductivity. The interpretation of its in-plane anisotropy has remained unsettled due to structural twinning, which superimposes orthogonal magnetic responses. Here we combine controlled crystal growth with magnetization and rotational magnetometry to disentangle the effects of twinning. Nearly untwinned high-quality single crystals reveal the intrinsic in-plane anisotropy: The in-plane easy axis saturates at $M_{\text{easy}}(4~\text{T}) \approx 1.8~μ_{\text{B}}$/Ce, while the in-plane hard axis magnetization is strongly suppressed, nearly linear, and comparable to the out-of-plane response. These results resolve long-standing discrepancies in reported magnetic measurements, in which in-plane metamagnetic transition fields and saturation magnetization varied significantly across previous studies. Growth experiments demonstrate that avoiding the proposed $α$-$β$ structural transition $-$ through Sb-rich flux and slower cooling $-$ systematically reduces twinning. However, powder X-ray diffraction and differential thermal analysis measurements show no clear evidence of a distinct $β$ phase. Our results establish a consistent magnetic phase diagram and provide essential constraints for crystal-electric field models, enabling a clearer understanding of the interplay between anisotropic magnetism and unconventional superconductivity in CeSb$_2$.

cond-mat.str-el↗

arXiv2026

Observation of Iso-Symmetric Structural and Lifshitz Transitions in Quasi-one-dimensional CrNbSe$_5$

Chalcogenides-rich transition metal compounds host a rich landscape of emergent quantum phenomena that are intimately governed by their quasi-one-dimensional chemical-bonding frameworks and their response to external perturbations such as pressure. Here, we report a pressure-induced iso-symmetric structural transition in the quasi-one-dimensional compound CrNbSe$_5$, in which the electronic ground state is controlled not by symmetry breaking but by a continuous reorganization of local bonding interactions. Applied pressure reversibly tunes CrNbSe$_5$ between semiconducting and semimetallic states, enabling access to low- and high-carrier electronic regimes through direct modulation of metal-chalcogen bonding. High-pressure single-crystal X-ray diffraction directly resolves the evolution of Cr-Se and Nb-Se bond distances, coordination polyhedra, and connectivity, revealing a fully reversible semimetal-semiconductor-semimetal transition driven by gradual yet cooperative bond rearrangements within a preserved crystallographic symmetry. In contrast to chemical substitution, which irreversibly alters composition and introduces disorder, pressure acts as a clean, continuous control parameter that reshapes the bonding landscape without disrupting structural symmetry. These results establish CrNbSe$_5$ as a model system for electronically driven phase switching via tunable chemical bonding, highlighting iso-symmetric bond reorganization as a powerful design principle for pressure-controlled electronic and spintronic functionalities.

cond-mat.mtrl-sci↗

arXiv2026

Direct observation of vortex liquid droplets in the iron pnictide superconductor CaKAs$_4$Fe$_4$ at $0.5T$_c$

Type-II superconductors under magnetic fields are in a quantum coherent non-dissipative state as long as vortices remain pinned. Dissipation appears when vortices depin, eventually driven by thermal fluctuations. This can be associated to a melting transition between a vortex solid and a vortex liquid. This transition is almost always observed very close to T$_c$ when probed by macroscopic experiments. However, it remains unclear how the vortex solid responds to thermal fluctuations at the scale of individual vortices far from the melting transition. Here we use scanning tunneling microscopy (STM) to visualize vortices in CaKAs$_4$Fe$_4$ (T$_c \approx$ 35 K). We find vortex liquid droplets-localized regions in space where vortices strongly fluctuate due to thermal exctiation-at temperatures as low as 0.5\,T$_c$. Our results show that the onset of dissipation at the local scale occurs at temperatures considerably below T$_c$ in type-II superconductors.

cond-mat.supr-con↗

arXiv2026

Growth, discovery and characterization of single crystalline Eu$_{0.8}$Pt$_6$Al$_{16.4}$

We report the discovery of a ternary compound, Eu$_{0.8}$Pt$_6$Al$_{16.4}$. We determine its chemical and structural characteristics based on energy-dispersive X-ray spectroscopy as well as both powder and single-crystal X-ray diffraction, demonstrating that it crystallizes in a hexagonal structure type EuPt$_6$Al$_{17}$ with no reported structural analog. The electronic and magnetic properties are characterized by temperature- and field-dependent magnetization, and temperature-dependent resistance measurements, revealing that the Eu$^{2+}$ magnetic moments order antiferromagnetically below 2.8 K.

cond-mat.mtrl-sci↗

arXiv2026

Suppression of the valence transition in solution-grown single crystals of Eu$_2$Pt$_6$Al$_{15}$

The study of Eu intermetallic compounds has allowed the exploration of valence fluctuations and transitions in 4f electron systems. Recently, a Eu$_2$Pt$_6$Al$_{15}$ phase synthesized by arc-melting followed by a thermal treatment was reported [M. Radzieowski \textit{et al.}, J Am Chem Soc 140(28), 8950-8957 (2018)], which undergoes a transition upon cooling below 45~K that was interpreted as a valence transition from Eu$^{2+}$ to Eu$^{3+}$. In this paper, we present the discovery of another polymorph of Eu$_2$Pt$_6$Al$_{15}$ obtained by high temperature solution growth, which presents different physical properties than the arc-melted polycrystalline sample. Despite the similarities in crystal structure and chemical composition, the Eu valence transition is almost fully suppressed in the solution-grown crystals, allowing the moments associated with the Eu$^{2+}$ state to order antiferromagnetically at around 14~K. A detailed analysis of the crystal structure using single crystal X-ray diffraction reveals that, although the solution grown crystals are built from the same constituent layers as the arc-melted samples, these layers present a different stacking. The effect of different thermal treatments is also studied. Different anneal procedures did not result in significant changes in the intrinsic properties, and only by arc-melting and quenching the crystals we were able to convert them into the previously reported polymorph.

cond-mat.mtrl-sci↗

arXiv2026

Quantum critical point followed by Kondo-like behavior due to Cu substitution in itinerant, antiferromagnet ${\text{La}_{2}\text{(Cu}_{x}\text {Ni}_{1-x})_7}$

$\text{La}_2 \text{Ni}_7$ is an itinerant magnetic system with a small ordered moment of $\sim$ 0.1 $μ_{B}/\text{Ni}$ and a series of antiferromagnetic (AFM) transitions at $T_1$ = 61.0 K, $T_2$ = 56.5 K and $T_3$ = 42.2 K. $M(H)$, and $ρ(H)$ isotherms as well as constant field $M(T)$ and $ρ(T)$ measurements on single crystalline samples manifest a complex, anisotropic $H-T$ phase diagram with multiple phase lines. Here we present the growth and characterization of single crystals of the ${\text{La}_{2}\text{(Cu}_{x}\text {Ni}_{1-x})_7}$ series for 0 $\leq x \leq$ 0.181. We measured powder x-ray diffraction, and composition, as well as anisotropic $R(H,T)$, $M(H,T)$, and $C_p(T)$ on these single crystals. Using the measured data, we infer a $(T-x)$ phase diagram to study the evolution of the AFM ordering upon Cu substitution. For ${0 \leq x \leq 0.097}$, the system remains magnetically ordered at base temperature with $x \leq$ 0.012, showing signs of multiple AFM ordering temperatures. For the higher substitution levels, ${0.125 \leq x \leq 0.181}$, there are no signatures of magnetic ordering, but anomalous features in $R(T)$ and $C_p(T)$ data are observed which are consistent with the Kondo effect in this system. The intermediate $x$ = 0.105 sample lies between the magnetic ordered and the Kondo regime and is in the vicinity of the AFM-quantum critical point (QCP). Thus, ${\text{La}_{2}\text{(Cu}_{x}\text {Ni}_{1-x})_7}$ is an example of a small moment system that can be tuned through a QCP. Given these data combined with the fact that the $\text{La}_2 \text{Ni}_7$ structure has kagome-like, Ni-sublattices running perpendicular to the crystallographic $c$ axis, and a predicted $3d$-electron flat band that contributes to the density of states near the Fermi energy, ${\text{La}_{2}\text{(Cu}_{x}\text {Ni}_{1-x})_7}$ becomes a promising system to host and study exotic physics.

cond-mat.str-el↗

arXiv2025

Structural phase transitions in the van der Waals ferromagnets Fe$_x$Pd$_{y}$Te$_2$

Here, we provide a detailed study of the crystal structure and physical properties of the recently discovered vdW ferromagnet FePd$_2$Te$_2$. We find this compound has a relatively wide width of formation, and grow single crystals with compositions Fe$_x$Pd$_{y}$Te$_2$ where $x$ ranges from 0.9 to 1.1 and $y$ from 1.8 to 2.5, respectively. Temperature-dependent X-ray diffraction and transport measurements reveal that a first-order structural transition occurs in the range of $T$ = 360-420 K. Above the transition, the compounds with Pd fraction $y>2$ adopt a disordered derivative of the tetragonal FeTe structure, with the Fe layer showing mixed Fe/Pd occupancy and the extra Pd atoms partially occupying interstitial sites. Below 370 K, the structure is incommensurately modulated. For $y<2$, the composition Fe$_{1.1}$Pd$_{1.8}$Te$_2$ has monoclinic symmetry at room temperature that is consistent with the reported structure of FePd$_2$Te$_2$. This phase undergoes a structural transition at 420 K for which the high temperature structure is yet to be determined; however, based on the similarities with the $y > 2$ compounds, we speculate that this composition also adopts a tetragonal structure above 420\,K. All compounds investigated in the Fe$_x$Pd$_{y}$Te$_2$ series show metallic behavior, with magnetic characterization indicating that they are easy-plane, hard, ferromagnets with $T_C$ spanning 98--180 K. Both the critical temperature for the structural transition and the Curie temperature are moderately suppressed with increasing Pd fraction $y$ and corresponding decreasing Fe fraction $x$, indicating that synthetic control over $x$ and $y$ paves way for the further exploration of these compounds.

cond-mat.str-el↗

arXiv2025

Hidden symmetry-breaking in a kagome Ising ferromagnet

Kagome metals can host unconventional electronic phenomena that emerge from their frustrated lattice geometry and associated band topology. Correlated electronic orders, such as charge-density waves and superconductivity, are observed to intertwine with subtle time-reversal symmetry breaking whose microscopic origin is not currently understood. Here, we provide evidence for such time-reversal symmetry breaking in the kagome metal TbV$_6$Sn$_6$ arising from staggered magnetic moments within the kagome layers. TbV$_6$Sn$_6$ consists of metallic V kagome layers separated by Tb triangular layers that host Ising ferromagnetic order. Deep in the ferromagnetic state, the Tb Ising doublet ground state should display a single, dispersionless spin-flip excitation. Instead, inelastic neutron scattering reveals two sharp excitations associated with inequivalent Tb sites, demonstrating that a symmetry-broken phase coexists with Ising ferromagnetism. No additional structural or magnetic phase transitions are detected, and first-principles calculations rule out lattice distortions as the origin of the splitting. We attribute this effect to time-reversal symmetry breaking encoded by small V moments that couple to the Tb sublattice and leave a measurable spectral fingerprint. Our results establish rare-earth local moment spectroscopy as a sensitive probe of subtle broken symmetries and highlight an unexpected interplay between kagome magnetism and rare-earth local moment magnetism.

cond-mat.str-el↗