arXiv subjects

Paul C. Canfield

Publications and source records attributed to Paul C. Canfield.

At least 163 records · Page 9Linked to original sources

arXiv2017

On magnetic structure of CuFe2Ge2: constrains from the 57Fe Mossbauer spectroscopy

57Fe Mossbauer spectroscopy measurements were performed on a powdered CuFe2Ge2 sample that orders antiferromagnetically at ~ 175 K. Whereas a paramagnetic doublet was observed above the Neel temperature, a superposition of paramagnetic doublet and magnetic sextet (in approximately 0.5 : 0.5 ratio) was observed in the magnetically ordered state, suggesting a magnetic structure similar to a double-Q spin density wave with half of the Fe paramagnetic and another half bearing static moment of ~ 0.5 - 1 mu_B. These results call for a re-evaluation of the recent neutron scattering data and band structure calculations.

cond-mat.mtrl-sci↗

arXiv2017

GdPtPb: A non collinear antiferromagnet with distorted Kagomé lattice

In the spirit of searching for Gd-based, frustrated, rare earth magnets, we have found antiferomagnetism (AF) in GdPtPb which crystallizes in the ZrNiAl-type structure that has a distorted Kagomé lattice of Gd-triangles. Single crystals were grown and investigated using structural, magnetic, transport and thermodynamic measurements. GdPtPb orders antiferromagnetically at 15.5 K arguably with a planar, non-collinear structure. The high temperature magnetic susceptibility data reveal an "anti-frustration" behavior having a frustration parameter, $|f|$ = $|Θ|$/ $T_N$ = 0.25, which can be explained by mean field theory (MFT) within a two sub-lattice model. Study of the magnetic phase diagram down to $T$ = 1.8 K reveals a change of magnetic structure through a metamagnetic transition at around 20 kOe and the disappearance of the AF ordering near 140 kOe. In total, our work indicates that, GdPtPb can serve as an example of a planar, non collinear, AF with a distorted Kagomé magnetic sub-lattice.

cond-mat.str-el↗

arXiv2017

Discovery of Orbital-Selective Cooper Pairing in FeSe

The superconductor FeSe is of intense interest thanks to its unusual non-magnetic nematic state and potential for high temperature superconductivity. But its Cooper pairing mechanism has not been determined. Here we use Bogoliubov quasiparticle interference imaging to determine the Fermi surface geometry of the bands surrounding the $Γ= (0,0)$ and $X=(π/ a_{Fe}, 0)$ points of FeSe, and to measure the corresponding superconducting energy gaps. We show that both gaps are extremely anisotropic but nodeless, and exhibit gap maxima oriented orthogonally in momentum space. Moreover, by implementing a novel technique we demonstrate that these gaps have opposite sign with respect to each other. This complex gap configuration reveals the existence of orbital-selective Cooper pairing which, in FeSe, is based preferentially on electrons from the $d_{yz}$ orbitals of the iron atoms.

cond-mat.supr-con↗

arXiv2017

Dome of magnetic order inside the nematic phase of sulfur-substituted FeSe under pressure

The pressure dependence of the structural, magnetic and superconducting transitions and of the superconducting upper critical field were studied in sulfur-substituted Fe(Se$_{1-x}$S$_{x}$). Resistance measurements were performed on single crystals with three substitution levels ($x$=0.043, 0.096, 0.12) under hydrostatic pressures up to 1.8 GPa and in magnetic fields up to 9 T, and compared to data on pure FeSe. Our results illustrate the effects of chemical and physical pressure on Fe(Se$_{1-x}$S$_{x}$). On increasing sulfur content, magnetic order in the low-pressure range is strongly suppressed to a small dome-like region in the phase diagrams. However, $T_s$ is much less suppressed by sulfur substitution and $T_c$ of Fe(Se$_{1-x}$S$_{x}$) exhibits similar non-monotonic pressure dependence with a local maximum and a local minimum present in the low pressure range for all $x$. The local maximum in $T_c$ coincides with the emergence of the magnetic order above $T_c$. At this pressure the slope of the upper critical field decreases abruptly. The minimum of $T_c$ correlates with a broad maximum of the upper critical field slope normalized by $T_c$.

cond-mat.supr-con↗

arXiv2017

Vortex creep at very low temperatures in single crystals of the extreme type-II superconductor Rh$_9$In$_4$S$_4$

We image vortex creep at very low temperatures using Scanning Tunneling Microscopy (STM) in the superconductor Rh$_9$In$_4$S$_4$ ($T_c$=2.25 K). We measure the superconducting gap of Rh$_9$In$_4$S$_4$, finding $Δ\approx 0.33$meV and image a hexagonal vortex lattice up to close to H$_{c2}$, observing slow vortex creep at temperatures as low as 150 mK. We estimate thermal and quantum barriers for vortex motion and show that thermal fluctuations likely cause vortex creep, in spite of being at temperatures $T/T_c<0.1$. We study creeping vortex lattices by making images during long times and show that the vortex lattice remains hexagonal during creep with vortices moving along one of the high symmetry axis of the vortex lattice. Furthermore, the creep velocity changes with the scanning window suggesting that creep depends on the local arrangements of pinning centers. Vortices fluctuate on small scale erratic paths, indicating that the vortex lattice makes jumps trying different arrangements during its travel along the main direction for creep. The images provide a visual account of how vortex lattice motion maintains hexagonal order, while showing dynamic properties characteristic of a glass.

cond-mat.supr-con↗

arXiv2017

Phonon-Induced Topological Transition to a Type-II Weyl Semimetal

Given the importance of crystal symmetry for the emergence of topological quantum states, we have studied, as exemplified in NbNiTe2, the interplay of crystal symmetry, atomic displacements (lattice vibration), band degeneracy, and band topology. For NbNiTe2 structure in space group 53 (Pmna) - having an inversion center arising from two glide planes and one mirror plane with a 2-fold rotation and screw axis - a full gap opening exists between two band manifolds near the Fermi energy. Upon atomic displacements by optical phonons, the symmetry lowers to space group 28 (Pma2), eliminating one glide plane along c, the associated rotation and screw axis, and the inversion center. As a result, twenty Weyl points emerge, including four type-II Weyl points in the G-X direction at the boundary between a pair of adjacent electron and hole bands. Thus, optical phonons may offer control of the transition to a Weyl fermion state.

cond-mat.mtrl-sci↗

arXiv2017

Pressure induced change in the electronic state of Ta$_4$Pd$_3$Te$_{16}$

We present measurements of superconducting transition temperature, resistivity, magnetoresistivity and temperature dependence of the upper critical field of Ta$_4$Pd$_3$Te$_{16}$ under pressures up to 16.4 kbar. All measured properties have an anomaly at $\sim 2 - 4$ kbar pressure range, in particular there is a maximum in $T_c$ and upper critical field, $H_{c2}(0)$, and minimum in low temperature, normal state resistivity. Qualitatively, the data can be explained considering the density of state at the Fermi level as a dominant parameter.

cond-mat.supr-con↗

arXiv2017

Pressure induced half-collapsed-tetragonal phase in CaKFe$_4$As$_4$

We report the temperature-pressure phase diagram of CaKFe$_4$As$_4$ established using high pressure electrical resistivity, magnetization and high energy x-ray diffraction measurements up to 6 GPa. With increasing pressure, both resistivity and magnetization data show that the bulk superconducting transition of CaKFe$_4$As$_4$ is suppressed and then disappears at $p$ $\gtrsim$ 4 GPa. High pressure x-ray data clearly indicate a phase transition to a collapsed tetragonal phase in CaKFe$_4$As$_4$ under pressure that coincides with the abrupt loss of bulk superconductivity near 4 GPa. The x-ray data, combined with resistivity data, indicate that the collapsed tetragonal transition line is essentially vertical, occuring at 4.0(5) GPa for temperatures below 150 K. Band structure calculations also find a sudden transition to a collapsed tetragonal state near 4 GPa, as As-As bonding takes place across the Ca-layer. Bonding across the K-layer only occurs for $p$ $\geq$ 12 GPa. These findings demonstrate a new type of collapsed tetragonal phase in CaKFe$_4$As$_4$: a half-collapsed-tetragonal phase.

cond-mat.supr-con↗

arXiv2017

Highly responsive ground state of PbTaSe$_2$: structural phase transition and evolution of superconductivity under pressure

Transport and magnetic studies of PbTaSe$_2$ under pressure suggest existence of two superconducting phases with the low temperature phase boundary at $\sim 0.25$ GPa that is defined by a very sharp, first order, phase transition. The first order phase transition line can be followed via pressure dependent resistivity measurements, and is found to be near 0.12 GPa near room temperature. Transmission electron microscopy and x-ray diffraction at elevated temperatures confirm that this first order phase transition is structural and occurs at ambient pressure near $\sim 425$ K. The new, high temperature / high pressure phase has a similar crystal structure and slightly lower unit cell volume relative to the ambient pressure, room temperature structure. Based on first-principles calculations this structure is suggested to be obtained by shifting the Pb atoms from the $1a$ to $1e$ Wyckoff position without changing the positions of Ta and Se atoms. PbTaSe$_2$ has an exceptionally pressure sensitive, structural phase transition with $ΔT_s/ΔP \approx - 1700$ K/GPa near 4 K, this first order transition causes an $\sim 1$ K ($\sim 25 \%$) step - like decrease in $T_c$ as pressure is increased through 0.25 GPa.

cond-mat.supr-con↗

arXiv2017

$^{57}$Fe Mössbauer study of stoichiometric iron based superconductor CaKFe$_4$As$_4$: a comparison to KFe$_2$As$_2$ and CaFe$_2$As$_2$

$^{57}$Fe Mössbauer spectra at different temperatures between $\sim 5$ K and $\sim 300$ K were measured on an oriented mosaic of single crystals of CaKFe$_4$As$_4$ . The data indicate that CaKFe$_4$As$_4$ is a well formed compound with narrow spectral lines, no traces of other, Fe - containing, secondary phases in the spectra and no static magnetic order. There is no discernible feature at the superconducting transition temperature in any of the hyperfine parameters. The temperature dependence of the quadrupole splitting approximately follows the empirical "$T^{3/2}$ law". The hyperfine parameters of CaKFe$_4$As$_4$ are compared with those for KFe$_2$As$_2$ measured in this work, and the literature data for CaFe$_2$As$_2$, and were found to be in between those for these two, ordered, 122 compounds, in agreement with the gross view of CaKFe$_4$As$_4$ as a structural analog of KFe$_2$As$_2$ and CaFe$_2$As$_2$ that has alternating Ca - and K - layers in the structure.

cond-mat.supr-con↗

arXiv2016

Discovery of ferromagnetism with large magnetic anisotropy in ZrMnP and HfMnP

ZrMnP and HfMnP single crystals are grown by a self-flux growth technique and structural as well as temperature dependent magnetic and transport properties are studied. Both compounds have an orthorhombic crystal structure. ZrMnP and HfMnP are ferromagnetic with Curie temperatures around $370$~K and $320$~K respectively. The spontaneous magnetizations of ZrMnP and HfMnP are determined to be $1.9$~$μ_\textrm{B}$/f.u. and $2.1$~$μ_\textrm{B}$/f.u. respectively at $50$~K. The magnetocaloric effect of ZrMnP in term of entropy change ($ΔS$) is estimated to be $-6.7$ kJm$^{-3}$K$^{-1}$ around $369$~K. The easy axis of magnetization is [100] for both compounds, with a small anisotropy relative to the [010] axis. At $50$~K, the anisotropy field along the [001] axis is $\sim4.6$~T for ZrMnP and $\sim10$~T for HfMnP. Such large magnetic anisotropy is remarkable considering the absence of rare-earth elements in these compounds. The first principle calculation correctly predicts the magnetization and hard axis orientation for both compounds, and predicts the experimental HfMnP anisotropy field within 25 percent. More importantly, our calculations suggest that the large magnetic anisotropy comes primarily from the Mn atoms suggesting that similarly large anisotropies may be found in other 3d transition metal compounds.

cond-mat.mtrl-sci↗

arXiv2016

Anisotropic physical properties and pressure dependent magnetic ordering of CrAuTe$_4$

Systematic measurements of temperature dependent magnetization, resistivity and angle-resolved photoemission spectroscopy (ARPES) at ambient pressure as well as resistivity under pressures up to 5.25 GPa were conducted on single crystals of CrAuTe$_4$. Magnetization data suggest that magnetic moments are aligned antiferromagnetically along the crystallographic $c$-axis below $T_\textrm{N}$ = 255 K. ARPES measurements show band reconstruction due to the magnetic ordering. Magnetoresistance data show clear anisotropy, and, at high fields, quantum oscillations. The Neel temperature decreases monotonically under pressure, decreasing to $T_\textrm{N}$ = 236 K at 5.22 GPa. The pressure dependencies of (i) $T_\textrm{N}$, (ii) the residual resistivity ratio, and (iii) the size and power-law behavior of the low temperature magnetoresistance all show anomalies near 2 GPa suggesting that there may be a phase transition (structural, magnetic, and/or electronic) induced by pressure. For pressures higher than 2 GPa a significantly different quantum oscillation frequency emerges, consistent with a pressure induced change in the electronic states.

cond-mat.str-el↗

arXiv2016

Tricritical wings and modulated magnetic phases in LaCrGe$ _{3}$ under pressure

We determined on the temperature-pressure-magnetic field ($T$-$p$-$H$) phase diagram of the ferromagnet LaCrGe$_3$ from electrical resistivity measurements on single crystals. In ferromagnetic systems, quantum criticality is avoided either by a change of the transition order, becoming of the first order at a tricritical point, or by the appearance of modulated magnetic phases. In the first case, the application of a magnetic field reveals a wing-structure phase diagram as seen in itinerant ferromagnets such as ZrZn$_2$ and UGe$_2$. In the second case, no tricritical wings have been observed so far. Our investigation of LaCrGe$_3$ reveals a double-wing structure indicating strong similarities with ZrZn$_2$ and UGe$_2$. But, unlike these, simpler systems, LaCrGe$_3$ is thought to exhibit a modulated magnetic phase under pressure which already precludes it from a pressure-driven paramagnetic-ferromagnetic quantum phase transition in zero field. As a result, the $T$-$p$-$H$ phase diagram of LaCrGe$_3$ shows both the wing structure as well as the appearance of new magnetic phases, providing the first example of this new possibility for the phase diagram of metallic quantum ferromagnets.

cond-mat.str-el↗

arXiv2016

BaSn$_2$: A new, wide-gap, strong topological insulator

BaSn$_2$ has been shown to form as layers of buckled stanene intercalated by barium ions~\cite{Kim_2008}. However, despite an apparently straightforward synthesis and significant interest in stanene as a topological material, BaSn$_2$ has been left largely unexplored, and has only recently been recognized as a potential topological insulator. Belonging to neither the lead nor bismuth chalcogenide families, it would represent a unique manifestation of the topological insulating phase. Here we present a detailed investigation of BaSn$_2$, using both {\it ab initio} and experimental methods. First-principles calculations demonstrate that this overlooked material is a indeed strong topological insulator with a bulk band gap of 360meV, among the largest observed for topological insulators. We characterize the surface state dependence on termination chemistry, providing guidance for experimental efforts to measure and manipulate its topological properties. Additionally, through {\it ab initio} modeling and synthesis experiments we explore the stability and accessibility of this phase, revealing a complicated phase diagram that indicates a challenging path to obtaining single crystals.

cond-mat.mtrl-sci↗

arXiv2016

Tuning the Kondo effect in Yb(Fe$_{1-x}$Co$_{x}$)$_{2}$Zn$_{20}$

We study the evolution of the Kondo effect in heavy fermion compounds, Yb(Fe$_{1-x}$Co$_{x}$)$_{2}$Zn$_{20}$ (0$\leqslant$ x $\leqslant$ 1), by means of temperature-dependent electric resistivity and specific heat. The ground state of YbFe$_2$Zn$_{20}$ can be well described by a Kondo model with degeneracy $N$ = 8 and a $T_K\sim$30 K. In the presence of a very similar total CEF splitting with YbFe$_2$Zn$_{20}$, the ground state of YbCo$_2$Zn$_{20}$ is close to a Kondo state with degeneracy $N$ = 2 and a much lower $T_K\sim$ 2 K. Upon Co substitution, the coherence temperature of YbFe$_2$Zn$_{20}$ is suppressed, accompanied by an emerging Schottky-like feature in specific heat associated with the thermal depopulation of CEF levels upon cooling. For 0.4$\lesssim$ x $\lesssim$ 0.9, the ground state remains roughly the same which can be qualitatively understood by Kondo effect in the presence of CEF splitting. There is no clear indication of Kondo coherence observable in resistivity within this substitution range down to 500 mK. The coherence re-appears at around x$\gtrsim$ 0.9 and the coherence temperature increases with higher Co concentration levels.

cond-mat.str-el↗

arXiv2016

Physical properties of single crystalline $R$Mg$_{2}$Cu$_{9}$ ($R$ = Y, Ce-Nd, Gd-Dy, Yb) and the search for in-plane magnetic anisotropy in hexagonal systems

Single crystals of $R$Mg$_{2}$Cu$_{9}$ ($R$=Y, Ce-Nd, Gd-Dy, Yb) were grown using a high-temperature solution growth technique and were characterized by measurements of room-temperature x-ray diffraction, temperature-dependent specific heat and temperature-, field-dependent resistivity and anisotropic magnetization. YMg$_{2}$Cu$_{9}$ is a non-local-moment-bearing metal with an electronic specific heat coefficient, $γ\sim$ 15 mJ/mol K$^2$. Yb is divalent and basically non-moment bearing in YbMg$_{2}$Cu$_{9}$. Ce is trivalent in CeMg$_{2}$Cu$_{9}$ with two magnetic transitions being observed at 2.1 K and 1.5 K. PrMg$_{2}$Cu$_{9}$ does not exhibit any magnetic phase transition down to 0.5 K. The other members being studied ($R$=Nd, Gd-Dy) all exhibits antiferromagnetic transitions at low-temperatures ranging from 3.2 K for NdMg$_{2}$Cu$_{9}$ to 11.9 K for TbMg$_{2}$Cu$_{9}$. Whereas GdMg$_{2}$Cu$_{9}$ is isotropic in its paramagnetic state due to zero angular momentum ($L$=0), all the other local-moment-bearing members manifest an anisotropic, planar magnetization in their paramagnetic states. To further study this planar anisotropy, detailed angular-dependent magnetization was carried out on magnetically diluted (Y$_{0.99}$Tb$_{0.01}$)Mg$_{2}$Cu$_{9}$ and (Y$_{0.99}$Dy$_{0.01}$)Mg$_{2}$Cu$_{9}$. Despite the strong, planar magnetization anisotropy, the in-plane magnetic anisotropy is weak and field-dependent. A set of crystal electric field parameters are proposed to explain the observed magnetic anisotropy.

cond-mat.mtrl-sci↗

arXiv2016

Ferromagnetic Quantum Critical Point Avoided by the Appearance of Another Magnetic Phase in LaCrGe$_3$ under Pressure

The temperature-pressure phase diagram of the ferromagnet LaCrGe$_3$ is determined for the first time from a combination of magnetization, muon-spin-rotation and electrical resistivity measurements. The ferromagnetic phase is suppressed near $2.1$~GPa, but quantum criticality is avoided by the appearance of a magnetic phase, likely modulated, AFM$_Q$. Our density functional theory total energy calculations suggest a near degeneracy of antiferromagnetic states with small magnetic wave vectors $Q$ allowing for the potential of an ordering wave vector evolving from $Q=0$ to finite $Q$, as expected from the most recent theories on ferromagnetic quantum criticality. Our findings show that LaCrGe$_3$ is a very simple example to study this scenario of avoided ferromagnetic quantum criticality and will inspire further study on this material and other itinerant ferromagnets.

cond-mat.str-el↗

arXiv2016

Anisotropic magnetoresistance and upper critical fields up to 63 T in CaKFe$_4$As$_4$ single crystals

We report the temperature dependencies of the upper critical fields $H_{c\text{2}}^{\text{c}}(T)$ parallel to the c-axis and $H_{c\text{2}}^{\text{ab}}(T)$ parallel to the ab-plane of single crystalline CaKFe$_4$As$_4$ inferred from the measurements of the temperature-dependent resistance in static magnetic fields up to 14 T and magnetoresistance in pulsed fields up to 63 T. We show that the observed decrease of the anisotropy parameter $γ(T)=H_{c\text{2}}^{\text{ab}}/H_{c\text{2}}^{\text{c}}$ from $\simeq 2.5$ at $T_c$ to $\simeq 1.5$ at 25 K can be explained by interplay of paramagnetic pairbreaking and orbital effects in a multiband theory of $H_{c2}$. The slopes of $dH_{c\text{2}}^{\text{c}}/dT\simeq-4.4$ T/K and $dH_{c\text{2}}^{\text{ab}}/dT \simeq-10.9$ T/K at $T_c$ yield an electron mass anisotropy of $m_{ab}/m_c\simeq 1/6$ and short coherence lengths $ξ_c\simeq 5.8\,\textÅ$ and $ξ_{ab}\simeq 14.3\,\textÅ$. The behavior of $H_{c\text{2}}(T)$ turns out to be similar to that of the optimal doped (Ba,K)Fe$_2$As$_2$, with $H_{c\text{2}}^{\text{ab}}(0)$ extrapolating to $\simeq 92$ T, well above the BCS paramagnetic limit.

cond-mat.supr-con↗