arXiv subjects

Valentin Taufour

Publications and source records attributed to Valentin Taufour.

At least 55 records · Page 3Linked to original sources

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

Constraints on the merging of the transition lines at the tricritical point in a wing-structure phase diagram

We consider the phase diagram of a ferromagnetic system driven to a quantum phase transition with a tuning parameter $p$. Before being suppressed, the transition becomes of the first order at a tricritical point, from which wings emerge under application of the magnetic field $H$ in the $T$-$p$-$H$ phase diagram. We show that the edge of the wings merge with tangent slopes at the tricritical point.

cond-mat.str-el↗

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

Superconducting properties of Rh$_{9}$In$_4$S$_4$ single crystals

The synthesis and crystallographic, thermodynamic and transport properties of single crystalline Rh$_{9}$In$_4$S$_4$ were studied. The resistivity, magnetization and specific heat measurements all clearly indicate bulk superconductivity with a critical temperature, $T_{c}\sim$ 2.25 K. The Sommerfeld coefficient $γ$ and the Debye temperature($\varTheta_{\textrm{D}}$) were found to be 34 mJ mol$^{-1}$ K$^{-2}$ and 217 K respectively. The observed specific heat jump, $ΔC/γT_{c}$ = 1.66, is larger than the expected BCS weak coupling value of 1.43. Ginzburg-Landau (GL) ratio of the low temperature GL-penetration depth, $λ_\text{GL}$$\approx$ 5750Å, to the GL-coherence length, $ξ_\text{GL}$$\approx$ 94Å, is large: $κ$ $\sim$ 60. Furthermore, we observed a peak effect in the resistivity measurement as a function of both temperature and magnetic field.

cond-mat.supr-con↗

arXiv2016

Isotope Effect on Electron-Phonon Interaction in Multiband Superconductor MgB2

We investigate the effect of isotope substitution on the electron-phonon interaction in the multi-band superconductor MgB2 using tunable laser based Angle Resolved Photoemission Spectroscopy. The kink structure around 70 meV in the $σ$ band, which is caused by electron coupling to the E2g phonon mode, is shifted to higher binding energy by ~3.5 meV in Mg(10)B2 and the shift is not affected by superconducting transition. These results serve as the benchmark for investigations of isotope effects in known, unconventional superconductors and newly discovered superconductors where the origin of pairing is unknown.

cond-mat.supr-con↗

arXiv2016

Non-monotonic pressure evolution of the upper critical field in superconducting FeSe

The pressure dependence of the upper critical field, $H_\textrm{c2,c}$, of single crystalline FeSe was studied using measurements of the inter-plane resistivity, $ρ_{\textrm{c}}$ in magnetic fields parallel to tetragonal $c$-axis. $H_\textrm{c2,c}(T)$ curves obtained under hydrostatic pressures up to $1.56$ GPa, the range over which the superconducting transition temperature, $T_\textrm{c}$, of FeSe exhibits a non-monotonic dependence with local maximum at $p_1\approx$ 0.8 GPa and local minimum at $p_2\approx$ 1.2 GPa. The slope of the upper critical field at $T_\textrm{c}$, $\left(\textrm{d}H_\text{c2,c}/\textrm{d}T\right)_{T_\textrm{c}}$, also exhibits a non-monotonic pressure dependence with distinct changes at $p_1$ and $p_2$. For $p p_2$ the slope is in good quantitative agreement with a single band, orbital Helfand-Werthamer theory with Fermi velocities determined from Shubnikov-de Haas measurements. This finding indicates that Fermi surface changes are responsible for the local minimum of $T_\textrm{c}(p)$ at $p_2\approx$ 1.2 GPa.

cond-mat.supr-con↗

arXiv2015

Imaging the magnetic nano-domains in Nd2Fe14B

We study magnetic domains in Nd2Fe14B single crystals using high resolution magnetic force mi- croscopy (MFM). Previous MFM studies and small angle neutron scattering experiments suggested the presence of nano-scale domains in addition to optically detected micrometer-scale ones. We find, in addition to the elongated, wavy nano-domains reported by a previous MFM study, that the micrometer size, star shape fractal pattern is constructed of an elongated network of nano-domains ~20 nm in width, with resolution-limited domain walls thinner than 2 nm. While the microscopic domains exhibit significant resilience to an external magnetic field, some of the nano-domains are sensitive to the magnetic field of the MFM tip.

cond-mat.mes-hall↗

arXiv2015

A study of the physical properties of single crystalline Fe5B2P

Single crystals of Fe5B2P were grown by self-flux growth technique. Structural and magnetic properties are studied. The Curie temperature of Fe5B2P is determined to be 655$pm$2K. The saturation magnetization is determined to be 1.72 MuB/Fe at 2K. The temperature variation of the anisotropy constant K1 is determined for the first time, reaching ~0.50 MJ/m3 at 2K, and it is comparable to that of hard ferrites. The saturation magnetization is found to be larger than the hard ferrites. The first principle calculations of saturation magnetization and anisotropy constant are found to be consistent with the experimental results.

cond-mat.mtrl-sci↗

arXiv2015

Structural and ferromagnetic properties of an orthorhombic phase of MnBi stabilized with Rh additions

The article addresses the possibility of alloy elements in MnBi which may modify the thermodynamic stability of the NiAs-type structure without significantly degrading the magnetic properties. The addition of small amounts of Rh and Mn provides an improvement in the thermal stability with some degradation of the magnetic properties. The small amounts of Rh and Mn additions in MnBi stabilize an orthorhombic phase whose structural and magnetic properties are closely related to the ones of the previously reported high-temperature phase of MnBi (HT~MnBi). To date, the properties of the HT~MnBi, which is stable between $613$ and $719$~K, have not been studied in detail because of its transformation to the stable low-temperature MnBi (LT~MnBi), making measurements near and below its Curie temperature difficult. The Rh-stabilized MnBi with chemical formula Mn$_{1.0625-x}$Rh$_{x}$Bi [$x=0.02(1)$] adopts a new superstructure of the NiAs/Ni$_2$In structure family. It is ferromagnetic below a Curie temperature of $416$~K. The critical exponents of the ferromagnetic transition are not of the mean-field type but are closer to those associated with the Ising model in three dimensions. The magnetic anisotropy is uniaxial; the anisotropy energy is rather large, and it does not increase when raising the temperature, contrary to what happens in LT~MnBi. The saturation magnetization is approximately $3$~$μ_B$/f.u. at low temperatures. While this exact composition may not be application ready, it does show that alloying is a viable route to modifying the stability of this class of rare-earth-free magnet alloys.

cond-mat.mtrl-sci↗

arXiv2015

Superconductivity versus structural phase transition in the closely related Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ and Bi$_{2} $Rh$ _{3}$S$ _{2}$

Single crystals of Bi$_{2} $Rh$ _{3}$S$ _{2}$ and Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ were synthesized by solution growth and the crystal structures, thermodynamic and transport properties of both compounds were studied. In the case of Bi$_{2} $Rh$ _{3}$S$ _{2}$, a structural first-order transition at around 165 K is identified by single crystal diffraction experiments, with clear signatures visible in resistivity, magnetization and specific heat data. No superconducting transition for Bi$_{2} $Rh$ _{3}$S$ _{2}$ was observed down to 0.5 K. In contrast, no structural phase transition at high temperature was observed for Bi$_{2} $Rh$ _{3.5}$S$ _{2}$, however bulk superconductivity with a critical temperature, $T_{c}\approx$ 1.7 K was observed. The Sommerfeld coefficient $γ$\ and the Debye temperature($\varTheta_{\textrm{D}}$) were found to be 9.4 mJ mol$^{-1}$ K$^{-2}$ and 209 K respectively for Bi$_{2} $Rh$ _{3}$S$ _{2}$, and 22 mJ mol$^{-1}$ K$^{-2}$ and 196 K respectively for Bi$_{2} $Rh$ _{3.5}$S$ _{2}$. Study of the specific heat in the superconducting state of Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ suggests that Bi$_{2} $Rh$ _{3.5}$S$ _{2}$ is a weakly coupled, BCS superconductor.

cond-mat.supr-con↗

arXiv2015

Momentum dependence of the superconducting gap and in-gap states in MgB2 multi-band superconductor

We use tunable laser based Angle Resolved Photoemission Spectroscopy to study the electronic structure of the multi-band superconductor, MgB2. These results form the base line for detailed studies of superconductivity in multi-band systems. We find that the magnitude of the superconducting gap on both sigma bands follows a BCS-like variation with temperature with Delta0 ~7 meV. The value of the gap is isotropic within experimental uncertainty and in agreement with pure a s-wave pairing symmetry. We also observe in-gap states confined to kF of the sigma band that occur at some locations of the sample surface. The energy of this excitation, ~3 meV, is inconsistent with scattering from the pi band.

cond-mat.supr-con↗

arXiv2015

Strong interaction between electrons and collective excitations in multiband superconductor MgB2

We use a tunable laser ARPES to study the electronic properties of the prototypical multiband BCS superconductor MgB2. Our data reveal a strong renormalization of the dispersion (kink) at ~65 meV, which is caused by coupling of electrons to the E2g phonon mode. In contrast to cuprates, the 65 meV kink in MgB2 does not change significantly across Tc. More interestingly, we observe strong coupling to a second, lower energy collective mode at binding energy of 10 meV. This excitation vanishes above Tc and is likely a signature of the elusive Leggett mode.

cond-mat.supr-con↗

arXiv2015

Origin of the spin reorientation transitions in (Fe$_{1-x}$Co$_{x}$)$_{2}$B alloys

Low-temperature measurements of the magnetocrystalline anisotropy energy $K$ in (Fe$_{1-x}$Co$_{x}$)$_{2}$B alloys are reported, and the origin of this anisotropy is elucidated using a first-principles electronic structure analysis. The calculated concentration dependence $K(x)$ with a maximum near $x=0.3$ and a minimum near $x=0.8$ is in excellent agreement with experiment. This dependence is traced down to spin-orbital selection rules and the filling of electronic bands with increasing electronic concentration. At the optimal Co concentration, $K$ depends strongly on the tetragonality and doubles under a modest 3% increase of the $c/a$ ratio, suggesting that the magnetocrystalline anisotropy can be further enhanced using epitaxial or chemical strain.

cond-mat.mtrl-sci↗

arXiv2014

Upper critical field of KFe2As2 under pressure: A test for the change in the superconducting gap structure

We report measurements of electrical resistivity under pressure to 5.8 GPa, magnetization to 6.7 GPa,and ac susceptibility to 7.1 GPa in KFe2As2. The previously reported change of slope in the pressure dependence of the superconducting transition temperature Tc(p) at a pressure p0=1.8 GPa is confirmed, and Tc(p) is found to be nearly constant above p0 up to 7.1 GPa. The T-p phase diagram is very sensitive to the pressure conditions as a consequence of the anisotropic uniaxial pressure dependence of Tc. Across p0, a change in the behavior of the upper critical field is revealed through a scaling analysis of the slope of Hc2 with the effective mass as determined from the A coefficient of the T2 term of the temperature-dependent resistivity. We show that this scaling provides a quantitative test for the changes of the superconducting gap structure and suggests the development of a kz modulation of the superconducting gap above p0 as a most likely explanation.

cond-mat.supr-con↗

arXiv2013

Thermodynamic and transport properties of single crystalline RCo$_{2}$Ge$_{2}$ (R = Y, La-Nd, Sm-Tm)

Single crystals of RCo$_{2}$Ge$_{2}$ (R = Y, La-Nd, Sm-Tm) were grown using a self-flux method and were characterized by room-temperature powder x-ray diffraction; anisotropic, temperature and field dependent magnetization; temperature and field dependent, in-plane resistivity; and specific heat measurements. In this series, the majority of the moment-bearing members order antiferromagnetically; YCo$_{2}$Ge$_{2}$ and LaCo$_{2}$Ge$_{2}$ are non-moment-bearing. Ce is trivalent in CeCo$_{2}$Ge$_{2}$ at high temperatures, and exhibits an enhanced electronic specific heat coefficient due to Kondo effect at low temperatures. In addition, CeCo$_{2}$Ge$_{2}$ shows two low-temperature anomalies in temperature-dependent magnetization and specific heat measurements. Three members (R = Tb-Ho) have multiple phase transitions above 1.8 K. Eu appears to be divalent with total angular momentum L = 0. Both EuCo$_{2}$Ge$_{2}$ and GdCo$_{2}$Ge$_{2}$ manifest essentially isotropic paramagnetic properties consistent with J = S = 7/2. Clear magnetic anisotropy for rare-earth members with finite L was observed, with ErCo$_{2}$Ge$_{2}$ and TmCo$_{2}$Ge$_{2}$ manifesting planar anisotropy and the rest members manifesting axial anisotropy. The experimentally estimated crystal electric field (CEF) parameters B$_{2}^{0}$ were calculated from the anisotropic paramagnetic $θ_{ab}$ and $θ_{c}$ values and follow a trend that agrees well with theoretical predictions. The ordering temperatures, T$_{N}$, as well as the polycrystalline averaged paramagnetic Curie-Weiss temperature, $Θ_{avg}$, for the heavy rare-earth members deviate from the de Gennes scaling, as the magnitude of both are the highest for Tb, which is sometimes seen for extremely axial systems. Except for SmCo$_{2}$Ge$_{2}$, metamagnetic transitions were observed at 1.8 K for all members that ordered antiferromagnetically.

cond-mat.mtrl-sci↗

arXiv2013

Suppression of ferromagnetism in the La(V$_x$Cr$_{1-x}$)Sb$_3$ system

To explore the possibility of quantum phase transitions and even quantum criticality in LaCrSb$_3$ based compounds, we performed measurements under pressure as well as a vanadium substitution study. The Curie temperature of LaCrSb$_3$ was found to be invariant under pressure. Although pressure was not able to suppress the ferromagnetism, chemical substitution was used as another parameter to tune the magnetism. We grew La(V$_x$Cr$_{1-x}$)Sb$_3$ (\textit{x} = 0 -- 1.0) single crystals, and studied the series by measurements of temperature and field dependent magnetic susceptibility, magnetization, resistivity, and specific heat. Ferromagnetism has been observed for $x \leq 0.22$, and the system manifests a strong anisotropy in its ordered state. The Curie temperature decreases monotonically as the V concentration increases. For $0.42 \leq x \leq 0.73$, the system enters a new magnetic state at low temperatures, and no magnetic ordering above 1.8 K can be observed for $x \geq 0.88$. The effective moment $μ_{\rm eff}$/Cr varies only slightly as the V concentration increases, from 3.9 $μ_{\rm B}$ for $x$ = 0 to 2.9 $μ_{\rm B}$ for $x$ = 0.88. Features related to quantum criticality have not been observed in the La(V$_x$Cr$_{1-x}$)Sb$_3$ system.

cond-mat.mtrl-sci↗

arXiv2013

Suppression of ferromagnetism in the LaVxCr1-xGe3 system

We report synthesis of hexagonal LaVxCr1-xGe3 (x = 0-0.21, 1.00) single crystals, and present a systematic study of this series by measurements of temperature and field dependent magnetic susceptibility, magnetization, resistivity, and specific heat. Ferromagnetism has been observed for x = 0-0.21, and the system manifests a strong axial anisotropy in its ordered state. The decrease of the saturated moment and the effective moment per Cr with the increase of V-concentration suggests this is an itinerant ferromagnetic system. The Curie temperature declines monotonically as the V-concentration increases. Single crystalline samples could only be grown for x-values up to 0.21 for which the transition temperature was suppressed down to 36 K. Although we could not fully suppress TC via V-substitution, for x = 0.16, we performed magnetization measurements under pressure. The ferromagnetic state is suppressed under pressure at an initial rate of dTC/dp = -11.7 K/GPa and vanishes by 3.3 GPa.

cond-mat.str-el↗

arXiv2012

Field-induced Phenomena in Ferromagnetic Superconductors UCoGe and URhGe

We review our recent studies on ferromagnetic superconductors, UGe2, URhGe and UCoGe, where the spin-triplet state with the so-called equal spin pairing is realized. We focus on experimental results of URhGe and UCoGe in which the superconductivity occurs already at ambient pressure. The huge upper critical field Hc2 on UCoGe for the field along the hard magnetization axis (b-axis) is confirmed by the AC susceptibility measurements by the fine tuning of field angle. Contrary to the huge Hc2 along the hard-magnetization axis, Hc2 along the easy-magnetization axis (c-axis) is relatively small in value. However, the initial slope of Hc2, namely dHc2/dT (H -> 0) both in UCoGe and in URhGe indicates the large value, which can be explained by the magnetic domain effect detected in the magnetization measurements. The specific heat measurements using a high quality single crystal of UCoGe demonstrate the bulk superconductivity, which is extended under magnetic field for the field along c-axis.

cond-mat.str-el↗