arXiv subjects

Hubertus Luetkens

Publications and source records attributed to Hubertus Luetkens.

At least 73 records · Page 4Linked to original sources

arXiv2020

Piezoelectric-driven uniaxial pressure cell for muon spin relaxation and neutron scattering experiments

We present a piezoelectric-driven uniaxial pressure cell that is optimized for muon spin relaxation and neutron scattering experiments, and that is operable over a wide temperature range including cryogenic temperatures. To accommodate the large samples required for these measurement techniques, the cell is designed to generate forces up to 1000 N, and to minimize the background signal the space around the sample is kept as open as possible. We demonstrate here that by mounting plate-like samples with epoxy, a uniaxial stress exceeding 1 GPa can be achieved in an active volume of 5 mm3. We show that for practical operation it is important to monitor both the force and displacement applied to the sample. Also, because time is critical during facility experiments, samples are mounted in detachable holders that can be rapidly exchanged. The piezoelectric actuators are likewise contained in an exchangeable cartridge.

physics.app-ph↗

arXiv2020

Thermal disorder driven magnetic phases in van der Waals magnet CrI3

Magnetic phase transitions often occur spontaneously at specific critical temperatures. The presence of more than one critical temperature (Tc) has been observed in several compounds where the coexistence of competing magnetic orders highlights the importance of phase separation driven by different factors such as pressure, temperature and chemical composition. However, it is unknown whether recently discovered two-dimensional (2D) van der Walls (vdW) magnetic materials show such intriguing phenomena that can result in rich phase diagrams with novel magnetic features to be explored. Here we show the existence of three magnetic phase transitions at different Tc's in 2D vdW magnet CrI3 revealed by a complementary suite of muon spin relaxation-rotation, superconducting quantum interference device magnetometry, and large-scale atomistic simulations including higher-order exchange interactions. We find that the traditionally identified Curie temperature of bulk CrI3 at 61 K does not correspond to the long-range order in the full volume (VM) of the crystal but rather a partial transition with less than 25% of VM being magnetically spin-ordered. This transition is composed of highly disordered domains with the easy-axis component of the magnetization Sz not being fully spin-polarized but disordered by in-plane components (Sx, Sy) over the entire layer. As the system cools down, two additional phase transitions at 50 K and 25 K drive the system to 80% and nearly 100% of the magnetically ordered volume, respectively, where the ferromagnetic ground state has a marked Sz character yet also displaying finite contributions of Sx and Sy to the total magnetization. Our results indicate that volume-wise competing electronic phases play an important role in the magnetic properties of CrI3 which set a much lower threshold temperature for exploitation in magnetic device-platforms than initially considered.

cond-mat.mes-hall↗

arXiv2020

Short-range magnetic interactions and spin-glass behavior in the quasi-2D nickelate Pr4Ni3O8

The nickelate Pr4Ni3O8 features quasi-two-dimensional layers consisting of three stacked square-planar NiO2 planes, in a similar way to the well-known cuprate superconductors. The mixed-valent nature of Ni and its metallic properties makes it a candidate for potentially unconventional superconductivity. We have synthesized Pr4Ni3O8 by topotactic reduction of Pr4Ni3O10 in 10 percent hydrogen gas, and report on measurements of powder-neutron diffraction, magnetization and muon-spin rotation (uSR). We find that Pr4Ni3O8 shows complicated spin-glass behavior with a distinct magnetic memory effect in the temperature range from 2 to 300 K and a freezing temperature T_s ~ 68 K. Moreover, the analysis of uSR spectra indicates two magnetic processes characterized by remarkably different relaxation rates: a slowly-relaxing signal, resulting from paramagnetic fluctuations of Pr/Ni ions, and a fast-relaxing signal, whose relaxation rate increases substantially below ~ 70 K which can be ascribed to the presence of short-range correlated regions. We conclude that the complex spin-freezing process in Pr4Ni3O8 is governed by these multiple magnetic interactions. It is possible that the complex magnetism in Pr4Ni3O8 is detrimental to the occurrence of superconductivity.

cond-mat.supr-con↗

arXiv2020

Penetration depth and gap structure in the antiperovskite oxide superconductor Sr$_{3-x}$SnO revealed by $μ$SR

We report a $μ$SR study on the antiperovskite oxide superconductor Sr$_{3-x}$SnO. With transverse-field $μ$SR, we observed the increase of the muon relaxation rate upon cooling below the superconducting transition temperature $T_{\mathrm{c}}=5.4$ K, evidencing bulk superconductivity. The exponential temperature dependence of the relaxation rate $σ$ at low temperatures suggests a fully gapped superconducting state. We evaluated the zero-temperature penetration depth $λ(0)\propto1/\sqrt{σ(0)}$ to be around 320-1020 nm. Such a large value is consistent with the picture of a doped Dirac semimetal. Moreover, we revealed that the ratio $T_{\mathrm{c}}/λ(0)^{-2}$ is larger than those of ordinary superconductors and is comparable to those of unconventional superconductors. The relatively high $T_{\mathrm{c}}$ for small carrier density may hint at an unconventional pairing mechanism beyond the ordinary phonon-mediated pairing. In addition, zero-field $μ$SR did not provide evidence of broken time-reversal symmetry in the superconducting state. These features are consistent with the theoretically proposed topological superconducting state in Sr$_{3-x}$SnO, as well as with $s$-wave superconductivity.

cond-mat.supr-con↗

arXiv2020

Split superconducting and time-reversal symmetry-breaking transitions, and magnetic order in Sr$_2$RuO$_4$ under uniaxial stress

Among unconventional superconductors, Sr$_2$RuO$_4$ has become a benchmark for experimentation and theoretical analysis because its normal-state electronic structure is known with exceptional precision, and because of experimental evidence that its superconductivity has, very unusually, a spontaneous angular momentum, i.e. a chiral state. This hypothesis of chirality is however difficult to reconcile with recent evidence on the spin part of the order parameter. Measurements under uniaxial stress offer an ideal way to test for chirality, because under uniaxial stress the superconducting and chiral transitions are predicted to split, allowing the empirical signatures of each to be identified separately. Here, we report zerofield muon spin relaxation (ZF-$μ$SR) measurements on crystals placed under uniaxial stresses of up to 1.05 GPa. We report a clear stress-induced splitting between the onset temperatures of superconductivity and time-reversal symmetry breaking, consistent with qualitative expectations for chiral superconductivity. We also report the appearance of unexpected bulk magnetic order under a uniaxial stress of ~ 1.0 GPa in clean Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2019

Self-consistent two-gap approach in studying multi-band superconductivity in NdFeAsO$_{0.65}$F$_{0.35}$

High-quality single crystals of NdFeAsO$_{0.65}$F$_{0.35}$ (the transition temperature $T_{\rm c} \simeq 30.6$~K) were studied in zero-field (ZF) and transverse-field (TF) muon-spin rotation/relaxation ($μ$SR) experiments. An upturn in muon-spin depolarization rate at $T\lesssim 3$~K was observed in ZF-$μ$SR measurements and it was associated with the onset of ordering of Nd electronic moments. Measurements of the magnetic field penetration depth ($λ$) were performed in the TF geometry. By applying the external magnetic field $B_{\rm ex}$ parallel to the crystallographic $c$-axis ($B_{\rm ex}\| c$) and parallel to the $ab$-plane ($B_{\rm ex}\| ab$), the temperature dependencies of the in-plane component ($λ_{ab}^{-2}$) and the combination of the in-plane and the out of plane components ($λ_{ab,c}^{-2}$) of the superfluid density were determined, respectively. The out-of-plane superfluid density component ($λ_{c}^{-2}$) was further obtained by combining the results of $B_{\rm ex} \| c$ and $B_{\rm ex} \| {ab}$ set of experiments. The temperature dependencies of $λ_{ab}^{-2}$, $λ_{ab,c}^{-2}$, and $λ_{c}^{-2}$ were analyzed within the framework of a self-consistent two-gap model despite of using the traditional $α$-model. Interband coupling was taken into account, instead of assuming it to be zero as it stated in the $α$-model. A relatively small value of the interband coupling constant $Λ_{12} \simeq 0.01$ was obtained, thus indicating that the energy bands in NdFeAsO$_{0.65}$F$_{0.35}$ are only weakly coupled. In spite of their small magnitude, the coupling between the bands leads to the single value of the superconducting transition temperature $T_{\rm c}$.

cond-mat.supr-con↗

arXiv2019

Relevance of magnetism to cuprate superconductivity: Lanthanides versus charge-compensated cuprates

We address what seemed to be a contradiction between the lanthanide series REBa$_2$Cu$_3$O$_y$ (RE123) and the charge-compensated series (Ca$_{x}$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x} $)Cu$_{3}$O$_{y}$ (CLBLCO) regarding the superexchange ($J$) dependence of the maximum superconductivity (SC) critical temperature $T_c^{max}(J)$; RE and $x$ are implicit variables. This is done by measuring the Néel temperature and the temperature dependence of the magnetic order parameter for RE=Nd, Sm, Eu, Gd, Dy, Yb, Y, and for Y(BaSr)Cu$_3$O$_y$, at various very light dopings. The doping is determined by thermopower, and the magnetic properties by muon spin rotation. We find that the normalized-temperature dependence of the order parameter is identical for all RE123 in the undoped limit (with the exception of Gd123) implying identical out-of-plane magnetic coupling. The extrapolation of $T_N$ to zero doping suggests that, despite the variations in ionic radii, $J$ varies too weakly in this system to test the relation between SC and magnetism. This stands in contrast to CLBLCO where both $T_c^{max}$ and $T_N^{max}$ vary considerably in the undoped limit, and a positive correlation between the two quantities was observed.

cond-mat.supr-con↗

arXiv2019

Structural phases of elemental Gallium: universal relations in type-I superconductors

The temperature dependent measurements of the thermodynamic critical field and the specific heat for the pressure stabilized Ga-II phase of elemental Gallium are presented. The discussion of these and other Ga phases data in the context of elemental and binary phonon-mediated type-I superconductors allowed to establish simple scaling relations between BCS quantities such as $B_{\rm c}(0)/T_{\rm c}\sqrt{γ_{\rm e}}$ and the specific heat jump at $T_{\rm c}$ versus the coupling strength $2Δ/k_{\rm B} T_{\rm c}$ [$Δ$ and $B_{\rm c}(0)$ are the zero-temperature values of the superconducting energy gap and the thermodynamic critical field, respectively, $T_{\rm c}$ is the transition temperature, and $γ_{\rm e}$ is the electronic specific heat]. The scaling relations can be analytically expressed by taking into account strong-coupling corrections to BCS theory. Such correlations can naturally explain the linear relation between $B_{\rm c}(0)$ and $T_{\rm c}$, which holds for type-I superconducting materials.

cond-mat.supr-con↗

arXiv2019

Muon spin rotation study of type-I superconductivity: elemental $β-$Sn

The application of the muon-spin rotation/relaxation ($μ$SR) technique for studying type-I superconductivity is discussed. In the intermediate state, i.e. when a type-I superconducting sample with non-zero demagnetization factor $N$ is separated into normal state and Meissner state (superconducting) domains, the $μ$SR technique allows to determine with very high precision the value of the thermodynamic critical field $B_{\rm c}$, as well as the volume of the sample in the normal and the superconducting state. Due to the microscopic nature of $μ$SR technique, the $B_{\rm c}$ values are determined directly via measurements of the internal field inside the normal state domains. No assumptions or introduction of any type of measurement criteria are needed. Experiments performed on a 'classical' type-I superconductor, a cylindrically shaped $β-$Sn sample, allowed to reconstruct the full $B-T$ phase diagram. The zero-temperature value of the thermodynamic critical field $B_{\rm c}(0)=30.578(6)$ mT and the transition temperature $T_{\rm c}=3.717(3)$ K were determined and found to be in a good agreement with the literature data. An experimentally obtained demagnetization factor is in very good agreement with theoretical calculations of the demagnetization factor of a finite cylinder. The analysis of $B_{\rm c}(T)$ dependence within the framework of the phenomenological $α-$model allow to obtain the value of the superconducting energy gap $Δ=0.59(1)$ meV, of the electronic specific heat $γ_e=1.781(3)$ ${\rm mJ}/{\rm mol}\; {\rm K}^2$ and of the jump in the heat capacity ${ΔC(T_c)}/{γT_{\rm c}}=1.55(2)$.

cond-mat.supr-con↗

arXiv2019

Local study of the insulating quantum kagome antiferromagnets YCu3(OH)6OxCl3-x (x=0,1/3)

The quantum kagome antiferromagnets YCu3(OH)6OxCl3-x (x=0,1/3) are produced using a unified solid state synthesis route for polycrystalline samples. From structural refinements based on neutron diffraction data, we clarify the structure of the Y3Cu9(OH)18OCl8 (x=1/3) compound and provide a revised chemical formula. We use muon spin relaxation, as a local probe of magnetism, to investigate the exotic low temperature properties in the two compounds. In agreement with the low temperature neutron diffraction data, we find no evidence for long range ordering in both materials but they exhibit distinct ground states: while disordered static magnetism develops in the x=0 compound, we conclude on the stabilization of a quantum spin liquid in the x=1/3 one, since the local fields remain fully dynamical. Our findings are in contrast to previous reports based on thermodynamical measurements only. We then discuss their origin on the basis of structural details and specific heat measurements. In particular, the x=1/3 compound appears to realize an original spatially anisotropic kagome model.

cond-mat.str-el↗

arXiv2019

On the superconducting nature of the Bi-II phase of elemental Bismuth

The superconductivity in the Bi-II phase of elemental Bismuth (transition temperature $T_{\rm c}\simeq3.92$ K at pressure $p\simeq 2.80$ GPa) was studied experimentally by means of the muon-spin rotation as well as theoretically by using the Eliashberg theory in combination with Density Functional Theory calculations. Experiments reveal that Bi-II is a type-I superconductor with a zero temperature value of the thermodynamic critical field $B_{\rm c}(0)\simeq31.97$~mT. The Eliashberg theory approach provides a good agreement with the experimental $T_{\rm c}$ and the temperature evolution of $B_{\rm c}$. The estimated value for the retardation (coupling) parameter $k_{\rm B}T_{\rm c}/ω_{\rm ln} \approx 0.07$ ($ω_{\rm ln}$ is the logarithmically averaged phonon frequency) suggests that Bi-II is an intermediately-coupled superconductor.

cond-mat.supr-con↗

arXiv2019

Superconductivity of Bi-III phase of elemental Bismuth: insights from Muon-Spin Rotation and Density Functional Theory

Using muon-spin rotation the pressure-induced superconductivity in the Bi-III phase of elemental Bismuth (transition temperature $T_{\rm c}\simeq7.05$ K) was investigated. The Ginzburg-Landau parameter $κ=λ/ξ=30(6)$ ($λ$ is the magnetic penetration depth, $ξ$ is the coherence length) was estimated which is the highest among single element superconductors. The temperature dependence of the superconducting energy gap [$Δ(T)$] reconstructed from $λ^{-2}(T)$ deviates from the weak-coupled BCS prediction. The coupling strength $2Δ/k_{\rm B}T_{\rm c}\simeq 4.34$ was estimated thus implying that Bi-III stays within the strong coupling regime. The Density Functional Theory calculations suggest that superconductivity in Bi-III could be described within the Eliashberg approach with the characteristic phonon frequency $ω_{\rm ln}\simeq 5.5$ meV. An alternative pairing mechanism to the electron-phonon coupling involves the possibility of Cooper pairing induced by the Fermi surface nesting.

cond-mat.supr-con↗

arXiv2019

Anisotropy induced vortex lattice rearrangement in CaKFe$_4$As$_4$

The magnetic penetration depth anisotropy $γ_λ=λ_{c}/λ_{ab}$ ($λ_{ab}$ and $λ_{c}$ are the in-plane and the out-of-plane components of the magnetic penetration depth) in a CaKFe$_4$As$_4$ single crystal sample (the critical temperature $T_{\rm c}\simeq 35$ K) was studied by means of muon-spin rotation ($μ$SR). $γ_λ$ is almost temperature independent for $T\lesssim 20$ K ($γ_λ\simeq 1.9$) and it reaches $\simeq 3.0$ by approaching $T_{\rm c}$. The change of $γ_λ$ induces the corresponding rearrangement of the flux line lattice (FLL), which is clearly detected via enhanced distortions of the FLL $μ$SR response. Comparison of $γ_λ$ with the anisotropy of the upper critical field ($γ_{H_{\rm c2}}$) studied in Phys. Rev B {\bf 94}, 064501 (2016), reveals that $γ_λ$ is systematically higher than $γ_{H_{\rm c2}}$ at low-temperatures and approaches $γ_{H_{\rm c2}}$ for $T \rightarrow T_{\rm c}$. The anisotropic properties of $λ$ are explained by the multi-gap nature of superconductivity in CaKFe$_4$As$_4$ and are caused by anisotropic contributions of various bands to the in-plane and the out-of-plane components of the superfluid density.

cond-mat.supr-con↗

arXiv2018

Magnetic tricritical point and nematicity in FeSe under pressure

Magnetism induced by external pressure ($p$) was studied in a FeSe crystal sample by means of muon-spin rotation. The magnetic transition changes from second-order to first-order for pressures exceeding the critical value $p_{\rm c}\simeq2.4-2.5$ GPa. The magnetic ordering temperature ($T_{\rm N}$) and the value of the magnetic moment per Fe site ($m_{\rm Fe}$) increase continuously with increasing pressure, reaching $T_{\rm N}\simeq50$~K and $m_{\rm Fe}\simeq0.25$ $μ_{\rm B}$ at $p\simeq2.6$ GPa, respectively. No pronounced features at both $T_{\rm N}(p)$ and $m_{\rm Fe}(p)$ are detected at $p\simeq p_{\rm c}$, thus suggesting that the stripe-type magnetic order in FeSe remains unchanged above and below the critical pressure $p_{\rm c}$. A phenomenological model for the $(p,T)$ phase diagram of FeSe reveals that these observations are consistent with a scenario where the nematic transitions of FeSe at low and high pressures are driven by different mechanisms.

cond-mat.supr-con↗

arXiv2018

In-plane magnetic penetration depth of superconducting CaKFe$_4$As$_4$

The temperature dependence of the in-plane magnetic penetration depth ($λ_{ab}$) in an extensively characterized sample of superconducting CaKFe$_4$As$_4$ ($T_{\rm c}\simeq35$ K) was investigated using muon-spin rotation ($μ$SR). A comparison of $λ_{ab}^{-2}(T)$ measured by $μ$SR with the one inferred from ARPES data confirms the presence of multiple gaps at the Fermi level. An agreement between $μ$SR and ARPES requires the presence of additional bands, which are not resolved by ARPES experiments. These bands are characterized by small supercondcting gaps with an average zero-temperature value of $Δ_{0} =$ 2.4(2) meV. Our data suggest that in CaKFe$_4$As$_4$ the $s^\pm$ order parameter symmetry acquires a more sophisticated form by allowing a sign change not only between electron and hole pockets, but also within pockets of similar type.

cond-mat.supr-con↗

arXiv2017

Coulomb spin liquid in anion-disordered pyrochlore Tb$_2$Hf$_2$O$_7$

The charge ordered structure of ions and vacancies characterizing rare-earth pyrochlore oxides serves as a model for the study of geometrically frustrated magnetism. The organization of magnetic ions into networks of corner-sharing tetrahedra gives rise to highly correlated magnetic phases with strong fluctuations, including spin liquids and spin ices. It is an open question how these ground states governed by local rules are affected by disorder. In the pyrochlore Tb$_2$Hf$_2$O$_7$, we demonstrate that the vicinity of the disordering transition towards a defective fluorite structure translates into a tunable density of anion Frenkel disorder while cations remain ordered. Quenched random crystal fields and disordered exchange interactions can therefore be introduced into otherwise perfect pyrochlore lattices of magnetic ions. We show that disorder can play a crucial role in preventing long-range magnetic order at low temperatures, and instead induces a strongly-fluctuating Coulomb spin liquid with defect-induced frozen magnetic degrees of freedom.

cond-mat.str-el↗

arXiv2016

Physical realization of a quantum spin liquid based on a novel frustration mechanism

Unlike conventional magnets where the magnetic moments are partially or completely static in the ground state, in a quantum spin liquid they remain in collective motion down to the lowest temperatures. The importance of this state is that it is coherent and highly entangled without breaking local symmetries. Such phenomena is usually sought in simple lattices where antiferromagnetic interactions and/or anisotropies that favor specific alignments of the magnetic moments are "frustrated" by lattice geometries incompatible with such order e.g. triangular structures. Despite an extensive search among such compounds, experimental realizations remain very few. Here we describe the investigation of a novel, unexplored magnetic system consisting of strong ferromagnetic and weaker antiferromagnetic isotropic interactions as realized by the compound Ca$_{10}$Cr$_7$O$_{28}$. Despite its exotic structure we show both experimentally and theoretically that it displays all the features expected of a quantum spin liquid including coherent spin dynamics in the ground state and the complete absence of static magnetism.

cond-mat.str-el↗

arXiv2015

Microscopic coexistence of superconductivity and magnetism in Ca1-xNaxFe2As2

We present a detailed investigation of the magnetic and superconducting properties of Ca1-xNaxFe2As2 single crystals with x = 0.00, 0.35, 0.50, and 0.67 by means of the local probe techniques Moessbauer spectroscopy and muon spin relaxation experiments. With increasing Na substitution level, the magnetic order parameter as well as the magneto-structural phase transition are suppressed. For x = 0.50 we find a microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature Tc. A systematic comparison with other 122 pnictides reveals a square-root correlation between the reduction of the magnetic order parameter and the ratio of the transition temperatures, Tc/TN, which can be understood in the framework of a Landau theory. In the optimally doped sample with Tc = 34 K, diluted magnetism is found and the temperature dependence of the penetration depth and superfluid density are obtained, proving the presence of two superconducting s-wave gaps

cond-mat.supr-con↗