arXiv subjects

H. Luetkens

Publications and source records attributed to H. Luetkens.

At least 91 records · Page 5Linked to original sources

arXiv2018

Magnetic inhomogeneity in the copper pseudochalcogenide CuNCN

Copper carbodiimide, CuNCN, is a geometrically frustrated nitrogen-based analogue of cupric oxide, whose magnetism remains ambiguous. Here, we employ a combination of local-probe techniques, including $^{63,\, 65}$Cu nuclear quadrupole resonance, $^{13}$C nuclear magnetic resonance and muon spin rotation to show that the magnetic ground state of the Cu$^{2+}$ ($S=1/2$) spins is frozen and disordered. Moreover, these complementary experiments unequivocally establish an onset of intrinsically inhomogeneous magnetic state at $T_h=80$ K. Below $T_h$, the low-temperature frozen component coexist with the remnant high-temperature dynamical component down to $T_l = 20$ K, where the latter finally ceases to exist. Based on a scaling of internal magnetic fields of both components we conclude that the two components coexist on a microscopic level.

cond-mat.str-el↗

arXiv2018

Close proximity of FeSe to a magnetic quantum critical point as revealed by high-resolution $μ$SR measurements

A nematic transition preceding a long-range spin density wave antiferromagnetic phase is a common feature of many Fe based superconductors. However, in the FeSe system with a nematic transition at $T_{\rm s} \approx$ 90 K no evidence for long-range static magnetism down to very low temperature was found. The lack of magnetism is a challenge for the theoretical description of FeSe. Here, we investigated high-quality single crystals of FeSe using high-field (up to 9.5 Tesla) muon spin rotation ($μ$SR) measurements. The $μ$SR Knight shift and the bulk susceptibility linearly scale at high temperatures but deviate from this behavior around $T^{*} \sim 10$ K, where the Knight shift exhibits a kink. This behavior hints to an essential change of the electronic and/or magnetic properties crossing the region near $T^{*}$. In the temperature range $T_{\rm s} \gtrsim T \gtrsim T^{*}$ the muon spin depolarization rate follows a critical behavior $Λ\propto T^{-0.4}$. The observed non-Fermi liquid behavior with a cutoff at $T^{*}$ indicates that FeSe is in the vicinity to a antiferromagnetic quantum critical point. Our analysis is suggestive for $T^{*}$ triggered by the Lifshitz transition.

cond-mat.supr-con↗

arXiv2017

Magnetism in Semiconducting Molybdenum Dichalcogenides

Transition metal dichalcogenides (TMDs) are interesting for understanding fundamental physics of two-dimensional materials (2D) as well as for many emerging technologies, including spin electronics. Here, we report the discovery of long-range magnetic order below TM = 40 K and 100 K in bulk semiconducting TMDs 2H-MoTe2 and 2H-MoSe2, respectively, by means of muon spin-rotation (muSR), scanning tunneling microscopy (STM), as well as density functional theory (DFT) calculations. The muon spin rotation measurements show the presence of a large and homogeneous internal magnetic fields at low temperatures in both compounds indicative of long-range magnetic order. DFT calculations show that this magnetism is promoted by the presence of defects in the crystal. The STM measurements show that the vast majority of defects in these materials are metal vacancies and chalcogen-metal antisites which are randomly distributed in the lattice at the sub-percent level. DFT indicates that the antisite defects are magnetic with a magnetic moment in the range of 0.9-2.8 mu_B. Further, we find that the magnetic order stabilized in 2H-MoTe2 and 2H-MoSe2 is highly sensitive to hydrostatic pressure. These observations establish 2H-MoTe2 and 2H-MoSe2 as a new class of magnetic semiconductors and opens a path to studying the interplay of 2D physics and magnetism in these interesting semiconductors.

cond-mat.mtrl-sci↗

arXiv2017

The new versatile general purpose surface-muon instrument (GPS) based on silicon photomultipliers for $μ$SR measurements on a continuous-wave beam

We report on the design and commissioning of a new spectrometer for muon-spin relaxation/rotation studies installed at the Swiss Muon Source (S$μ$S) of the Paul Scherrer Institute (PSI, Switzerland). This new instrument is essentially a new design and replaces the old general-purpose surface-muon instrument (GPS) which has been for long the workhorse of the $μ$SR user facility at PSI. By making use of muon and positron detectors made of plastic scintillators read out by silicon photomultipliers (SiPMs), a time resolution of the complete instrument of about 160 ps (standard deviation) could be achieved. In addition, the absence of light guides, which are needed in traditionally built $μ$SR instrument to deliver the scintillation light to photomultiplier tubes located outside magnetic fields applied, allowed us to design a compact instrument with a detector set covering an increased solid angle compared to the old GPS.

physics.ins-det↗

arXiv2017

Magnetic order and spin dynamics in the helical magnetic system Fe$_3$PO$_4$O$_3$

The 3$d$-electronic spin dynamics and the magnetic order in Fe$_3$PO$_4$O$_3$ were investigated by muon spin rotation and relaxation ($μ$SR) and $^{57}$Fe Mössbauer spectroscopy. Zero-field (ZF)-$μ$SR and $^{57}$Fe Mössbauer studies confirm static long range magnetic ordering below $T_{\mathrm{N}}$ $\approx$ 164\,K. Both transverse-field (TF) and ZF-$μ$SR results evidence 100\% magnetic volume fraction in the ordered state. The ZF-$μ$SR time spectra can be best described by a Bessel function, which is consistent with the helical magnetic structure proposed by neutron scattering experiments. The Mössbauer spectra are described in detail by considering the specific angular distribution of the local hyperfine field $B_{\mathrm{hyp}}$ with respect to the local electric field gradient. The $μ$SR spin-lattice relaxation rate exhibits two peaks: One at the magnetic ordering temperature related to critical magnetic fluctuations and another peak at 35\,K signaling the presence of a secondary low energy scale in Fe$_3$PO$_4$O$_3$.

cond-mat.str-el↗

arXiv2017

Superconductivity with broken time reversal symmetry in ion irradiated Ba$_{0.27}$K$_{0.73}$Fe$_2$As$_2$ single crystals

Over the last years a lot of theoretical and experimental efforts have been made to find states with broken time reversal symmetry (BTRS) in multi-band superconductors. In particular, it was theoretically proposed that in the Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ system either an $s+is$ or an $s+id$ BTRS state may exist at high doping levels in a narrow region of the phase diagram. Here we report the observation of an enhanced zero field muon spin relaxation rate below the superconducting transition temperature for a high quality crystalline sample with $x \approx$ 0.73. This indicates that indeed the time reversal symmetry is broken in superconducting Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ at this doping level.

cond-mat.supr-con↗

arXiv2017

Possible topologically non-trivial superconducting order parameter in type-II Weyl semimetal T_d-MoTe_2

MoTe_2, with the orthorhombic T_d phase, is a new type (type-II) of Weyl semimetal, where the Weyl Fermions emerge at the boundary between electron and hole pockets. Non-saturating magnetoresistance (MR), and superconductivity were also observed in T_d-MoTe_2. Understanding the superconductivity in T_d-MoTe_2, which was proposed to be topologically non-trivial, is of eminent interest. Here, we report high-pressure (p_max = 1.3 GPa) muon spin rotation experiments on the temperature-dependent magnetic penetration depth in T_d-MoTe_2. A substantial increase of the superfluid density n_s/m^* and a linear scaling with T_c is observed under pressure. Moreover, the superconducting order parameter in T_d-MoTe_2 is determined to be two gap (s+s)-wave symmetric. We also excluded time reversal symmetry breaking in the SC state with sensitive zero-field $μ$SR experiments. Considering the previous report \cite{Balicas1} on the strong suppression of T_c in T_d-MoTe_2 by disorder, we suggest that s^{+-} (topological order parameter) state is more likely to be realized in MoTe_2 than the s^{++} (trivial) state. Should s^{+-} be the SC gap symmetry, the T_d-MoTe_2 is, to our knowledge, the first known example of a time reversal invariant topological (Weyl) superconductor.

cond-mat.supr-con↗

arXiv2017

Magnetic order and spin dynamics across a ferromagnetic quantum critical point: $μ$SR investigations of YbNi$_4$(P$_{1-x}$As$_x$)$_2$

In the quasi-1D heavy-fermion system YbNi$_4$(P$_{1-x}$As$_x$)$_2$ the presence of a ferromagnetic (FM) quantum critical point (QCP) at $x_c$ $\approx 0.1$ with unconventional quantum critical exponents in the thermodynamic properties has been recently reported. Here, we present muon-spin relaxation ($μ$SR) experiments on polycrystals of this series to study the magnetic order and the low energy 4$f$-electronic spin dynamics across the FM QCP. The zero field $μ$SR measurements on pure YbNi$_4$(P$_{2}$ proved static long range magnetic order and suggested a strongly reduced ordered Yb moment of about 0.04$μ_B$. With increasing As substitution the ordered moment is reduced by half at $x = 0.04$ and to less than 0.005 $μ_B$ at $x=0.08$. The dynamic behavior in the $μ$SR response show that magnetism remains homogeneous upon As substitution, without evidence for disorder effect. In the paramagnetic state across the FM QCP the dynamic muon-spin relaxation rate follows 1/$T_{1}T\propto T^{-n}$ with $1.01 \pm 0.04 \leq n \leq 1.13 \pm 0.06$. The critical fluctuations are very slow and are even becoming slower when approaching the QCP.

cond-mat.str-el↗

arXiv2017

A new magnetic phase in the nickelate perovskite TlNiO$_3$

The RNiO$_3$ perovskites are known to order antiferromagnetically below a material-dependent Néel temperature $T_\text{N}$. We report experimental evidence indicating the existence of a second magnetically-ordered phase in TlNiO$_3$ above $T_\text{N} = 104$ K, obtained using nuclear magnetic resonance and muon spin rotation spectroscopy. The new phase, which persists up to a temperature $T_\text{N}^* = 202$ K, is suppressed by the application of an external magnetic field of approximately 1 T. It is not yet known if such a phase also exists in other perovskite nickelates.

cond-mat.str-el↗

arXiv2017

Magnetic states of MnP: muon-spin rotation studies

Muon-spin rotation data collected at ambient pressure ($p$) and at $p=2.42$ GPa in MnP were analyzed to check their consistency with various low- and high-pressure magnetic structures reported in the literature. Our analysis confirms that in MnP the low-temperature and low-pressure helimagnetic phase is characterised by an increased value of the average magnetic moment compared to the high-temperature ferromagnetic phase. An elliptical double-helical structure with a propagation vector ${\bf Q}=(0,0,0.117)$, an $a-$axis moment elongated by approximately 18% and an additional tilt of the rotation plane towards $c-$direction by $\simeq 4-8^{\rm o}$ leads to a good agreement between the theory and the experiment. The analysis of the high-pressure $μ$SR data reveals that the new magnetic order appearing for pressures exceeding $1.5$ GPa can not be described by keeping the propagation vector ${\bf Q} \parallel c$. Even the extreme case -- decoupling the double-helical structure into four individual helices -- remains inconsistent with the experiment. It is shown that the high-pressure magnetic phase which is a precursor of superconductivity is an incommensurate helical state with ${\bf Q} \parallel b$.

cond-mat.supr-con↗

arXiv2016

Spin dynamics of the ordered dipolar-octupolar pseudospin-$^1/_2$ pyrochlore Nd$_2$Zr$_2$O$_7$ probed by muon spin relaxation

We present a muon spin relaxation study on the Ising pyrochlore Nd$_2$Zr$_2$O$_7$ which develops an "all-in-all-out" magnetic order below 0.4~K. At 20~mK far below the ordering transition temperature, the zero-field muon spin relaxation spectra show no static features and can be well described by a dynamical Gaussian-broadened Gaussian Kubo-Toyabe function indicating strong fluctuations of the ordered state. The spectra of the paramagnetic state (below 4.2~K) reveal anomalously slow paramagnetic spin dynamics and show only small difference with the spectra of the ordered state. We find that the fluctuation rate decreases with decreasing temperature and becomes nearly temperature independent below the transition temperature indicating persistent slow spin dynamics in the ground state. The field distribution width shows a small but sudden increase at the transition temperature and then becomes almost constant. The spectra in applied longitudinal fields are well fitted by the conventional dynamical Gaussian Kubo-Toyabe function, which further supports the dynamical nature of the ground state. The fluctuation rate shows a peak as a function of external field which is associated with a field-induced spin-flip transition. The strong dynamics in the ordered state are attributed to the transverse coupling of the Ising spins introduced by the multipole interactions.

cond-mat.mtrl-sci↗

arXiv2016

$μ$SR Insight into the Impurity Problem in Quantum Kagome Antiferromagnets

Impurities, which are inherently present in any real material, may play an important role in the magnetism of frustrated spin systems with spin-liquid ground states. We address the impurity issue in quantum kagome antiferromagnets by investigating ZnCu$_3$(OH)$_6$SO$_4$ (Zn-brochantite) by means of muon spin spectroscopy. We show that muons couple to the impurity magnetism, originating from Cu-Zn intersite disorder, and that the impurities are highly correlated with the kagome spins, allowing us to probe the intrinsic kagome physics via a Kondo-like effect. The low-temperature plateau in local susceptibility identifies the spin-liquid ground state as being gapless. The corresponding spin fluctuations exhibit an unconventional spectral density and an intriguing field dependence.

cond-mat.str-el↗

arXiv2016

Fully gapped superconductivity in the topological superconductor beta-PdBi2

The recent discovery of the topologically protected surface states in the beta-phase of PdBi2 has reignited the research interest in this class of superconductors. Here, we show results of our muon spin relaxation and rotation (muSR) measurements carried out to investigate the superconducting and magnetic properties and the topological effect in the superconducting ground state of beta-PdBi2. Zero-field muSR data reveal that no sizeable spontaneous magnetization arises with the onset of superconductivity implying that the time reversal symmetry is preserved in the superconducting state of beta-PdBi2. Further, a strong diamagnetic shift of the applied field has been observed in the transverse-field (TF) muSR experiments, indicating that any triplet-pairing channel, if present, does not dominate the superconducting condensate. Using TF-muSR, we estimate that the magnetic penetration depth is 263(10) nm at zero temperature. Temperature dependence of the magnetic penetration depth provides evidence for the existence of a nodeless single s-wave type isotropic energy gap of 0.78(1) meV at zero temperature. Our results further suggest that the topologically protected surface states have no effect on the bulk of the superconductor.

cond-mat.supr-con↗

arXiv2016

Distinct Magnetic Phases in Structurally Uniform SrCoO$_{3-y}$

Two magnetic phase transitions have been noted for SrCoO$_{3-y}$ for near-stoichiometric oxygen concentrations (small y). Using muon spin rotation and neutron scattering experiments, we have established that the two transitions represent separate, spatially distinct magnetic phases that coexist in a two-phase equilibrium mixture. The two phases most likely represent areas of the sample with different effective valence charge density. Further, the phases exist over regions with a length scale intermediate between nanoscale charge inhomogeneity and systems such as manganites or super-oxygenated cuprates with large length scale phase separation.

cond-mat.mtrl-sci↗

arXiv2016

High pressure magnetic state of MnP probed by means of muon-spin rotation

We report a detailed $μ$SR study of the pressure evolution of the magnetic order in the manganese based pnictide MnP, which has been recently found to undergo a superconducting transition under pressure once the magnetic ground state is suppressed. Using the muon as a volume sensitive local magnetic probe, we identify a ferromagnetic state as well as two incommensurate helical states (with propagation vectors ${\bf Q}$ aligned along the crystallographic $c-$ and $b-$directions, respectively) which transform into each other through first order phase transitions as a function of pressure and temperature. Our data appear to support that the magnetic state from which superconductivity develops at higher pressures is an incommensurate helical phase.

cond-mat.supr-con↗

arXiv2016

Instabilities of Spin-Liquid States in a Quantum Kagome Antiferromagnet

The emergent behavior of spin liquids that are born out of geometrical frustration makes them an intriguing state of matter. We show that in the quantum kagome antiferromagnet ZnCu$_3$(OH)$_6$SO$_4$ several different correlated, yet fluctuating states exist. By combining complementary local-probe techniques with neutron scattering, we discover a crossover from a critical regime into a gapless spin-liquid phase with decreasing temperature. An additional unconventional instability of the latter phase leads to a second, distinct spin-liquid state that is stabilized at the lowest temperatures. We advance such complex behavior as a feature common to different frustrated quantum magnets.

cond-mat.str-el↗

arXiv2016

Probing the pairing symmetry in the over-doped Fe-based superconductor Ba_0.35Rb_0.65Fe_2As_2 as a function of hydrostatic pressure

We report muon spin rotation experiments on the magnetic penetration depth lambda and the temperature dependence of lambda^{-2} in the over-doped Fe-based high-temperature superconductor (Fe-HTS) Ba_{1-x}Rb_ xFe_2As_2 (x = 0.65) studied at ambient and under hydrostatic pressures up to p = 2.3 GPa. We find that in this system lambda^{-2}(T) is best described by d-wave scenario. This is in contrast to the case of the optimally doped x = 0.35 system which is known to be a nodeless s^{+-}-wave superconductor. This suggests that the doping induces the change of the pairing symmetry from s^{+-} to d-wave in Ba_{1-x}Rb_{x}Fe_{2}As_{2}. In addition, we find that the d-wave order parameter is robust against pressure, suggesting that d is the common and dominant pairing symmetry in over-doped Ba_{1-x}Rb_{x}Fe_{2}As_{2}. Application of pressure of p = 2.3 GPa causes a decrease of lambda(0) by less than 5 %, while at optimal doping x = 0.35 a significant decrease of lambda(0) was reported. The superconducting transition temperature T_c as well as the gap to T_c ratio 2Delta/k_BT_c show only a modest decrease with pressure. By combining the present data with those previously obtained for optimally doped system x = 0.35 and for the end member x = 1 we conclude that the SC gap symmetry as well as the pressure effects on the SC quantities strongly depend on the Rb doping level. These results are discussed in the light of the putative Lifshitz transition, i.e., a disappearance of the electron pockets in the Fermi surface of Ba_{1-x}Rb_{x}Fe_{2}As_{2} upon hole doping.

cond-mat.supr-con↗

arXiv2016

Coexistence of low moment magnetism and superconductivity in tetragonal FeS and suppression of $T_\mathrm{c}$ under pressure

We report local probe ($μ$SR) measurements on the recently discovered tetragonal FeS superconductor which has been predicted to be electronically very similar to superconducting FeSe. Most remarkably, we find that low moment ($10^{-2}-10^{-3}μ_\mathrm{B})$ disordered magnetism with a transition temperature of $T_\mathrm{N}\approx 20$ K microscopically coexists with bulk superconductivity below $T_\mathrm{c}=4.3(1)$ K. From transverse field $μ$SR we obtain an in-plane penetration depth $λ_\mathrm{ab}(0)=223(2)$ nm for FeS. The temperature dependence of the corresponding superfluid density $λ_\mathrm{ab}^{-2}(T)$ indicates a fully gapped superconducting state and is consistent with a two gap s-wave model. Additionally, we find that the superconducting $T_\mathrm{c}$ of FeS continuously decreases for hydrostatic pressures up to 2.2 GPa.

cond-mat.supr-con↗