arXiv subjects

A. Keren

Publications and source records attributed to A. Keren.

At least 19 recordsLinked to original sources

arXiv2020

Magnetic properties of $(Fe_{1-x}Mn_x)_2AlB_2$ and the impact of substitution on the magnetocaloric effect

In this work, we investigate the magnetic structures of $(Fe_{1-x}Mn_x)_2AlB_2$ solid-solution quaternaries in the $x = 0$ to $1$ range using x-ray and neutron diffraction, magnetization measurements, and mean-field theory calculations. While $Fe_2AlB_2$ and $Mn_2AlB_2$ are known to be ferromagnetic (FM) and antiferromagnetic (AFM), respectively, herein we focused on the magnetic structure of their solid solutions, which is not well understood. The FM ground state of $Fe_2AlB_2$ becomes a canted AFM at $x \approx 0.2$, with a monotonically diminishing FM component until $x \approx 0.5$. The FM transition temperature ($T_C$) decreases linearly with increasing $x$. These changes in magnetic moments and structures are reflected in anomalous expansions of the lattice parameters, indicating a magnetoelastic coupling. Lastly, the magnetocaloric properties of the solid solutions were explored. For $x = 0.2$ the isothermal entropy change is smaller by 30% than it is for $Fe_2AlB_2$, while the relative cooling power is larger by 6%, due to broadening of the temperature range of the transition.

cond-mat.mtrl-sci↗

arXiv2019

Adiabatic variation of the charge-density-wave phase diagram in the 123 cuprate (Ca$_x$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x}$)Cu$_3$O$_y$

In order to explain the emergence of the anomalous pseudogap state and high-temperature superconductivity in the cuprates, intense research activity over three decades has focused on unravelling the connection between the various instabilities of the underdoped regime. In the high-temperature superconductor (Ca$_x$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x}$)Cu$_3$O$_y$ (CLBLCO) isovalent chemical substitution produces smooth changes to the CuO$_2$ plane buckling and the Cu(II)-to-apical-oxygen distance, allowing us to study the interdependence of charge-density-wave (CDW) order, superconductivity and the pseudogap at constant hole doping in two adiabatically connected representations of the 123 cuprate structure. In this study, resonant soft x-ray scattering measurements reveal the first observation of incommensurate CDW correlations in CLBLCO and demonstrate a lack of correlation between $T_{\text{CDW}}$ and the pseudogap crossover temperature ($T^{\ast}$). This result disfavours a scenario in which the opening of the pseudogap at $T^{\ast}$ results from fluctuating CDW correlations.

cond-mat.supr-con↗

arXiv2018

RIXS studies of magnons and bimagnons in the lightly doped cuprate La2-xSrxCuO4

We investigated the doping dependence of magnetic excitations in the lightly doped cuprate La2-xSrxCuO4 via combined studies of resonant inelastic x-ray scattering (RIXS) at the Cu L3-edge and theoretical calculations. With increasing doping, the magnon dispersion is found to be essentially unchanged, but the spectral width broadens and the spectral weight varies differently at different momenta. Near the Brillouin zone center, we directly observe bimagnon excitations which possess the same energy scale and doping dependence as previously observed by Raman spectroscopy. They disperse weakly in energy-momentum space, and are consistent with a bimagnon dispersion that is renormalized by the magnon-magnon interaction at the zone center.

cond-mat.supr-con↗

arXiv2017

Zero Energy States at a Normal--Cuprate-Superconductor Interface Probed by Shot Noise

We report measurements of the current noise generated by the optimally doped, x=0.15, ${\rm Au- La_{2-x}Sr_xCuO_4}$ junctions. For high transmission junctions on (110) surface, we observed split zero-bias conductance peak (ZBCP), accompanied by enhanced shot noise. We attribute the enhanced noise to the Cooper pair transport through the junction. The ZBCP disappears and the noise decreases to the one expected for the charge $e$ with heating at temperatures well below ${\rm T_c}$, and at voltages much smaller than the bulk superconducting gap, setting a new energy scale of 0.5 mV. We attribute this scale to the existence of an $id_{xy}$ or $is$ order parameter at the sample surface.

cond-mat.supr-con↗

arXiv2017

Rotation symmetry breaking in La2-xSrxCuO4 revealed by ARPES

Using angle-resolved photoemission spectroscopy it is revealed that in the vicinity of optimal doping the electronic structure of La2-xSrxCuO4 cuprate undergoes an electronic reconstruction associated with a wave vector q_a=(pi, 0). The reconstructed Fermi surface and folded band are distinct to the shadow bands observed in BSCCO cuprates and in underdoped La2-xSrxCuO4 with x <= 0.12, which shift the primary band along the zone diagonal direction. Furthermore the folded bands appear only with q_a=(pi, 0) vector, but not with q_b= (0, pi). We demonstrate that the absence of q_b reconstruction is not due to the matrix-element effects in the photoemission process, which indicates the four-fold symmetry is broken in the system.

cond-mat.supr-con↗

arXiv2014

17O-NMR Knight shift study of the interplay between superconductivity and pseudogap in (Ca_xLa_{1-x})(Ba_{1.75 - x}La_{0.25 + x})Cu_3O_y

We report systematic 17O-NMR measurements on the high-Tc cuprate (Ca_xLa_{1-x})(Ba_{1.75 - x}La_{0.25 + x})Cu_3O_y, for four different families (different x). Using Knight shift data, we show that the pseudogap lines for all families are inconsistent with a quantum critical point inside the superconducting dome. In addition, at constant doping the pseudogap temperature does not vary with x, in contrast to Tc. We therefore argue that pseudogap and superconductivity are separate phenomena in these cuprates. Using Knight shift data, we show that the pseudogap opening temperature T* is much higher than Tc near optimal doping, unlike structurally similar YBCO. In addition, at constant doping the pseudogap temperature does not vary with x, in contrast to Tc. This puts constraints on the nature of the pseudogap and position of the quantum critical point inside the superconducting dome.

cond-mat.supr-con↗

arXiv2012

Evolution from a nodeless gap to d(x2-y2) form in underdoped La(2-x)SrxCuO4

Using angle-resolved photoemission (ARPES), it is revealed that the low-energy electronic excitation spectra of highly underdoped superconducting and non-superconducting La(2-x)SrxCuO4 cuprates are gapped along the entire underlying Fermi surface at low temperatures. We show how the gap function evolves to a d(x2-y2) form as increasing temperature or doping, consistent with the vast majority of ARPES studies of cuprates. Our results provide essential information for uncovering the symmetry of the order parameter(s) in strongly underdoped cuprates, which is a prerequisite for understanding the pairing mechanism and how superconductivity emerges from a Mott insulator.

cond-mat.supr-con↗

arXiv2012

Anisotropy of the upper critical fields and the paramagnetic Meissner effect in La1.85Sr0.15CuO4 single Crystals

Optimally-doped La1.85Sr0.15CuO4 single crystals have been investigated by dc and ac magnetic measurements. These crystals have rectangular needle-like shapes with the long needle axis parallel to the crystallographic c axis (c-crystal) or parallel to the basal planes (a-crystal). In both crystals, the temperature dependence of the upper critical fields (HC2) and the surface critical field (HC3) were measured. The H-T phase diagram is presented. Close to TC =35 K, for the c-crystal, γc = / = 1.80(2), whereas for the a-crystal the γa = / =4.0(2) obtained, is much higher than the theoretical value 1.69. At low applied dc fields, positive field-cooled branches known as the "paramagnetic Meissner effect" (PME) are observed, their magnitude is inversely proportional to H. The anisotropic PME is observed in both a- and c-crystals, only when the applied field is along the basal planes. It is speculated that the high γa and the PME are connected to each other.

cond-mat.supr-con↗

arXiv2010

Muon spin rotation study of the magnetic penetration depth in the intercalated graphite superconductor CaC6

We report temperature- and magnetic field-dependent bulk muon spin rotation measurements in a c-axis oriented superconductor CaC6 in the mixed state. Using both a simple second moment analysis and the more precise analytical Ginzburg-Landau model, we obtained a field independent in-plane magnetic penetration depth λab (0) = 72(3) nm. The temperature dependencies of the normalized muon spin relaxation rate and of the normalized superfluid density result to be identical, and both are well represented by the clean limit BCS model with 2Δ/kB Tc = 3.6(1), suggesting that CaC6 is a fully gapped BCS superconductor in the clean limit regime.

cond-mat.supr-con↗

arXiv2004

Anomaly in YBa2Cu4O8 charge distribution below Tc: a zero-field muSR study

Zero-field $μ$SR measurements in $^{63}$Cu isotope enriched and natural YBa$_2$Cu$_4$O$_8$ powders are presented. The temperature dependence of the $μ^+$ relaxation rate is characterized by a sizeable enhancement below T$_c$. The comparison of the asymmetry decay in the two samples reveals that the $μ^+$ relaxation is driven by nuclear dipole interaction from 300 K down to 4.2 K. It is argued that the increase in the relaxation below T$_c$ originates from a change in $μ^+$ site, possibly due to a modification in the charge distribution within CuO chains.

cond-mat.str-el↗

arXiv2004

Dynamic properties of a diluted pyrochlore cooperative paramagnet (Tb_pY_{1-p})_2Ti_2O_7

Investigations of the spin dynamics of the geometrically frustrated pyrochlore (Tb_pY_{1-p})_2Ti_2O_7, using muon spin relaxation and neutron spin echo, as a function of magnetic coverage p, have been carried out. Our major finding is that paramagnetic fluctuations prevail as T->0 for all values of p, and that they are sensitive to dilution, indicating a cooperative spin motion. However, the percolation threshold p_c is not a critical point for the fluctuations. We also find that the low temperatures spectral density has a 1/f behavior, and that dilution slows down the spin fluctuations.

cond-mat.str-el↗

arXiv2003

Muon Spin Relaxation Measurements in Na_xCoO_2*yH_2O

Using the transverse field muon spin relaxation technique we measure the temperature dependence of the magnetic field penetration depth $λ$, in the Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O system. We find that $λ,$ which is determined by superfluid density $n_{s}$ and the effective mass $m^{\ast}$, is very small and on the edge of the TF-$μ$SR sensitivity. Nevertheless, the results indicate that the order parameter in this system has nodes and that it obeys the Uemura relation. By comparing $λ$ with the normal state electron density we conclude that $m^{\ast}$ of the superconductivity carrier is 70 times larger than the mass of bare electrons.

cond-mat.str-el↗

arXiv2003

Muon Spin Relaxation and Susceptibility Studies of Pure and Doped Spin 1/2 Kagomé-like system (Cu$_x$Zn$_{1-x}$)$_{3}$V$_{2}$O$_7$(OH)$_{2}$ 2H$_2$O

Muon spin relaxation ($μ$SR) and magnetic susceptibility measurements have been performed on the pure and diluted spin 1/2 kagomé system (Cu$_x$Zn$_{1-x}$)$_{3}$V$_{2}$O$_7$(OH)$_{2}$ 2H$_2$O. In the pure $x=1$ system we found a slowing down of Cu spin fluctuations with decreasing temperature towards $T \sim 1$ K, followed by slow and nearly temperature-independent spin fluctuations persisting down to $T$ = 50 mK, indicative of quantum fluctuations. No indication of static spin freezing was detected in either of the pure ($x$=1.0) or diluted samples. The observed magnitude of fluctuating fields indicates that the slow spin fluctuations represent an intrinsic property of kagomé network rather than impurity spins.

cond-mat.str-el↗

arXiv2003

Local Magnetic Susceptibility of the Positive Muon in the Quasi 1D S=1/2 Antiferromagnet dichlorobis (pyridine) copper (II)

We report muon spin rotation measurements of the local magnetic susceptibility around a positive muon in the paramagnetic state of the quasi one-dimensional spin 1/2 antiferromagnet dichlorobis (pyridine) copper (II). Signals from three distinct sites are resolved and have a temperature dependent frequency shift which is significantly different than the magnetic susceptibility. This difference is attributed to a muon induced perturbation of the spin 1/2 chain. The obtained frequency shifts are compared with Transfer Matrix DMRG numerical simulations.

cond-mat.str-el↗

arXiv2002

Universal superconducting and magnetic properties of the (CaLa)(BaLa)CuO system: a MuSR investigation

The (Ca_xLa_(1-x))(Ba_(1.75-x)La_(0.25+x))Cu_(3)O_(y) system is ideal for testing theories of high temperature superconductivity, since nearly the full range of doping is controlled by y, and T_(c)^max is continuously controlled by x, with minimal structural changes. We investigate this system with both transverse and longitudinal field MuSR. This allows us to re-examine the Uemura relation, the nature of the spontaneous magnetic fields below T_(c), and the relation between their appearance temperature T_(g) and T_(c)^max . Our major findings are: (I) the Uemura relation is respected by CLBLCO more adequately than by other cuprates, (II) T_(g) and T_(c) are controlled by the same energy scale, (III) the phase separation between hole poor and hole rich regions is a microscopic one, and (IV) spontaneous magnetic fields appear gradually with no moment size evolution.

cond-mat.supr-con↗

arXiv2000

Ga NMR study of the local susceptibility in SrCr8Ga4O19: pseudogap and paramagnetic defects

We present the first Ga(4f) NMR study of the Cr susceptibility in the archetype of Kagome based frustrated antiferromagnets, SrCr$_{8}$Ga$_{4}$O$_{19}$. Our major finding is that the susceptibility of the frustrated lattice goes through a maximum around 50 K. Our data also supports the existence of paramagnetic ``clusters'' of spins, responsible for the Curie behavior observed in the macroscopic susceptibility at low T. These results set novel features for the constantly debated physics of geometrically frustrated magnets.

cond-mat.str-el↗

arXiv2000

Magnetic dilution in the geometrically frustrated SrCr$_{9p}$Ga$_{12-9p}$O$_{19}$ and the role of local dynamics: a $μ$SR study

We investigate the spin dynamics of SrCr$_{9p}$Ga$_{12-9p}$O$_{19}$ for p below and above the percolation threshold p_c using muon spin relaxation. Our major findings are: (I) At T->0 the relaxation rate is T independent and proportional to p^3, (II) the slowing down of spin fluctuation is activated with an energy U which is also a linear function of p^3 and lim_{p-> 0}U = 8 K; this energy scale could stem only from a single ion anisotropy, and (III) the p dependence of the dynamical properties is identical below and above p_c, indicating that they are controlled by local excitation.

cond-mat.str-el↗

arXiv1998

Magnetic quantum tunneling in the half-integer high spin molecule CrNi6: a muSR study

In high spin molecules metal ions are coupled by ferro or antiferromagnetic short range interactions so that their magnetic moments are parallel or antiparallel to each other at temperatures (T) much smaller than the coupling constant J. This leads to a high spin (S) value in the ground state. In the crystal lattice, the molecules are well separated from each other and the active part of the Hamiltonian at k_B*T << J is H=-D*S_z^2, so that up and down spins states have identical energies, and S_z can escape from one state to the other via either over-the-barrier motion, or tunneling. So far, the escape rate Gamma in HSM was found to have smooth T dependence throughout the cooling process from DS^2 << k_B*T to k_B*T << D*S^2. In this case, the transition from the over-the-barrier motion to tunneling, is referred to as second-order transition. In the present work, we use the muon spin relaxation technique to show evidence for temperature-independent tunneling at k_B*T << D*S^2, in zero applied magnetic field (ZF), in the HSM CrNi6. Moreover, the crossover from the classical to quantum regime is sharp, and appears to be consistent with a first order transition of the escape rate. In a first order transition, over-the-barrier motion is abruptly replaced by tunneling between ground states, as the temperature is lowered down to the barrier height.

cond-mat↗