arXiv subjects

Amit Keren

Publications and source records attributed to Amit Keren.

At least 37 records · Page 2Linked to original sources

arXiv2014

Energy emanating from the molecular nanomagnet Fe$_{8}$ revisited

In the molecular nanomagnet Fe$_{8}$ tunneling can occur from a metastable state to an excited state followed by a transition to the ground state. This transition is accompanied by an energy release of $115.6$GHz. We constructed an experimental setup to measure whether this energy is released in the form of thermal or electromagnetic energy. Contrary to a previous publication we find no evidence for release of electromagnetic radiation. Our results for transitions between the first and second excited states to the ground state are consistent with a release of only thermal energy. This energy release extends for a longer time for the second excited state than for the first excited state.

cond-mat.mes-hall↗

arXiv2014

Quantum avalanche in the Fe$_{8}$ Molecular-Magnet

We report spatially resolved, time-dependent, magnetization reversal measurements of an Fe$_{8}$ single molecular magnet using a microscopic Hall bar array. We found that under some conditions the molecules reverse their spin direction at a resonance field in the form of an avalanche. The avalanche front velocity is of the order of $1$m/sec and is sensitive to field gradients and sweep rates. We also measured the propagation velocity of a heat pulse and found that it is much slower than the avalanche velocity. We therefore conclude that in Fe$_{8}$, the avalanche front propagates without thermal assistance.

cond-mat.mes-hall↗

arXiv2014

Spin-charge interplay in antiferromagnetic La$_{2-x}$Sr$_{x}$CuO$_{4}$ studied by the muons, neutrons, and ARPES techniques

Exploring whether a spin density wave (SDW) is responsible for the charge excitations gap in the high-temperature superconducting cuprates is difficult, since the region of the phase diagram where the magnetic properties are clearly exposed is different from the region where the band dispersion is visible. On the one hand, long range magnetic order disappears as doping approaches 2% from below, hindering our ability to perform elastic neutron scattering (ENS). On the other hand, cuprates become insulating at low temperature when the doping approaches 2% from above, thus restricting angle-resolved photoemission spectroscopy (ARPES). In fact, ARPES data for samples with doping lower than 3% are rare and missing the quasiparticle peaks in the energy distribution curves (EDCs). The main problem is the high resistivity of extremely underdoped samples, which is detrimental to ARPES due to charging effects. Nevertheless, the resistivity of La$_{2-x}$Sr$_{x}$CuO$_{4}$ as a function of temperature, at 2% doping, has a broad minimum around 100K. This minimum opens a window for both experiments. By preparing a series of LSCO single crystals with $\sim $0.2-0.3% doping steps around 2%, we managed to find one to which both techniques apply. This allows us to explore the cross talk between the magnetic and electronic properties of the material.

cond-mat.supr-con↗

arXiv2014

Linking Dynamic and Thermodynamic Properties of Cuprates; an ARPES study of (CaLa)(BaLa)$_{2}$Cu$_{3}$O$_{y}$

We report angle-resolved photoemission spectroscopy (ARPES) on two families of high temperature superconductors (Ca$_{x}$La$_{1-x}$)(Ba$_{1.75-x}$ La $% _{0.25+x}$)Cu$_{3}$O$_{y}$ with $x=0.1$ ($T_{c}^{max}=56$ K) and $x=0.4$ ($% T_{c}^{max}=82$ K). The Fermi surface (FS) is found to be independent of $x$ or $y$, and its size indicates extreme sample-surface overdoping. This universal FS allowes the comparison of dynamical properties between superconductors of similar structure and identical doping, but different $% T_{c}^{max}$. We find that the high-energy ($\left\vert E\right\vert >50$% meV) nodal velocity in the $x=0.4$ family is higher than in the $x=0.1$ family. The implied correlation between $T_{c}^{max}$ and the hopping rate $% t $ supports the notion of kinetic energy driven superconductivity in the cuprates. We also find that the antinodal gap is higher for the $x=0.4$ family.

cond-mat.supr-con↗

arXiv2013

Dynamic Spin Fluctuations at $T\rightarrow 0$ in a Spin-1/2 Ferromagnetic Kagome Lattice

We report magnetization, electron spin resonance (ESR), and muon spin relaxation ($μ$SR) measurements on single crystals of the $S=1/2$ (Cu$% ^{+2}$) kagomé compound Cu(1,3-benzendicarboxylate). The $μ$SR is carried to temperatures as low as 45 mK. The spin Hamiltonian parameters are determined from the analysis of the magnetization and ESR data. We find that this compound has anisotropic ferromagnetic interactions. Nevertheless, no spin freezing is observed even at temperatures two orders of magnitude lower than the coupling constants. In light of this finding, the relation between persistent spin dynamics and spin liquid are reexamined.

cond-mat.str-el↗

arXiv2013

Strong- versus Weak-Coupling Paradigms for Cuprate Superconductivity

Absolute resistivity measurements as a function of temperature from optimally doped YBa_2Cu_3O_(7), La_(2-x)Sr_xCuO_4, Bi_2Sr_2CaCu_2O_(8-x), and (Ca_0.1La_0.9)(Ba_1.65La_0.35)Cu_3O_y thin films are reported. Special attention is given to the measurement geometrical factors and the resistivity slope between Tc and T^{*}. The results are compared with a strong coupling theory for the resistivity derivative near T_c, which is based on hard core bosons (HCB), and with several weak coupling theories, which are BCS based. Surprisingly, our results agree with both paradigms. The implications of these findings and the missing calculations needed to distinguish between the two paradigms are discussed.

cond-mat.supr-con↗

arXiv2012

Parallel and Perpendicular Susceptibility Above $T_{c}$ in La$_{2-x}$Sr$_{x}$CuO$_{4}$ Single Crystals

We report direction-dependent susceptibility and resistivity measurements on La$_{2-x}$Sr$_{x}$CuO$_{4}$ single crystals. These crystals have rectangular needle-like shapes with the crystallographic "c" direction parallel or perpendicular to the needle axis,which, in turn, is in the applied field direction. At optimal doping we find finite diamagnetic susceptibility above $T_{c}$, namely fluctuating superconductivity (FSC), only when the field is perpendicular to the planes. In underdoped samples we could find FSC in both field directions. We provide a phase diagram showing the FSC region, although it is sample dependent in the underdoped cases. The variations in the susceptibility data suggest a different origin for the FSC between underdoping (below 10%) and optimal doping. Finally, our data indicates that the spontaneous vortex diffusion constant above $T_c$ is anomalously high.

cond-mat.supr-con↗

arXiv2012

New Perspectives for Cuprate Research: CLBLCO Single Crystals

We report the successful growth of a large (Ca$_{x}$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x}$)Cu$_{3}$O$_{y}$ (CLBLCO) single crystal. In this material, $x$ controls the maximum of Tc ($T_{c}^{\max}$), with minimal structural changes. Therefore, it allows a search for correlations between material properties and $T_{c}^{\max}$. We demonstrate that the crystals are good enough for neutron scattering and cleave well enough for Raman scattering. These results open new possibilities for cuprate research.

cond-mat.supr-con↗

arXiv2011

A Precise Measurement of the Oxygen Isotope Effect on the Néel temperature in Cuprates

A limiting factor in the ability to interpret isotope effect measurements in cuprates is the absence of sufficiently accurate data for the whole phase diagram; there is precise data for Tc, but not for the antiferromagnetic transition temperature T_N. Extreme sensitivity of T_N to small changes in the amount of oxygen in the sample is the major problem. This problem is solved here by using the novel compound (Ca 0.1 La 0.9)(Ba 1.65 La 0.35)Cu 3 O y for which there is a region where T_N is independent of oxygen doping. Meticulous measurements of T_N for samples with oxygen 16 and oxygen 18 find the absence of an oxygen isotope effect on T_N with unprecedented accuracy. A possible interpretation of our finding and existing data is that isotope substitution affects the normal state charge carrier density.

cond-mat.supr-con↗

arXiv2010

The Herbertsmithite Hamiltonian: $μ$SR measurements on single crystals

We present transverse field muon spin rotation/relaxation measurements on single crystals of the spin-1/2 kagome antiferromagnet Herbertsmithite. We find that the spins are more easily polarized when the field is perpendicular to the kagome plane. We demonstrate that the difference in magnetization between the different directions cannot be accounted for by Dzyaloshinksii-Moriya type interactions alone, and that anisotropic axial interaction is present.

cond-mat.str-el↗

arXiv2010

Critical-doping universality for cuprate superconductors: Oxygen nuclear-magnetic-resonance investigation of Ca(x)La(1-x)Ba(1.75-x)La(0.25+x)Cu(3)O(y)

The critical doping levels in cuprates, where the ground state changes its nature (from an antiferromagnet to a spin glass to superconductor to metal), are not universal. We investigate the origin of these critical doping variations by measuring the in-plane oxygen $p_σ$ hole density in the CuO$_{2}$ layers as a function of the oxygen density $y$ in (Ca$_{x}$ La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x}$)Cu$_{3}$O$_{y}$. This is done using the oxygen 17 nuclear quadrupole resonance parameter $ν_{Q}$. We compare compounds with $x=0.1$ and 0.4 which have significant critical $y$ variations and find that these variations can be explained by a change in the efficiency of hole injection into the $p_σ$ orbital. This allows us to generate a unified phase diagram for the CLBLCO system across the entire doping range, with no adjustable parameters.

cond-mat.supr-con↗

arXiv2010

Origin of magnetic freezing in the pyrochlore Y2Mo2O7

We investigated the nature of the spin glass-like phase transition in the geometrically frustrated pyrochlore lattices Y$_{2}$Mo$_{2}$O$_{7}$ using the local probes nuclear and muon magnetic resonances, and the field-dependent long range probes x-ray and neutron scattering. The long range probes indicated that Y$_{2}$Mo$_{2}$O$_{7}$ does not undergo any global symmetry changes, even in a field of $6$~T. In contrast, the local signal indicates a lattice distortion close to the critical temperature. The nuclei show at least two inequivalent Y sites, and the muons show sub-linear line broadening as a function of moment size, over a wide temperature range. The conclusion from all the measurements is that even in high field, the distortion of Y$_{2}$Mo$_{2}$O$%_{7}$ takes place within the unit-cell, while its global cubic symmetry is preserved. Moreover, the muon result clearly indicates the presence of magneto-elastic coupling.

cond-mat.str-el↗

arXiv2009

Nutation versus angular dependent NQR spectroscopy and the impact of underdoping on charge inhomogeneities in YBa$_2$Cu$_3$O$_y$

We describe two different nuclear quadrupole resonance (NQR) based techniques, designed to measure the local asymmetry of the internal electric field gradient, and the tilt angle of the main NQR principal axis z from the crystallographic axis c. These techniques use the dependence of the NQR signal on the duration of the radio frequency (rf) pulse and on the direction of the rf field H1 with respect to the crystal axis. The techniques are applied to oriented powder of YBa$_{2}$Cu$%_{3}$O$_{y}$ fully enriched with 63Cu. Measurements were performed at different frequencies, corresponding to different in-plane copper sites with respect to the dopant. Combining the results from both techniques, we conclude that oxygen deficiency in the chain layer lead to a rotation of the NQR main principal axis at the nearby Cu on the CuO2 planes by 20+-degrees. This occurs with no change to the asymmetry. The axis rotation associated with oxygen deficiency means that there must be electric field inhomogeneities in the CuO2 planes only in the vicinity of the missing oxygen.

cond-mat.supr-con↗

arXiv2009

Muon-Spin Spectroscopy of the organometallic spin 1/2 kagome-lattice compound Cu(1,3-benzenedicarboxylate)

Using muon spin resonance we examine the organometallic hybrid compound Cu(1,3-benzenedicarboxylate) [Cu(1,3-bdc)], which has structurally perfect spin 1/2 copper kagome planes separated by pure organic linkers. This compound has antiferromagnetic interactions with Curie-Weiss temperature of -33 K. We found slowing down of spin fluctuations starting at T=1.8 K, and that the state at T->0 is quasi-static with no long-range order and extremely slow spin fluctuations at a rate of 3.6 1/usec. This indicates that Cu(1,3-bdc) behaves as expected from a kagome magnet and could serve as a model kagome compound.

cond-mat.str-el↗

arXiv2009

Evidence of magnetic mechanism for cuprate superconductivity

A proper understanding of the mechanism for cuprate superconductivity can emerge only by comparing materials in which physical parameters vary one at a time. Here we present a variety of bulk, resonance, and scattering measurements on the (Ca_xLa_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_3O_y high temperature superconductors, in which this can be done. We determine the superconducting, Neel, glass, and pseudopage critical temperatures. In addition, we clarify which physical parameter varies, and, equally important, which does not, with each chemical modification. This allows us to demonstrate that a single energy scale, set by the superexchange interaction J, controls all the critical temperatures of the system. J, in-turn, is determined by the in plane Cu-O-Cu buckling angle.

cond-mat.supr-con↗

arXiv2009

The interaction between the magnetic and superconducting order parameters in a La1.94Sr0.06CuO4 wire

We investigate the coupling between the magnetic and superconducting order parameters in an 8 m long meander line ("wire") made of a La1.94Sr0.06CuO4 film with a cross section of 0.5 x 100 um^2. The magnetic order parameter is determined using the Low-Energy muon spin relaxation technique. The superconducting order parameter is characterized by transport measurements and modified by high current density. We find that, when the superconducting order parameter is suppressed by the current, the magnetic transition temperature, Tm, increases. The extracted sign and magnitude of the Ginzburg-Landau coupling constant indicate that the two orders are repulsive, and that our system is located close to the border between first and second order phase transition.

cond-mat.supr-con↗

arXiv2009

Magnetic quantum tunnelling in Fe8 with excited nuclei

We investigate the effect of dynamic nuclear spin fluctuation on quantum tunneling of the magnetization (QTM) in the molecular magnet Fe8 by increasing the nuclei temperature using radio frequency (RF) pulses before the hysteresis loop measurements. The RF pulses do not change the electrons spin temperature. Independently we show that the nuclear spin-spin relaxation time T2 has strong temperature dependence. Nevertheless, we found no effect of the nuclear spin temperature on the tunneling probability. This suggests that in our experimental conditions only the hyperfine field strength is relevant for QTM. We demonstrate theoretically how this can occur.

cond-mat.str-el↗

arXiv2009

NMR and muSR in Highly Frustrated Magnets

We present a brief overview on some of the most significant achievements obtained by means of NMR and muSR techniques in highly frustrated magnets. First the basic quantities measured by the two techniques will be presented and their connection to the microscopic static and dynamical spin susceptibility recalled. Then the main findings will be outlined, starting from the most simple frustrated units, the molecular nanomagnets, to artificially built frustrated systems as $^{3}$He on graphite, to magnets with a macroscopically degenerate ground-state as the ones on a pyrochlore or kagomé lattices.

cond-mat.str-el↗