arXiv subjects

Amit Keren

Publications and source records attributed to Amit Keren.

At least 55 records · Page 3Linked to original sources

arXiv2009

Electromagnetic radiation emanating from the molecular nanomagnet Fe8

Photons emitted by transition between the discrete levels of single molecular magnets might obay the elementary condition for Dicke's super-radiance. We investigate this possibility in the Fe8 molecule where magnetization jumps are known to occur at discreet magnetic field values. We found energy bursts each time the molecule undergoes a magnetization jump, confirming their quantum nature. A series of tests indicated that photons carry out the energy, and that indeed these photons obey the elementary conditions for super-radiance.

cond-mat.str-el↗

arXiv2009

The symmetry of the spin Hamiltonian in herbertsmithite, a spin-1/2 kagomé lattice

We present magnetization measurements on oriented powder of ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ along and perpendicular to the orienting field. We find a dramatic difference in the magnetization between the two directions. It is biggest at low measurement fields $H$ or high temperatures. We show that the difference at high temperatures must emerge from Ising-like exchange anisotropy. This allows us to explain muon spin rotation data at $T\to 0$ in terms of an exotic ferromagnetic ground state.

cond-mat.str-el↗

arXiv2008

Universal doping dependence of the ground state staggered magnetization in cuprates

Using muon spin rotation we determine the zero temperature staggered antiferromagnetic order parameter M0 versus hole doping measured from optimum Dp_m, in the (Ca_{x}La_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_{3}O_{y} system. In this system the maximum Tc and the superexchange J vary by 30% between families (x). M0(x,Dp_m) is found to be x-independent. Using neutron diffraction we also determine the lattice parameters variations for all x and doping. The oxygen buckling angle is found to change with x, implying a change in the holes kinetic energy. We discuss the surprising insensitivity of M0(x,Dp_m) to the kinetic energy variations in the framework of the t-J model.

cond-mat.supr-con↗

arXiv2008

Experimental investigation of the origin of the cross-over temperature in the cuprates

We investigate the cross-over temperature T* as a function of doping in (Ca_{x}La_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_3O_{y}, where the maximum Tc (Tc^max) varies continuously by 30% between families (x) with minimal structural changes. T* is determined by DC-susceptibility measurements. We find that T* scales with the maximum Neel temperature TN^max of each family. This result strongly supports a magnetic origin of T*, and indicates that three dimensional interactions play a role in its magnitude.

cond-mat.str-el↗

arXiv2007

Experimental Estimates of Dephasing Time in Molecular Magnets

Muon spin relaxation measurements in isotropic molecular magnets (MM) with a spin value S ranging from 7/2 to 27/2 are used to determine the magnitude and origin of dephasing time $τ_ϕ$ of molecular magnets. It is found that $τ_ϕ$ ~ 10 nsec with no S or ligand dependence. This indicates a nuclear origin for the stochastic field. Since $τ_ϕ$ is a property of the environment, we argue that it is a number common to similar types of MM. Therefore, $τ_ϕ$ is shorter than the Zener and tunneling times of anisotropic MM such as Fe8 or Mn4 for standard laboratory sweep rates. Our findings call for a stochastic Landau-Zener theory in this particular case.

cond-mat.str-el↗

arXiv2006

Ground state and excitation properties of the quantum kagomé system ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ investigated by local probes

We characterize the ground state and excitation spectrum of the $S=1/2$, nominally pure and perfect kagomé system ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ using the following measurements: magnetization, muon spin rotation frequency shift $K$, transverse relaxation time $T_{2}^{\ast}$, and zero field relaxation, and Cl nuclear spin-lattice relaxation $T_{1}$. We found no sign of singlet formation, no long range order or spin freezing, and no sign of spin-Peierls transition even at temperatures as low as 60 mK. The density of states has $E^{1/4}$ energy dependence with a negligible gap to excitation.

cond-mat.str-el↗

arXiv2006

Lack of static lattice distortion in $Tb_2 Ti_2 O_7$

We investigated the possibility of temperature dependent lattice distortions in the pyrochlore compound Tb$_{2}$Ti$_{2}$O$_{7}$ by measuring the internal magnetic field distribution, using muon spin resonance, and comparing it to the susceptibility. The measurements are done at temperatures as low as 70 mK and external fields up to 6 kG. We find that the evolution of the width of the field distribution can be explained by spin susceptibility only, thus ruling out a temperature dependent hyperfine coupling. We conclude that lattice deformations are absent in Tb$_{2}$Ti$_{2}$O$_{7}$.

cond-mat.str-el↗

arXiv2006

Evidence for two fluids in the ground state of cuprates

We report charge density measurements, using NMR, in the superconducting compound (Ca_{x}La_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_3O_{y}, which has two independent variables x (family) and y (oxygen). For underdoped samples we find the rate at which holes are introduced into the plane upon oxygenation to be family-independent. In contrast, not all carriers contribute to either antiferromagnetic or superconducting order parameters. This result is consistent with a two fluid phenomenology or intrinsic mesoscopic inhomogeneities in the bulk. We also discuss the impact of weak-chemical-disorder on T_{c}.

cond-mat.supr-con↗

arXiv2006

A magnetic analog of the isotope effect in cuprates

We present extensive magnetic measurements of the (Ca_xLa_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_{3}O_{y} (CLBLCO) system with its four different families (x) having a Tc^max(x) variation of 28% and minimal structural changes. For each family we measured the Neel temperature, the anisotropies of the magnetic interactions, and the spin glass temperature. Our results exhibit a universal relation Tc=c*J*n_s for all families, where c~1, J is the in plane Heisenberg exchange, and n_s is the carrier density. This relates cuprate superconductivity to magnetism in the same sense that phonon mediated superconductivity is related to atomic mass.

cond-mat.supr-con↗

arXiv2006

In-plane hole density in Ca(0.1)La(0.9)Ba(1.65)La(0.35)Cu(3)O(y) ; nuclear resonance study over the full doping range

We report in-plane Cu(63) Nuclear Magnetic Resonance measurements for a series of fully enriched Ca(0.1)La(0.9)Ba(1.65)La(0.35)Cu(3)O(y) powder samples, which belong to the YBCO family, but the doping could vary from very underdoped to extremely overdoped. From these measurements we determine the average nuclear quadrupole resonance frequency ν_Q, and its second moment Δ(ν_Q), both set by the in-plane hole density, n, as a function of oxygen level, y. We find that in the overdoped side n is saturated, but Δ(ν_Q) rapidly increases with increasing y. The relevance of these results to the increasing penetration depth in overdoped cuprates is discussed.

cond-mat.supr-con↗

arXiv2005

Charge-Inhomogeneity doping relations in YBCO detected by Angle Dependent Nuclear Quadrupole Resonance

The origin of charge inhomogeneity in YBa_{2}Cu_{3}O_{y} is investigated using a new experimental method designed to determine the nuclear quadrupole resonance (NQR) asymmetry parameter eta for very wide NQR lines at different positions on the line. The method is based on the measurement of the echo intensity as a function of the angle between the radio frequency field H1 and the principal axis of the electric field gradient. Static charge inhomogeneity deduced from eta>0 are found in this compound, but only in conjunction with oxygen deficiency. This limits considerably the possible forms of charge inhomogeneity in bulk YBa_{2}Cu_{3}O_{y}.

cond-mat.supr-con↗

arXiv2005

Demonstrating Multi-bit Magnetic Memory in the Fe8 High Spin Molecule by Muon Spin Rotation

We developed a method to detect the quantum nature of high spin molecules using muon spin rotation, and a three-step field cycle ending always with the same field. We use this method to demonstrate that the Fe8 molecule can remember 6 (possibly 8) different histories (bits). A wide range of fields can be used to write a particular bit, and the information is stored in discrete states. Therefore, Fe8 can be used as a model compound for Multi-bit Magnetic Memory. Our experiment also paves the way for magnetic quantum tunneling detection in films.

cond-mat.other↗

arXiv2005

A quest for frustration driven distortion in Y2Mo2O7

We investigated the nature of the freezing in the geometrically frustrated Heisenberg spin-glass Y2Mo2O7 by measuring the temperature dependence of the static internal magnetic field distribution above the spin-glass temperature, Tg, using the muSR technique. The evolution of the field distribution cannot be explained by changes in the spin susceptibility alone and suggests a lattice deformation. This possibility is addressed by numerical simulations of the Heisenberg Hamiltonian with magneto-elastic coupling at T>0.

cond-mat.str-el↗

arXiv2005

Universal linear relations between susceptibility and Tc in cuprates

We developed an experimental method for measuring the intrinsic susceptibility χof powder of cuprate superconductors in the zero field limit using a DC-magnetometer. The method is tested with lead spheres. Using this method we determine χfor a number of cuprate families as a function of doping. A universal linear (and not proportionality) relation between Tc and χis found. We suggest possible explanations for this phenomenon.

cond-mat.supr-con↗

arXiv2004

Pure nuclear quadrupole resonance determination of the electric field gradient asymmetry for broad lines (and application to YBa2Cu3O7)

We present an angle dependent nuclear quadrupole resonance (ADNQR) method to determine the electric field gradient asymmetry parameter eta in systems where the resonance line is so broad that the radio frequency field can excite only a portion of the nuclear spins. In this situation the recently developed spectroscopic methods are not applicable. ADNQR is useful for single crystals and oriented powders, and, for small eta determines eta^4. Therefore, it can be used to evaluate fluctuations in eta due to inhomogeneities. We demonstrate the application of ADNQR experimentally to oriented superconducting YBa2Cu3O7 powder.

cond-mat.supr-con↗

arXiv2004

Common energy scale for magnetism and superconductivity in cuprates

Many compounds based on CuO_2 planes (cuprates) super-conduct below a critical temperature Tc. Some of them show a second phase where a spontaneous static magnetic field appears below a critical temperature Tg, which is lower then Tc. By comparing Tc and Tg in numerous superconducting families, each with its own maximum Tc, we find that the same energy scale determines both critical temperatures. This clearly indicates that superconductivity and magnetism are intimately related in the cuprates.

cond-mat.supr-con↗

arXiv2002

Common energy scale for magnetism and superconductivity in underdoped cuprates: a MuSR investigation of (CaLa)(BaLa)CuO

We characterize the spontaneous magnetic field, and determine the associated temperature Tg, in the superconducting state of Ca(x)La(1-x)Ba(1.75-x)La(0.25+x)Cu(3)O(y) using zero and longitudinal field MuSR measurements for various values of x and y. Our major findings are: (I) Tg and Tc are controlled by the same energy scale, (II) the phase separation between hole poor and hole rich regions is a microscopic one, and (III) spontaneous magnetic fields appear gradually with no moment size evolution.

cond-mat.supr-con↗

arXiv2002

Dynamics at T -> 0 in the half-integer isotropic high spin molecules

We investigate the dynamical spin-spin auto-correlation function of the isotropic high spin molecules CrCu$_{6}$ (S=9/2), CrNi$_{6}$ (S=15/2) and CrMn$_{6}$ (S=27/2), using magnetization, $μ$SR and NMR measurements. We find that the field autocorrelation time $τ$ of the molecule's spin at zero and low fields is nearly temperature independent as $T\to 50$ mK. The high temperatures $τ$ is very different between the molecules. Surprisingly, it is identical ($\sim$ 10 nsec) at low temperature. This suggests that $τ$ is governed by hyperfine interactions.

cond-mat.mes-hall↗