arXiv subjects

M. P. Sarachik

Publications and source records attributed to M. P. Sarachik.

At least 37 records · Page 2Linked to original sources

arXiv2007

Ignition and Propagation of Magnetic Avalanches in Mn$_{12}$-Acetate: the effect of quantum tunneling

Using a wire heater to ignite magnetic avalanches in fixed magnetic field applied along the easy axis of single crystals of the molecular magnet Mn$_{12}$-acetate, we report fast local measurements of the temperature and time-resolved measurements of the local magnetization as a function of magnetic field. In addition to confirming maxima in the velocity of propagation, we find that avalanches trigger at a threshold temperature which exhibits pronounced minima at resonant magnetic fields, demonstrating that thermally assisted quantum tunneling plays an important role in the ignition as well as the propagation of magnetic avalanches in molecular magnets.

cond-mat.mes-hall↗

arXiv2007

Spatial Determination of Magnetic Avalanche Ignition Points

Using time-resolved measurements of local magnetization in the molecular magnet Mn12-ac, we report studies of the propagation of magnetic avalanches (fast magnetization reversals) that originate from points inside the crystals rather than at the edges. The curved nature of the fronts produced by avalanches is reflected in the time-of-arrival at micro-Hall sensors placed at the surface of the sample. Assuming that the avalanche interface is a spherical bubble that grows with a radius proportional to time, we are able to locate the approximate ignition point of each avalanche in a two-dimensional cross-section of the crystal. For the samples used in these studies, avalanches in a given crystal are found to originate in a small region with a radius of roughly 150 microns.

cond-mat.mtrl-sci↗

arXiv2005

Propagation of Avalanches in Mn$_{12}$-acetate: Magnetic Deflagration

Local time-resolved measurements of fast reversal of the magnetization of single crystals of Mn12-acetate indicate that the magnetization avalanche spreads as a narrow interface that propagates through the crystal at a constant velocity that is roughly two orders of magnitude smaller than the speed of sound. We argue that this phenomenon is closely analogous to the propagation of a flame front (deflagration) through a flammable chemical substance.

cond-mat.other↗

arXiv2005

Local Measurements of Magnetization in Mn12 Crystals

The spatial profile of the magnetization of Mn12 crystals in a swept magnetic field applied along the easy axis is determined from measurements of the local magnetic induction along the sample surface using an array of Hall sensors. We find that the magnetization is not uniform inside the sample, but rather shows some spatial oscillations which become more prominent around the resonance field values. Moreover, it appears that different regions of the sample are at resonance at different values of the applied field and that the sweep rate of the internal magnetic induction is spatially non-uniform. We present a model which describes the evolution of the non-uniformities as a function of the applied field. Finally we show that the degree of non-uniformity can be manipulated by sweeping the magnetic field back and forth through part of the resonance.

cond-mat.mes-hall↗

arXiv2004

Conductivity of Silicon Inversion Layers: comparison with and without in-plane magnetic field

A detailed comparison is presented of the temperature dependence of the conductivity of dilute, strongly interacting electrons in two-dimensional silicon inversion layers in the metallic regime in the presence and in the absence of a magnetic field. We show explicitly and quantitatively that a magnetic field applied parallel to the plane of the electrons reduces the slope of the conductivity versus temperature curves to near zero over a broad range of electron densities extending from $n_c$ to deep in the metallic regime where the high field conductivity is on the order of $10 e^2/h$. The strong suppression (or "quenching") of the metallic behavior by a magnetic field sets an important constraint on theory.

cond-mat.str-el↗

arXiv2003

Magnetoconductivity of Insulating Silicon Inversion Layers

The normalized in-plane magnetoconductivity of the dilute strongly interacting system of electrons in silicon MOSFET's scales with $B/T$ for low densities in the insulating phase. Pronounced deviations occur at higher metallic-like densities, where a new energy scale $k_B Δ$ emerges which is not associated with either magnetic field or thermal effects. $B/T$ scaling of the magnetoconductivity breaks down at the density $n_0$ where the energy scale $k_B Δ$ vanishes, near or at the critical density $n_c$ for the apparent metal-insulator transition. The different behavior of the magnetoconductivity at low and high densities suggests the existence of two distinct phases.

cond-mat.str-el↗

arXiv2003

Metal-insulator transition in two-dimensional electron systems

The interplay between strong Coulomb interactions and randomness has been a long-standing problem in condensed matter physics. According to the scaling theory of localization, in two-dimensional systems of noninteracting or weakly interacting electrons, the ever-present randomness causes the resistance to rise as the temperature is decreased, leading to an insulating ground state. However, new evidence has emerged within the past decade indicating a transition from insulating to metallic phase in two-dimensional systems of strongly interacting electrons. We review earlier experiments that demonstrate the unexpected presence of a metallic phase in two dimensions, and present an overview of recent experiments with emphasis on the anomalous magnetic properties that have been observed in the vicinity of the transition.

cond-mat.str-el↗

arXiv2002

More Evidence for a Distribution of Tunnel Splittings in Mn$_{12}$-acetate

In magnetic fields applied parallel to the anisotropy axis, the magnetization of Mn$_{12}$ has been measured in response to a field that is swept back and forth across the resonances corresponding to steps $N=4,5,...9$. The fraction of molecules remaining in the metastable well after each sweep through the resonance is inconsistent with expectations for an ensemble of identical molecules. The data are consistent instead with the presence of a broad distribution of tunnel splittings. A very good fit is obtained for a Gaussian distribution of the second-order anisotropy tunneling parameter $X_E=-\ln(\mid E\mid/2D)$. We show that dipolar shuffling is a negligible effect which cannot explain our data.

cond-mat.mes-hall↗

arXiv2002

Does $m^g^$ diverge at a finite electron density in silicon inversion layers?

For the two-dimensional electron system in silicon MOSFET's, the scaled magnetoconductivity has been shown to exhibit critical behavior at finite density $n_0$. Analysis of these magnetotransport experiments yields a product $g^*m^*$ that diverges at this density (here $g^*$ is the interaction-enhanced Landé $g$-factor and $m^*$ is the effective mass). This claim has been disputed based on direct determinations of the $g^*m^*$ obtained from Shubnikov-de Haas measurements. We briefly review these experiments, and possible sources of the discrepancies.

cond-mat.str-el↗

arXiv2002

In-plane Magnetoconductivity of Si-MOSFET's: A Quantitative Comparison between Theory and Experiment

For densities above $n=1.6 \times 10^{11}$ cm$^{-2}$ in the strongly interacting system of electrons in two-dimensional silicon inversion layers, excellent agreement between experiment and the theory of Zala, Narozhny and Aleiner is obtained for the response of the conductivity to a magnetic field applied parallel to the plane of the electrons. However, the Fermi liquid parameter $F_0^σ(n)$ and the valley splitting $Δ_V(n)$ obtained from fits to the magnetoconductivity, although providing qualitatively correct behavior (including sign), do not yield quantitative agreement with the temperature dependence of the conductivity in zero magnetic field. Our results suggest the existence of additional scattering processes not included in the theory in its present form.

cond-mat.str-el↗

arXiv2002

Spin polarization of strongly interacting 2D electrons: the role of disorder

In high-mobility silicon MOSFET's, the $g^*m^*$ inferred indirectly from magnetoconductance and magnetoresistance measurements with the assumption that $g^*μ_BH_s=2E_F$ are in surprisingly good agreement with $g^*m^*$ obtained by direct measurement of Shubnikov-de Haas oscillations. The enhanced susceptibility $χ^* \propto (g^*m^*)$ exhibits critical behavior of the form $χ^* \propto (n - n_0)^{-α}$. We examine the significance of the field scale $H_s$ derived from transport measurements, and show that this field signals the onset of full spin polarization only in the absence of disorder. Our results suggest that disorder becomes increasingly important as the electron density is reduced toward the transition.

cond-mat.str-el↗

arXiv2001

Microwave Rectification at the Boundary between Two-Dimensional Electron Systems

Rectification of microwave radiation (20-40 GHz) by a line boundary between two two-dimensional metals on a silicon surface was observed and investigated at different temperatures, in-plane magnetic fields and microwave powers. The rectified voltage $V_{dc}$ is generated whenever the electron densities $n_{1,2}$ of the two metals are different, changing polarity at $n_1 \approx n_2$. Very strong nonlinear response is found when one of the two 2D metals is close to the electron density corresponding to the reported magnetic instability in this system.

cond-mat.mes-hall↗

arXiv2001

One-Parameter Scaling of the Conductivity of Si:B: A Temperature-Independent Variable-Range Hopping Prefactor

For insulating Si:B with dopant concentrations from 0.75 n_c to the critical concentration n_c, the conductivity ranging over five orders of magnitude collapses using a single scaling parameter T* onto a universal curve of the form $σ(T) =σ_0 f (T^*/T)$ with a temperature-independent prefactor of the order of Mott's minimum metallic conductivity. The function $f (T^*/T) = e^{-(T^*/T)^β}$ with β= 1/2 when T^*/T>10, corresponding to Efros-Shklovskii variable-range hopping. For T^*/T < 8 the exponent β= 1/3, a value expected for Mott variable-range hopping in two rather than three dimensions. The temperature-independent prefactor implies hopping that is not mediated by phonons.

cond-mat↗

arXiv2001

On Ground State Tunneling in Mn$_{12}$-Acetate

We report Hall sensor measurements of the magnetic relaxation of Mn$_{12}$-acetate as a function of magnetic field applied along the easy axis of magnetization for a series of closely-spaced temperatures between 0.24 K and 1.9 K. We map out a region in the $H-T$ plane where ground state tunneling is observed, a region where tunneling proceeds via thermally-assisted tunneling, and a crossover region where both participate in the relaxation. We observe the occasional absence of ground-state tunneling under conditions where one would expect it to be present, and suggest a resolution to the enigma.

cond-mat↗

arXiv2001

A Distribution of Tunnel Splittings in Mn$_{12}$-Acetate

In magnetic fields applied parallel to the anisotropy axis, the relaxation of the magnetization of Mn$_{12}$ measured for different sweep rates is shown to collapse onto a single scaled curve. The form of the scaling implies that the dominant symmetry-breaking process that gives rise to tunneling is a locally varying second-order anisotropy, forbidden by tetragonal symmetry in the perfect crystal, which gives rise to a broad distribution of tunnel splittings in a real crystal of Mn$_{12}$-acetate. Different forms applied to even and odd-numbered steps provide a distinction between even step resonances (associated with crystal anisotropy) and odd resonances (which require a transverse component of magnetic field).

cond-mat↗

arXiv2001

Mobility-Dependence of the Critical Density in Two-Dimensional Systems: An Empirical Relation

For five different electron and hole systems in two dimensions (Si MOSFET's, p-GaAs, p-SiGe, n-GaAs and n-AlAs), the critical density, $n_c$ that marks the onset of strong localization is shown to be a single power-law function of the scattering rate $1/τ$ deduced from the maximum mobility. The resulting curve defines the boundary separating a localized phase from a phase that exhibits metallic behavior. The critical density $n_c \to 0$ in the limit of infinite mobility.

cond-mat.str-el↗

arXiv2001

Scaling of the magnetoconductivity of silicon MOSFET's: evidence for a quantum phase transition in two dimensions

For a broad range of electron densities $n$ and temperatures $T$, the in-plane magnetoconductivity of the two-dimensional system of electrons in silicon MOSFET's can be scaled onto a universal curve with a single parameter $H_σ(n,T)$, where $H_σ$ obeys the empirical relation $H_σ=A (n) [Δ(n)^2 +T^2]^{1/2}$. The characteristic energy $k_B Δ$ associated with the magnetic field dependence of the conductivity decreases with decreasing density, and extrapolates to 0 at a critical density $n_0$, signaling the approach to a zero-temperature quantum phase transition. We show that $H_σ=AT$ for densities near $n_0$.

cond-mat.str-el↗

arXiv2001

Spin Polarization of Two-Dimensional Electrons Determined from Shubnikov-de Haas Oscillations as a Function of Angle

Recent experiments in the two dimensional electron systems in silicon MOSFETs have shown that the in-plane magnetic field $H_{sat}$ required to saturate the conductivity to its high-field value and the magnetic field $H_s$ needed to completely align the spins of the electrons are comparable. By small-angle Shubnikov-de Haas oscillation measurements that allow separate determinations of the spin-up and spin-down subband populations, we show that $H_{sat}=H_s$ to an accuracy of 5% for electron densities $n_s > 3 \times 10^{11}$ cm$^{-2}$.

cond-mat.str-el↗