Search

This autobiographical narrative offers a brief account of my journey and adventures in condensed matter physics (a.k.a. solid state physics) and some of the personal events that shaped my life and my career: my early years in Europe, my family's escape from the Nazis, growing up in Cuba, the difficult road into a field that was essentially closed to women, a personal disaster that knocked the wind out of my sails for more than a decade, and my return to a successful career in physics. In closing, I argue that, although we have made remarkable progress, we know but a thimble-full in our inexhaustible search for an understanding of the laws of nature.

The enhancement of toroic motifs through coupling toroidal moments within molecular nanomagnets is a new, interesting and relevant approach for both fundamental research and potential quantum computation applications. We investigate a Dy8 molecular cluster and discover it has a antiferrotoroic ground state with slow magnetic relaxation. The experimental characterization of the magnetic anisotropy axes of each magnetic center and their exchange interactions represents a considerable challenge due to the non-magnetic nature of the toroidal motif. To overcome this and obtain access to the low energy states of Dy8 we establish a multi-orientation single-crystal micro Hall sensor magnetometry approach. Using an effective Hamiltonian model we then unpick the microscopic spin structure of Dy8, leading to a canted antiferrotoroidic tetramer molecular ground state. These findings are supported with electrostatic calculations that independently confirm the experimentally determined magnetic anisotropy axes for each DyIII ion within the molecule.

The enhancement of toroic motifs through coupling toroidal moments within molecular nanomagnets is a new, interesting and relevant approach for both fundamental research and potential quantum computation applications. We investigate a Dy8 molecular cluster and discover it has a antiferrotoroic ground state with slow magnetic relaxation. The experimental characterization of the magnetic anisotropy axes of each magnetic center and their exchange interactions represents a considerable challenge due to the non-magnetic nature of the toroidal motif. To overcome this and obtain access to the low energy states of Dy8 we establish a multi-orientation single-crystal micro Hall sensor magnetometry approach. Using an effective Hamiltonian model we then unpick the microscopic spin structure of Dy8, leading to a canted antiferrotoroidic tetramer molecular ground state. These findings are supported with electrostatic calculations that independently confirm the experimentally determined magnetic anisotropy axes for each DyIII ion within the molecule. ARO W911NF-13-1-1025 NSF-DMR-1309008 NSF-DMR-1309202 MAT2017-89528 CTQ2017-89993 MDM-2015-0538 ERC-CoG-647301 DECRESIM COST-MOLSPIN CA15128 Prometeo Program of excellence The enhancement of toroic motifs through coupling toroidal moments within molecular nanomagnets is a new, interesting and relevant approach for both fundamental research and potential quantum computation applications. We investigate a Dy8 molecular cluster and discover it has a antiferrotoroic ground state with slow magnetic relaxation. The experimental characterization of the magnetic anisotropy axes of each magnetic center and their exchange interactions represents a considerable challenge due to the non-magnetic nature of the toroidal motif. To overcome this and obtain access to the low energy states of Dy8 we establish a multi-orientation single-crystal micro Hall sensor magnetometry approach. Using an effective Hamiltonian model we then unpick the microscopic spin structure of Dy8, leading to a canted antiferrotoroidic tetramer molecular ground state. These findings are supported with electrostatic calculations that independently confirm the experimentally determined magnetic anisotropy axes for each DyIII ion within the molecule.

Ignited by the discovery of the metal-insulator transition, the behaviour of low-disorder two-dimensional (2D) electron systems is currently the focus of a great deal of attention. In the strongly interacting limit, electrons are expected to crystallize into a quantum Wigner crystal, but no definitive evidence for this effect has been obtained despite much experimental effort over the years. Here, studying the insulating state of a 2D electron system in silicon, we have found two-threshold voltage-current characteristics with a dramatic increase in noise between the two threshold voltages. This behaviour cannot be described within existing traditional models. On the other hand, it is strikingly similar to that observed for the collective depinning of the vortex lattice in type-II superconductors. Adapting the model used for vortexes to the case of an electron solid yields good agreement with our experimental results, favouring the quantum electron solid as the origin of the low-density state.

The in-plane magnetoconductance of the strongly interacting two-dimensional electron system in a silicon MOSFET (metal-oxide-semiconductor-field-effect transistor) exhibits an unmistakeable kink at a well-defined electron density, $n_k$. The kink at $n_k$ is near, but not at the critical density $n_c$ determined from resistivity measurements, and the density at which $n_k$ occurs varies with temperature. These features are inconsistent with expectations for a quantum phase transition. We suggest instead that this is a percolation transition and present a detailed model based on the formation of a mixed insulating and metallic phase within which a metal-insulator transition takes place by percolation., Comment: 8 pages, 7 figures

We review our studies of the effect of transverse fields on the susceptibility and magnetization of single crystals of the prototype single molecule magnet (SMM), Mn$_{12}$-acetate, and of a new high-symmetry variant, Mn$_{12}$-acetate-MeOH. SMM single crystals can exhibit long range ferromagnetic order associated with intermolecular dipole interactions. Transverse fields increase quantum spin fluctuation and quantum tunneling of the magnetization suppressing long range order. However, we have found that suppression of the Curie temperature by a transverse field in Mn$_{12}$-acetate is far more rapid than predicted by the Transverse-Field Ising Ferromagnetic Model (TFIFM). It appears that solvent disorder in Mn$_{12}$-acetate that results in an intrinsic distribution of small discrete tilts of the molecular magnetic easy axis from the global easy axis of the crystal ($\approx \pm 1^\circ$) gives rise to a distribution of random-fields that further suppresses long-range order. Semiquantitative agreement with the predictions of a Random-Field Ising Ferromagnet Model is found. Subsequent susceptibility studies we have conducted of the high symmetry Mn$_{12}$ variant, Mn$_{12}$-acetate-MeOH, with the same spin structure and similar lattice constants but without the same solvent disorder as Mn$_{12}$-acetate, agrees with the TFIFM. An important implication of our studies is that long-range order in these two chemically very similar SMMs are described by distinct physical models., arXiv admin note: text overlap with arXiv:1202.4963

The resistivities of the dilute, strongly interacting two-dimensional electron systems in the insulating phase of a silicon MOSFET are the same for unpolarized electrons in the absence of magnetic field and for electrons that are fully spin polarized by the presence of an in-plane magnetic field. In both cases the resistivity obeys Efros-Shklovskii variable range hopping $\ensuremath{\rho}(T)={\ensuremath{\rho}}_{0}\text{exp}[{({T}_{ES}/T)}^{1/2}]$, with ${T}_{ES}$ and $1/{\ensuremath{\rho}}_{0}$ mapping onto each other if one applies a shift of the critical density ${n}_{\text{c}}$ reported earlier. With and without magnetic field, the parameters ${T}_{ES}$ and $1/{\ensuremath{\rho}}_{0}={\ensuremath{\sigma}}_{0}$ exhibit scaling consistent with critical behavior approaching a metal-insulator transition.

The discovery of a metallic state and a metal-insulator transition (MIT) in two-dimensional (2D) electron systems challenges one of the most influential paradigms of modern mesoscopic physics, namely, that "there is no true metallic behavior in two dimensions". However, this conclusion was drawn for systems of noninteracting or weakly interacting carriers, while in all 2D systems exhibiting the metal-insulator transition, the interaction energy greatly exceeds all other energy scales. We review the main experimental findings and show that, although significant progress has been achieved in our understanding of the MIT in 2D, many open questions remain., Comment: Review article for "50 Years of Anderson Localization". A paragraph briefly describing a theory, based on the Pomeranchuk effect, has been added.

STUDIES OF STRONGLY-INTERACTING 2D ELECTRON SYSTEMS – There is a great deal of current interest in the properties of systems in which the interaction between electrons (their potential energy) is large compared to their kinetic energy. We have investigated an apparent, unexpected metal-insulator transition inferred from the behavior of the temperature-dependence of the resistivity; moreover, detailed analysis of the behavior of the magnetoresistance suggests that the electrons’ effective mass diverges, supporting this scenario. Whether this is a true phase transition or crossover behavior has been strenuously debated over the past 20 years. Our measurements have now shown that the thermoelectric power of these 2D materials diverges at a finite density, providing clear evidence that this is, in fact, a phase transition to a new low-density phase which may be a precursor or a direct transition to the long sought-after electronic crystal predicted by Eugene Wigner in 1934.

Single molecule magnets display fascinating quantum mechanical behavior, and may have important technological applications for information storage and quantum computation. A brief review is given of the physical properties of Mn12-acetate, one of the two prototypical molecular nanomagnets that have been most intensively investigated. Descriptions and discussions are given of the Mn12 magnetic cluster and the fundamental process of quantum tunneling of a nanoscopic spin magnetization; the distinction between thermally-assisted tunneling and pure quantum tunneling, and a study of the crossover between the two regimes; and a review of earlier investigations that suggest that the tunneling in this system is due to locally varying second-order crystal anisotropy which gives rise to a distribution of tunnel splittings. In the second part of the paper, we report results obtained by a new experimental method that confirm our earlier conclusion that the tunnel splittings in Mn12 are distributed rather than single-valued, as had been generally assumed.

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 Lande 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.

For insulating Si:B with dopant concentrations from 0.75nc to the critical concentration nc, the conductivity ranging over five orders of magnitude collapses using a single scaling parameter T* onto a universal curve of the form ?(T) = ?0f(T*/T) with a prefactor ?0 that is independent of temperature and dopant concentration. The function f(T*/T) = e?(T*/T)? with ? = 1/2 when T*/T > 10, corresponding to Efros-Shklovskii variable-range hopping. For T*/T < 8 the exponent ? = 1/3, a value expected for Mott variable-range hopping in two rather than three dimensions.

For Si : B with dopant concentrations ranging from 0.75n c to the critical concentration for the metal-insulator transition, the conductivity ranging over five orders of magnitude is shown to collapse onto a single universal curve of the form a(T) = σ 0 f(T * /T) with a prefactor σ 0 that is independent of temperature and dopant concentration. The function f(T * /T) = exp[-(T * /T) β ] with β = 1/2 when T * > 10T, corresponding to Efros-Shklovskii variable-range hopping. For T * < 8T the exponent β is 1/3, the 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.

For a broad range of electron densities n and temperatures T, the in-plane magnetoconductivity of the two-dimensional system of electrons in silicon MOSFETs 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 no, signaling the approach to a zero-temperature quantum phase transition. We show that H σ = AT for densities near n 0 .

Measurements of the thermoelectric power of the dilute, strongly-interacting two-dimensional electron system in high-mobility, low-disorder silicon MOSFETs were obtained at low temperatures down to 0.2 K. With decreasing density n_s, the thermopower was found to exhibit a sharp increase by more than an order of magnitude, tending to a divergence at a finite, disorder-independent density n_t. The critical behavior of the thermopower observed in our experiments provides clear evidence for an interaction-induced quantum phase transition to a new phase at low density in a strongly interacting 2D electron system, thereby settling a 20-year debate.

For a broad range of electron densities n and temperatures T, data for the in-plane magnetoconductance of the two-dimensional system of electrons in silicon MOSFET's is shown to collapse onto a universal curve using a single parameter Hσ(n,T); the scaling parameter Hσ obeys the empirical relation Hσ=A(n)[Δ(n)2+T2]1/2. The characteristic energy kBΔ associated with the magnetic field dependence of the conductivity decreases with decreasing density, and extrapolates to 0 at a critical density n0, signaling the approach to a zero-temperature quantum phase transition. For densities near n0 Hσ=AT so that the magnetoconductance scales with H/T. We suggest that this behavior is associated with a ferromagnetic instability.

We report point-by-point measurements below the blocking temperature of the magnetic relaxation of ${\mathrm{Mn}}_{12}$-acetate as a function of magnetic field applied along the easy axis of magnetization. Unexpectedly complex structure is observed which we attribute to the effect of higher-order terms of the spin Hamiltonian on the tunneling process.

The electrical resistivity of the shallow double-donor system Si:P,Bi, prepared by ion implantation, was investigated in the temperature range from 1.7 to 300 K. Good agreement was obtained between the measured resistivities and resistivities calculated by a generalized Drude approach for the same temperatures and dopant concentrations. The critical impurity concentration for the metal-nonmetal transition for the double-doped Si:P,Bi system was found to lie between the critical concentrations of the two single-doped systems, Si:P and Si:Bi. @S0163-1829~99!11747-8#

The low-temperature dc conductivities of barely metallic samples of $p$-type Si:B are compared for a series of samples with different dopant concentrations, $n,$ in the absence of stress (cubic symmetry), and for a single sample driven from the metallic into the insulating phase by uniaxial compression, $S.$ For all values of temperature and stress, the conductivity of the stressed sample collapses onto a single universal scaling curve, $\ensuremath{\sigma}(S,T)={\ensuremath{\sigma}}_{0}(\ensuremath{\Delta}{S/S}_{c}{)}^{\ensuremath{\mu}}G[{T/T}^{*}(S)],$ with ${T}^{*}\ensuremath{\propto}(\ensuremath{\Delta}{S)}^{z\ensuremath{\nu}}.$ The scaling fit indicates that the conductivity of Si:B is $\ensuremath{\propto}{T}^{1/2}$ in the critical range. Our data yield a critical conductivity exponent $\ensuremath{\mu}=1.6,$ considerably larger than the value reported in earlier experiments where the transition was crossed by varying the dopant concentration. The larger exponent is based on data in a narrow range of stress near the critical value within which scaling holds. We show explicitly that the temperature dependences of the conductivity of stressed and unstressed Si:B are different, suggesting that a direct comparison of the critical behavior and critical exponents for stress-tuned and concentration-tuned transitions may not be warranted.

Using uniaxial stress to tune the critical density near that of the sample, we have studied in detail the low-temperature conductivity of p-type Si:B in the insulating phase very near the metal-insulator transition. For all values of temperature and stress, the conductivity collapses onto a single universal scaling curve. For large values of the argument, the scaling function is well fit by the exponentially activated form associated with variable range hopping when electron-electron interactions cause a soft Coulomb gap in the density of states at the Fermi energy. The temperature dependence of the prefactor, corresponding to the T-dependence of the critical curve, has been determined reliably for this system, and is proportional to the square-root of T. We show explicitly that nevlecting the prefactor leads to substantial errors in the determination of the scaling parameters and the critical exponents derived from them. The conductivity is not consistent with Mott variable-range hopping in the critical region nor does it obey this form for any range of the parameters. Instead, for smaller argument of the scaling function, the conductivity of Si:B is well fit by an exponential form with exponent 0.31 related to the critical exponents of the system at the metal- insulator transition., Comment: 13 pages, 6 figures

Within the scaling theory of localization for noninteracting electrons, all two dimensional systems are insulating in the absence of a magnetic field when examined at sufficiently large length scales, or in the limit of zero temperature. Contrary to this two-decades-old expectation, recent experiments have shown there exists a conducting phase at low temperatures for low electron (hole) densities, a regime where electron-electron interaction energies are large compared to the Fermi energy. We briefly review experiments in silicon MOSFETs that provide evidence of an apparent conductor-insulator transition in two dimensions.

The anomalous conducting phase that has been shown to exist in zero magnetic field in dilute two-dimensional electron systems in silicon is driven into a strongly insulating state by a magnetic field of about 2 T applied at any angle to the 2D plane. Once the conducting phase is quenched, the behavior of this system in a magnetic field is similar to that of disordered GaAs/AlGaAs heterostructures. Our results demonstrate that the suppression of the conducting phase is related only to the electrons' spin.

We present the results of a detailed study of the thermally-assisted-resonant-tunneling relaxation rate of Mn12 acetate as a function of an external, longitudinal magnetic field and find that the data can be fit extremely well to a Lorentzian function. No hint of inhomogeneous broadening is found, even though some is expected from the Mn nuclear hyperfine interaction. This inconsistency implies that the tunneling mechanism cannot be described simply in terms of a random hyperfine field.

The zero-temperature magnetoconductivity of just-metallic Si:P scales with magnetic field {ital H} and dopant concentration {ital n} lying on a single universal curve: {sigma}(n,H)/{sigma}(n,0)=G[H{sup {minus}{delta}}{Delta}n] with a magnetic-field crossover exponent {delta}{approx}2. We note that Si:P, Si:B, and Si:As all have unusually large crossover exponents near 2, and suggest that this anomalously weak response to a magnetic field, {Delta}n{sub c}{proportional_to}H{sup {delta}}, is a common feature of uncompensated doped semiconductors. {copyright} {ital 1998} {ital The American Physical Society}

The reversal of spins in a magnetic material as they relax toward equilibrium is accompanied by the release of Zeeman energy which can lead to accelerated spin relaxation and the formation of a well-defined self-sustained propagating spin-reversal front known as magnetic deflagration. To date, studies of Mn$_{12}$-acetate single crystals have focused mainly on deflagration in large longitudinal magnetic fields and found a fully spin-reversed final state. We report a systematic study of the effect of transverse magnetic field on magnetic deflagration and demonstrate that in small longitudinal fields the final state consists of only partially reversed spins. Further, we measured the front speed as a function of applied magnetic field. The theory of magnetic deflagration, together with a modification that takes into account the partial spin reversal, fits the transverse field dependence of the front speed but not its dependence on longitudinal field. The most significant result of this study is the finding of a partially spin-reversed final state, which is evidence that the spins at the deflagration front are also only partially reversed., 8 pages, 5 figures

The reversal of the magnetization of crystals of molecular magnets that have a large spin and high anisotropy barrier generally proceeds below the blocking temperature by quantum tunneling. This is manifested as a series of controlled steps in the hysteresis loops at resonant values of the magnetic field where energy levels on opposite sides of the barrier cross. An abrupt reversal of the magnetic moment of the entire crystal can occur instead by a process commonly referred to as a magnetic avalanche, where the molecular spins reverse along a deflagration front that travels through the sample at subsonic speed. In this chapter, we review experimental results obtained to date for magnetic deflagration in molecular nanomagnets.

We report measurements in natural horse-spleen ferritin that provide a detailed mapping of the blocking temperature, TB , as a function of magnetic field over a broad range up to 20 kOe. Unlike most superparamagnetic materials where it decreases with applied field, TB increases with increasing field at small fields, reaching a maximum at ’3 kOe before exhibiting the expected decrease. The hysteresis loops are anomalously ‘‘pinched’’ near zero field. Both observations are consistent with an effective energy barrier that is smaller at zero field than in small finite fields. This may arise from tunneling between pairs of states on opposite sides of the anisotropy barrier that are in resonance in zero magnetic field, regardless of particle size. However, direct measurements of the magnetic viscosity yield ambiguous results, leaving open other possible explanations. @S0163-1829~97!06441-2#

We have measured the dc magnetization at low temperatures of tetragonal crystals of ${\mathrm{Mn}}_{12}$ acetate complex [${\mathrm{Mn}}_{12}$ ${\mathrm{O}}_{12}$ (${\mathrm{CH}}_{3}$ COO${)}_{16}$ (${\mathrm{H}}_{2}$ O${)}_{4}$ ], a material composed of a large (Avogadro's) number of identical magnetic molecules, each of spin 10. Exchange coupling between Mn ions within each molecule is very strong, while the interaction between molecules is negligible. A large, uniaxial anisotropy (\ensuremath{\sim}60 K) gives rise to a doubly degenerate ground state corresponding to spin projections of \ifmmode\pm\else\textpm\fi{}10 along the easy axis (c axis); hysteretic behavior is found below a blocking temperature ${\mathrm{T}}_{\mathrm{b}}$ \ensuremath{\sim}3 K. Based on measurements of oriented crystallites at temperatures between 1.7 and 3.2 K, we report strong evidence for resonant tunneling of the magnetization: periodic steps in the hysteresis loop, and periodic marked increases in the magnetic relaxation rate at the magnetic fields corresponding to these steps. A total of seven increases in the relaxation rate were found within the temperature range of our experiments with a period of 0.46 T; we suggest that many more such steps would be found at lower temperatures. We attribute these observations to thermally assisted resonant tunneling of the magnetization and propose a detailed model to account for our results.

The zero-temperature conductivity of Si:B with dopant concentrations near the metal-insulator transition exhibits scaling as a function of magnetic field with an anomalously large crossover exponent. The large value of {delta} is associated with unusual behavior of the magnetoconductance, which vanishes as a power law approaching the transition. This demonstrates that Si:B, which has an anomalous critical conductivity exponent in zero field, also exhibits unusual behavior in response to a magnetic field. {copyright} {ital 1997} {ital The American Physical Society}

The nonlinear (electric field-dependent) resistivity of the 2D electron system in silicon exhibits scaling as a function of electric field and electron density in both the metallic and insulating phases, providing further evidence for a true metal-insulator transition in this 2D system at $B\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$. Comparison with the temperature scaling yields separate determinations of the correlation length exponent, $\ensuremath{\nu}\ensuremath{\approx}1.5$, and the dynamical exponent, $z\ensuremath{\approx}0.8$, close to the theoretical value $z\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$.

We report the observation of steps at regular intervals of magnetic field in the hysteresis loop of a macroscopic sample of oriented M${\mathrm{n}}_{12}$${\mathrm{O}}_{12}$(C${\mathrm{H}}_{3}$COO${)}_{16}$(${\mathrm{H}}_{2}$O${)}_{4}$ crystals. The magnetic relaxation rate increases substantially when the field is tuned to a step. We propose that these effects are manifestations of thermally assisted, field-tuned resonant tunneling between quantum spin states, and attribute the observation of quantum-mechanical phenomena on a macroscopic scale to tunneling in a large (Avogadro's) number of magnetically identical molecules.

The energy released in a magnetic material by reversing spins as they relax toward equilibrium can lead to a dynamical instability that ignites self-sustained rapid relaxation along a deflagration front that propagates at a constant subsonic speed. Using a trigger heat pulse and transverse and longitudinal magnetic fields, we investigate and control the crossover between thermally driven magnetic relaxation and magnetic deflagration in single crystals of Mn$_{12}$-acetate., Comment: 5 pages and 4 figures

Conductivity data for Si:B samples with dopant concentrations 1.01 ${n}_{c}lnl{1.22n}_{c}$ at temperatures between 0.07 and 0.5 K in magnetic fields from 0 to 9.0 T collapse onto a single universal curve $\ensuremath{\Delta}\ensuremath{\sigma}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}KT}^{1/2}F(H/T)$, the form expected for the magnetoconductance due to electron-electron interactions. This suggests that the metal-insulator transition is predominantly driven by electron correlations, and that localization, spin-flip scattering, and spin-orbit scattering are unimportant despite strong spin-orbit effects in Si:B.

The conductivity between 0.05 and 3.0 K of five samples of insulating CdSe:In containing dif- ferent dopant concentrations 0.75[ital n][sub [ital c]][lt][ital n][lt]0.95[ital n][sub [ital c]] is consistent with two-parameter scaling, [sigma]=[sigma][sup *]([ital n])[ital f][[ital T]/[ital T][sup *]([ital n])], where the parameter [ital T][sup *] goes to zero and [sigma][sup *] tends toward a finite value [sigma][sub [ital c]][sup *] at the metal-insulator transition. This form implies a minimum metallic conductivity, and must break down as [ital n][r arrow][ital n][sub [ital c]] if the transition is continuous.

With decreasing density $n_s$ the thermopower $S$ of a low-disorder 2D electron system in silicon is found to exhibit a sharp increase by more than an order of magnitude, tending to a divergence at a finite, disorder-independent density $n_t$ consistent with the critical form $(-T/S) \propto (n_s-n_t)^x$ with $x=1.0\pm 0.1$ ($T$ is the temperature). Our results provide clear evidence for an interaction-induced transition to a new phase at low density in a strongly-interacting 2D electron system., Published version - 5 pages, 3 figures

Single-molecule magnets straddle the classical and quantum mechanical worlds, displaying many fascinating phenomena. They may have important technological applications in information storage and quantum computation. We review the physical properties of two prototypical molecular nanomagnets, Mn12-acetate and Fe8: Each behaves as a rigid, spin-10 object and exhibits tunneling between up and down directions. As temperature is lowered, the spin-reversal process evolves from thermal activation to pure quantum tunneling. At low temperatures, magnetic avalanches occur in which the magnetization of an entire sample rapidly reverses. We discuss the important role that symmetry-breaking fields play in driving tunneling and in producing Berry-phase interference. Recent experimental advances indicate that quantum coherence can be maintained on timescales sufficient to allow a meaningful number of quantum computing operations to be performed. Efforts are under way to create monolayers and to address and manipulate in...

We report measurements of the magnetic susceptibility of single crystals of Mn$_{12}$-acetate-MeOH, a new high-symmetry variant of the original single molecule magnet Mn$_{12}$-acetate. A comparison of these data to theory and to data for the Mn$_{12}$ acetate material shows that Mn$_{12}$-acetate-MeOH is a realization of a transverse-field Ising ferromagnet in contrast to the original Mn$_{12}$ acetate material, in which solvent disorder leads to effects attributed to random field Ising ferromagnetism., 6 pages, 4 figures

We present a systematic experimental investigation of the photoluminescence of heavily doped, compensated Si:P,B. The spectra broaden as the doping levels are increased, as expected. The spectral weights of the no-phonon peak and the phonon-assisted transitions vary with compensation and with excitation intensity. In contrast with the simple superposition of high-level and low-level spectra found for uncompensated material, increasing the input power for heavily doped, compensated Si produces a strong, continuous shift in energy of the entire spectrum in an essentially rigid way. A logarithmic power dependence found for the two most heavily doped samples is consistent with an exponential band tail. Deviations from this behavior are found for two samples with lower concentration and compensation. We suggest that these deviations may signal the onset of a minimum in the density of states that occurs when the acceptor (donor) band begins to separate from the host valence (conduction) band as the dopant concentration is reduced.

A negative magnetoconductance is found for insulating, uncompensated [ital n]-type Si:P and Si:As, uncompensated [ital p]-type Si:B, and compensated Si:P,B at temperatures between 1.6 and 4.2 K. The positive component expected for quantum interference in the hopping regime is absent or undetectably small in all these Si-based semiconductors even in magnetic fields as small as 200 G.

We briefly review the temperature dependence of hopping conduction in doped semiconductors near the metal-insulator transition, with emphasis on recent experimental results in Si:B at very low temperatures. Our main finding is that at sufficiently low temperature the conduction is simply activated in zero magnetic field, indicating the presence of a "hard" gap in the density of states. A magnetic field suppresses this unexpectedly strong temperature dependence, changing it to the variable-range-hopping form expected for a "soft" Coulomb gap. This suggests that the density of states is determined by electron correlations due to exchange as well as charge.

We report Hall sensor measurements of the magnetic relaxation of Mn12–acetate as a function of magnetic field applied along the easy axis of magnetization. Data taken at a series of closely spaced temperatures between 0.24 and 1.9 K provide new clues for understanding the physics of quantum tunneling of magnetization in Mn12–acetate.

We report measurements of the susceptibility in the temperature range from 3.5 to 6.0 K of a series of ${\text{Mn}}_{12}\text{-ac}$ and ${\text{Mn}}_{12}\text{-ac-MeOH}$ samples in the shape of rectangular prisms of length ${l}_{c}$ and square cross section of side ${l}_{a}$. The susceptibility obeys a Curie-Weiss law, $\ensuremath{\chi}=C/(T\ensuremath{-}\ensuremath{\theta})$, where $\ensuremath{\theta}$ varies systematically with sample aspect ratio. Using published demagnetization factors, we obtain $\ensuremath{\theta}$ for an infinitely long sample corresponding to intrinsic ordering temperatures ${T}_{c}\ensuremath{\approx}0.85\text{ }\text{K}$ and $\ensuremath{\approx}0.74\text{ }\text{K}$ for ${\text{Mn}}_{12}\text{-ac}$ and ${\text{Mn}}_{12}\text{-ac-MeOH}$, respectively. The difference in ${T}_{c}$ for two materials that have nearly identical unit cell volumes and lattice constant ratios suggests that, in addition to dipolar interactions, there is a nondipolar (exchange) contribution to the Weiss temperature that differs in the two materials because of the difference in ligand molecules.

The longitudinal magnetic susceptibility of single crystals of the molecular magnet ${\text{Mn}}_{12}$-acetate obeys a Curie-Weiss law, indicating a transition to a ferromagnetic phase at $\ensuremath{\sim}0.9\text{ }\text{K}$. With increasing magnetic field applied transverse to the easy axis, a marked change is observed in the temperature dependence of the susceptibility, and the suppression of ferromagnetism is considerably more rapid than predicted by mean-field theory for an ordered single crystal. Our results can instead be fit by a Hamiltonian for a random-field Ising ferromagnet in a transverse magnetic field, where the randomness derives from the intrinsic distribution of locally tilted magnetic easy axes known to exist in ${\text{Mn}}_{12}$-acetate crystals, suggesting that ${\text{Mn}}_{12}$-acetate is a realization of the random-field Ising model in which the random field may be tuned by a field applied transverse to the easy axis.

A theoretical model for the $Mn_{12}$ acetates is derived including a 'random field' effect arising from isomer effects in some families of host acetate materials. Estimates for the important energy scales are given. Phase boundaries are determined using a mean field approximation. Predictions for experiment are presented

Single molecule magnets straddle the classical and quantum mechanical worlds, displaying many fascinating phenomena. They may have important technological applications in information storage and quantum computation. We review the physical properties of two prototypical molecular nanomagnets, Mn_12-acetate and Fe_8: each behaves as a rigid, spin-10 object, and exhibits tunneling between up and down directions. As temperature is lowered, the spin reversal process evolves from thermal activation to pure quantum tunneling. At low temperatures, magnetic avalanches occur in which the magnetization of an entire sample rapidly reverses. We discuss the important role that symmetry-breaking fields play in driving tunneling and in producing Berry-phase interference. Recent experimental advances indicate that quantum coherence can be maintained on time scales sufficient to allow a meaningful number of quantum computing operations to be performed. Efforts are underway to create monolayers and to address and manipulate individual molecules., Comment: original version (40 pages, including 9 figures); access to published version via http://www3.amherst.edu/~jrfriedman/jrf%20publications.htm

The magnetoconductance of p-type Si:B samples with dopant concentrations just above the metalinsulator transition is negative (positive magnetoresistance) at all measured temperatures between 0.1 and 4.2 K and for magnetic fields up to 9 T. We attribute this to the effects of strong spin-orbit scattering associated with the valence bands in p-type materials. The magnetoconductivity varies as H 2 in small magnetic fields and approximately as H 1/2 at high fields, with deviations from this simple form which become increasingly significant as the metal-insulator transition is approached

Using dimensional analysis, we show that the variable-range-hopping resistivity \ensuremath{\rho} of disordered systems with Coulomb interactions obeys the scaling form ln(\ensuremath{\rho}/${\mathrm{\ensuremath{\rho}}}_{0}$)=Af(T/${\mathit{T}}_{\mathit{x}}$), where f(x) is a universal function and A and ${\mathit{T}}_{\mathit{x}}$ are sample-dependent constants. A simple heuristic calculation in three dimensions yields an explicit form, for f(x), which exhibits a smooth-crossover from the Mott (f\ensuremath{\propto}${\mathit{x}}^{\mathrm{\ensuremath{-}}1/4}$) to the Efros-Shklovskii (f\ensuremath{\propto}${\mathit{x}}^{\mathrm{\ensuremath{-}}1/2}$) behaviors. Data on five different samples of compensated n-type CdSe are shown to collapse onto this single function.