Bernd Lorenz

Bio: Bernd Lorenz is an academic researcher from University of Houston. The author has contributed to research in topics: Superconductivity & Magnetic field. The author has an hindex of 37, co-authored 183 publications receiving 5931 citations. Previous affiliations of Bernd Lorenz include Lawrence Berkeley National Laboratory & Colorado State University.

LiFeAs: An intrinsic FeAs-based superconductor with Tc=18 K

Abstract: The synthesis and properties of LiFeAs, a high-${T}_{c}$ Fe-based superconducting stoichiometric compound, are reported. Single crystal x-ray studies reveal that it crystallizes in the tetragonal PbFCl type (P4/nmm) with $a=3.7914(7)\text{ }\text{\AA{}}$ and $c=6.364(2)\text{ }\text{\AA{}}$. Unlike the known isoelectronic undoped intrinsic FeAs compounds, LiFeAs does not show any spin-density wave behavior but exhibits superconductivity at ambient pressures without chemical doping. It exhibits a respectable transition temperature of ${T}_{c}=18\text{ }\text{K}$ with electronlike carriers and a very high critical field, ${\text{H}}_{c2}(0)g80\text{ }\text{T}$. LiFeAs appears to be the chemical equivalent of the infinite layered compound of the high-${T}_{c}$ cuprates.

Superconducting Fe-based compounds (A1-xSrx)Fe2As2 with A=K and Cs with transition temperatures up to 37 K.

Abstract: New high-${T}_{c}$ Fe-based superconducting compounds, $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ with $A=\mathrm{K}$, Cs, $\mathrm{K}/\mathrm{Sr}$, and $\mathrm{Cs}/\mathrm{Sr}$, were synthesized. The ${T}_{c}$ of ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{CsFe}}_{2}{\mathrm{As}}_{2}$ is 3.8 and 2.6 K, respectively, which rises with partial substitution of Sr for K and Cs and peaks at 37 K for 50%--60% Sr substitution, and the compounds enter a spin-density-wave state with increasing electron number (Sr content). The compounds represent $p$-type analogs of the $n$-doped rare-earth oxypnictide superconductors. Their electronic and structural behavior demonstrate the crucial role of the (${\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$) layers in the superconductivity of the Fe-based layered systems, and the special feature of having elemental $A$ layers provides new avenues to superconductivity at higher ${T}_{c}$.

Field-driven hysteretic and reversible resistive switch at the Ag–Pr0.7Ca0.3MnO3 interface

Abstract: The hysteretic and reversible polarity-dependent resistive switch driven by electric pulses is studied in both Ag/Pr0.7Ca0.3MnO3/YBa2Cu3O7 sandwiches and single-layer Pr0.7Ca0.3MnO3 strips. The data demonstrate that the switch takes place at the Ag–Pr0.7Ca0.3MnO3 interface. A model, which describes the data well, is proposed. We further suggest that electrochemical migration is the cause for the switch.

Large Magnetodielectric Effects in Orthorhombic HoMnO3 and YMnO3

Abstract: We have found a remarkable increase (up to 60%) of the dielectric constant with the onset of magnetic order at $42\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the metastable orthorhombic structures of $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_{3}$ and $\mathrm{Ho}\mathrm{Mn}{\mathrm{O}}_{3}$ that proves the existence of a strong magnetodielectric coupling in the compounds. Magnetic, dielectric, and thermodynamic properties show distinct anomalies at the onset of the incommensurate magnetic order and thermal hysteresis effects are observed around the lock-in transition temperature at which the incommensurate magnetic order locks into a temperature independent wave vector. The ${\mathrm{Mn}}^{3+}$ spins and ${\mathrm{Ho}}^{3+}$ moments both contribute to the magnetodielectric coupling. A large magnetodielectric effect was observed in $\mathrm{Ho}\mathrm{Mn}{\mathrm{O}}_{3}$ at low temperature where the dielectric constant can be tuned by an external magnetic field resulting in a decrease of up to 8% at $7\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. By comparing data for $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_{3}$ and $\mathrm{Ho}\mathrm{Mn}{\mathrm{O}}_{3}$ the contributions to the coupling between the dielectric response and Mn and Ho magnetic moments are separated.

Multiferroic and magnetoelectric materials

Abstract: A ferroelectric crystal exhibits a stable and switchable electrical polarization that is manifested in the form of cooperative atomic displacements. A ferromagnetic crystal exhibits a stable and switchable magnetization that arises through the quantum mechanical phenomenon of exchange. There are very few 'multiferroic' materials that exhibit both of these properties, but the 'magnetoelectric' coupling of magnetic and electrical properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technological aspirations.

Revival of the Magnetoelectric Effect

Abstract: Recent research activities on the linear magnetoelectric (ME) effect?induction of magnetization by an electric field or of polarization by a magnetic field?are reviewed. Beginning with a brief summary of the history of the ME effect since its prediction in 1894, the paper focuses on the present revival of the effect. Two major sources for 'large' ME effects are identified. (i) In composite materials the ME effect is generated as a product property of a magnetostrictive and a piezoelectric compound. A linear ME polarization is induced by a weak ac magnetic field oscillating in the presence of a strong dc bias field. The ME effect is large if the ME coefficient coupling the magnetic and electric fields is large. Experiments on sintered granular composites and on laminated layers of the constituents as well as theories on the interaction between the constituents are described. In the vicinity of electromechanical resonances a ME voltage coefficient of up to 90?V?cm?1?Oe?1 is achieved, which exceeds the ME response of single-phase compounds by 3?5 orders of magnitude. Microwave devices, sensors, transducers and heterogeneous read/write devices are among the suggested technical implementations of the composite ME effect. (ii) In multiferroics the internal magnetic and/or electric fields are enhanced by the presence of multiple long-range ordering. The ME effect is strong enough to trigger magnetic or electrical phase transitions. ME effects in multiferroics are thus 'large' if the corresponding contribution to the free energy is large. Clamped ME switching of electrical and magnetic domains, ferroelectric reorientation induced by applied magnetic fields and induction of ferromagnetic ordering in applied electric fields were observed. Mechanisms favouring multiferroicity are summarized, and multiferroics in reduced dimensions are discussed. In addition to composites and multiferroics, novel and exotic manifestations of ME behaviour are investigated. This includes (i) optical second harmonic generation as a tool to study magnetic, electrical and ME properties in one setup and with access to domain structures; (ii) ME effects in colossal magnetoresistive manganites, superconductors and phosphates of the LiMPO4 type; (iii) the concept of the toroidal moment as manifestation of a ME dipole moment; (iv) pronounced ME effects in photonic crystals with a possibility of electromagnetic unidirectionality. The review concludes with a summary and an outlook to the future development of magnetoelectrics research.

Multiferroics: a magnetic twist for ferroelectricity

Abstract: Magnetism and ferroelectricity are essential to many forms of current technology, and the quest for multiferroic materials, where these two phenomena are intimately coupled, is of great technological and fundamental importance. Ferroelectricity and magnetism tend to be mutually exclusive and interact weakly with each other when they coexist. The exciting new development is the discovery that even a weak magnetoelectric interaction can lead to spectacular cross-coupling effects when it induces electric polarization in a magnetically ordered state. Such magnetic ferroelectricity, showing an unprecedented sensitivity to ap plied magnetic fields, occurs in 'frustrated magnets' with competing interactions between spins and complex magnetic orders. We summarize key experimental findings and the current theoretical understanding of these phenomena, which have great potential for tuneable multifunctional devices.

Memristive devices for computing

Abstract: Memristive devices are electrical resistance switches that can retain a state of internal resistance based on the history of applied voltage and current. These devices can store and process information, and offer several key performance characteristics that exceed conventional integrated circuit technology. An important class of memristive devices are two-terminal resistance switches based on ionic motion, which are built from a simple conductor/insulator/conductor thin-film stack. These devices were originally conceived in the late 1960s and recent progress has led to fast, low-energy, high-endurance devices that can be scaled down to less than 10 nm and stacked in three dimensions. However, the underlying device mechanisms remain unclear, which is a significant barrier to their widespread application. Here, we review recent progress in the development and understanding of memristive devices. We also examine the performance requirements for computing with memristive devices and detail how the outstanding challenges could be met.

Memristive switching mechanism for metal/oxide/metal nanodevices.

Abstract: Nanoscale metal/oxide/metal switches have the potential to transform the market for nonvolatile memory and could lead to novel forms of computing. However, progress has been delayed by difficulties in understanding and controlling the coupled electronic and ionic phenomena that dominate the behaviour of nanoscale oxide devices. An analytic theory of the ‘memristor’ (memory-resistor) was first developed from fundamental symmetry arguments in 1971, and we recently showed that memristor behaviour can naturally explain such coupled electron–ion dynamics. Here we provide experimental evidence to support this general model of memristive electrical switching in oxide systems. We have built micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching. We demonstrate that switching involves changes to the electronic barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under an applied electric field. Vacancy drift towards the interface creates conducting channels that shunt, or short-circuit, the electronic barrier to switch ON. The drift of vacancies away from the interface annilihilates such channels, recovering the electronic barrier to switch OFF. Using this model we have built TiO2 crosspoints with engineered oxygen vacancy profiles that predictively control the switching polarity and conductance. Nanoscale metal/oxide/metal devices that are capable of fast non-volatile switching have been built from platinum and titanium dioxide. The devices could have applications in ultrahigh density memory cells and novel forms of computing.