Papers by Marcelo Rozenberg
Scientific Reports, 2019
We introduce an ultra-compact electronic circuit that realizes the leaky-integrate-and-fire model... more We introduce an ultra-compact electronic circuit that realizes the leaky-integrate-and-fire model of artificial neurons. Our circuit has only three active devices, two transistors and a silicon controlled rectifier (SCR). We demonstrate the implementation of biologically realistic features, such as spike-frequency adaptation, a refractory period and voltage modulation of spiking rate. All characteristic times can be controlled by the resistive parameters of the circuit. We built the circuit with out-of-the-shelf components and demonstrate that our ultra-compact neuron is a modular block that can be associated to build multi-layer deep neural networks. We also argue that our circuit has low power requirements, as it is normally off except during spike generation. Finally, we discuss the ultimate ultra-compact limit, which may be achieved by further replacing the SCR circuit with Mott materials.
Nonvolatile Memory with Multilevel Switching: A Basic Model
Physical Review Letters, 2004
APL Materials
Neuromorphic computing approaches become increasingly important as we address future needs for ef... more Neuromorphic computing approaches become increasingly important as we address future needs for efficiently processing massive amounts of data. The unique attributes of quantum materials can help address these needs by enabling new energy-efficient device concepts that implement neuromorphic ideas at the hardware level. In particular, strong correlations give rise to highly non-linear responses, such as conductive phase transitions that can be harnessed for short- and long-term plasticity. Similarly, magnetization dynamics are strongly non-linear and can be utilized for data classification. This Perspective discusses select examples of these approaches and provides an outlook on the current opportunities and challenges for assembling quantum-material-based devices for neuromorphic functionalities into larger emergent complex network systems.
Applied Physics Letters, 2021
The coupling of electronic degrees of freedom in materials to create 'hybridized functionalities'... more The coupling of electronic degrees of freedom in materials to create 'hybridized functionalities' is a holy grail of modern condensed matter physics that may produce versatile mechanisms of control. Correlated electron systems often exhibit coupled degrees of freedom with a high degree of tunability which sometimes lead to hybridized functionalities based on external stimuli. However, the mechanisms of tunability and the sensitivity to external stimuli are determined by intrinsic material properties which are not always controllable. A Mott metalinsulator transition (MIT) is technologically attractive due to the large changes in resistance, tunable by doping, strain, electric fields, and orbital occupancy but not, in and of itself, controllable with light. Here an alternate approach is presented to produce optical functionalities using a properly engineered photoconductor/strongly-correlated hybrid heterostructure. This approach combines a photoconductor, which does not exhibit an MIT, with a strongly correlated oxide, which is not photoconducting. Due to the intimate proximity between the two materials, the heterostructure exhibits giant volatile and nonvolatile, photoinduced resistivity changes with substantial shifts in the MIT transition temperatures. This approach can be extended to other judicious combinations of strongly correlated materials.
Physical Review Applied, 2022
Mott materials such as vanadium oxides, when subject to a strong applied voltage, present an inho... more Mott materials such as vanadium oxides, when subject to a strong applied voltage, present an inhomogeneous insulator-to-metal transition with formation of metallic filaments within the insulating bulk. This property is enabling the development of compact and power-efficient neuromorphic devices known as Mott neurons. However, the nature of the transition has not been fully understood yet, as it may be attributed to different effects, including Joule self-heating and hot-carrier injection. Moreover, the experimental determination of the threshold voltage needed to induce the transition has proven to be challenging, as the transition becomes increasingly unpredictable when the threshold is approached. The physical understanding of these issues would not only deepen our understanding of Mott insulators, but would also be an important step toward the realization of neuromorphic devices based on such materials. In this work we use numerical simulations based on the Mott resistor network model to study the nature of the filament incubation and formation process. We show that both electronic and thermal effects, in the form of current density focusing and Joule self-heating, respectively, contribute to the filamentary incubation and growth. Remarkably, we find that the percolation of the metallic filaments near the threshold is intrinsically stochastic, qualitatively similar to the familiar Arrhenius activated behavior and to the stochastic firing of biological neurons. More precisely, we characterize the filament percolation as a Poisson point process, which has the same probability distribution as mathematical models of neuronal firing with an exponential escape rate. Finally, we support the numerical simulation results by performing experiments in VO 2 that are in agreement with the exponential escape rate behavior. Thus, we establish a functionality of Mott insulators that opens a path toward implementing neuromorphic hardware with quantum materials.
arXiv: Mesoscale and Nanoscale Physics, 2019
Nanoparticle solar cells have low carrier mobility. Key causes of this low mobility include that ... more Nanoparticle solar cells have low carrier mobility. Key causes of this low mobility include that (1) the nanoparticle solids are insulators; and (2) the Coulomb blockade. The insulating behavior can be overcome by driving the system across a Metal-Insulator Transition (MIT). However, the evolution of the Coulomb blockade across the MIT has not been analyzed. This paper focuses on the behavior of the Coulomb blockade by analyzing carrier transport in the insulating phase by our Hierarchical Nanoparticle Transport Simulator, and by Dynamical Mean-Field Theory in the metallic phase. Our unexpected result is that the Coulomb blockade persists across the MIT.
Nature Communications, 2021
Application of an electric stimulus to a material with a metal-insulator transition can trigger a... more Application of an electric stimulus to a material with a metal-insulator transition can trigger a large resistance change. Resistive switching from an insulating into a metallic phase, which typically occurs by the formation of a conducting filament parallel to the current flow, is a highly active research topic. Using the magneto-optical Kerr imaging, we found that the opposite type of resistive switching, from a metal into an insulator, occurs in a reciprocal characteristic spatial pattern: the formation of an insulating barrier perpendicular to the driving current. This barrier formation leads to an unusual N-type negative differential resistance in the current-voltage characteristics. We further demonstrate that electrically inducing a transverse barrier enables a unique approach to voltage-controlled magnetism. By triggering the metal-to-insulator resistive switching in a magnetic material, local on/off control of ferromagnetism is achieved using a global voltage bias applied t...
Nature Communications, 2020
Resistive switching can be achieved in a Mott insulator by applying current/voltage, which trigge... more Resistive switching can be achieved in a Mott insulator by applying current/voltage, which triggers an insulator-metal transition (IMT). This phenomenon is key for understanding IMT physics and developing novel memory elements and brain-inspired technology. Despite this, the roles of electric field and Joule heating in the switching process remain controversial. Using nanowires of two archetypal Mott insulators—VO2 and V2O3 we unequivocally show that a purely non-thermal electrical IMT can occur in both materials. The mechanism behind this effect is identified as field-assisted carrier generation leading to a doping driven IMT. This effect can be controlled by similar means in both VO2 and V2O3, suggesting that the proposed mechanism is generally applicable to Mott insulators. The energy consumption associated with the non-thermal IMT is extremely low, rivaling that of state-of-the-art electronics and biological neurons. These findings pave the way towards highly energy-efficient ap...
Journal of Applied Physics, 2018
This tutorial describes challenges and possible avenues for the implementation of the components ... more This tutorial describes challenges and possible avenues for the implementation of the components of a solid-state system, which emulates a biological brain. The tutorial is devoted mostly to a charge-based (i.e. electric controlled) implementation using transition metal oxides materials, which exhibit unique properties that emulate key functionalities needed for this application. In the Introduction, we compare the main differences between a conventional computational machine, based on the Turing-von Neumann paradigm, to a Neuromorphic machine, which tries to emulate important functionalities of a biological brain. We also describe the main electrical properties of biological systems, which would be useful to implement in a charge-based system. In Chapter II, we describe the main components of a possible solid-state implementation. In Chapter III, we describe a variety of Resistive Switching phenomena, which may serve as the functional basis for the implementation of key devices for Neuromorphic computing. In Chapter IV we describe why transition metal oxides, are promising materials for future Neuromorphic machines. Theoretical models describing different resistive switching mechanisms are discussed in Chapter V while existing implementations are described in Chapter VI. Chapter VII presents applications to practical problems. We list in Chapter VIII important basic research challenges and open issues. We discuss issues related to specific implementations, novel materials, devices and phenomena. The development of reliable, fault tolerant, energy efficient devices, their scaling and integration into a Neuromorphic computer may bring us closer to the development of a machine that rivals the brain.
Physical Review Applied, 2018
We consider the phenomenon of electric Mott transition (EMT), which is an electric induced insula... more We consider the phenomenon of electric Mott transition (EMT), which is an electric induced insulator to metal transition. Experimentally, it is observed that depending on the magnitude of the electric excitation the final state may show a short lived or a long lived resistance change. We extend a previous model for the EMT to include the effect of local structural distortions through an elastic energy term. We find that by strong electric pulsing the induced metastable phase may become further stabilized by the electro-elastic effect. We present a systematic study of the model by numerical simulations and compare the results to new experiments in Mott insulators of the AM4Q8 family. Our work significantly extends the scope of our recently introduced leaky-integrateand-fire Mott-neuron [P. Stoliar Adv Mat 2017] to bring new insight on the physical mechanism of its relaxation. This is a key feature for future neuromorphic circuit implementations.
Physical Review B, 1998
Optical conductivity spectra of single crystals of the perovskite-type 3d 1 metallic alloy system... more Optical conductivity spectra of single crystals of the perovskite-type 3d 1 metallic alloy system Ca 1−x SrxVO 3 have been studied to elucidate how the electronic behavior depends on the strength of the electron correlation without changing the nominal number of electrons. The reflectivity measurements were made at room temperature between 0.05 eV and 40 eV. The effective mass deduced by the analysis of the Drude-like contribution to the optical conductivity and the plasma frequency do not show critical enhancement, even though the system is close to the Mott transition. Besides the Drude-like contribution, two anomalous features were observed in the optical conductivity spectra of the intraband transition within the 3d band. These features can be assigned to transitions involving the incoherent and coherent bands near the Fermi level. The large spectral weight redistribution in this system, however, does not involve a large mass enhancement.
Physical Review B, 1999
We study a mean-field model of a Kondo alloy using numerical techniques and analytic approximatio... more We study a mean-field model of a Kondo alloy using numerical techniques and analytic approximations. In this model, randomly distributed magnetic impurities interact with a band of conduction electrons and have a residual RKKY coupling of strength J. This system has a quantum critical point at J = J c ∼ T 0 K , the Kondo scale of the problem. The T dependence of the spin susceptibility near the quantum critical point is singular with χ(0) − χ(T) ∝ T γ and non-integer γ. At J c , γ = 3/4. For J < ∼ J c there are two crossovers with decreasing T , first to γ = 3/2 and then to γ = 2, the Fermi-liquid value. The dissipative part of the time-dependent susceptibility χ ′′ (ω) ∝ ω as ω → 0 except at the quantum critical point where we find χ ′′ (ω) ∝ √ ω. The characteristic spin-fluctuation energy vanishes at the quantum critical point with ω sf ∼ (1 − J/J c) for J < ∼ J c , and ω sf ∝ T 3/2 at the critical coupling.
ACS applied materials & interfaces, Jan 17, 2015
Cross-point array (CPA) structure memories using a memristor are attracting a great deal of atten... more Cross-point array (CPA) structure memories using a memristor are attracting a great deal of attention due to their high density integration with a 4F2 cell. However, a common significant drawback of the CPA configuration is crosstalk between cells. To date, the CPA structure using a redox-based memristor has restrictions to minimize the operating current level due to their resistive switching mechanism. This study demonstrates suitable characteristics of a ferroelectric tunnel junction (FTJ) for the memristor of the CPA structure using an electrostatic model. From the FTJ of the Au/ p-type Pr0.98Ca0.02MnO3 (4 nm)/ BiTiO3 (4.3 nm)/ n-type Ca0.98Pr0.02MnO3 (3 nm)/ Pt(111) structure, which has a higher and thicker potential barrier, a good memristive effect for the CPA structure with a high non-linear I-V curve and low current operation, was obtained by Δ Fowler-Nordheim tunneling with effectively blocked direct tunneling and thermionic emission. The FTJ demonstrated reduced sneak curr...
Physical Review B, 2014
Mott insulator to metal transitions under an electric field are currently the subject of numerous... more Mott insulator to metal transitions under an electric field are currently the subject of numerous fundamental and applied studies. This puzzling effect, which involves nontrivial out-of-equilibrium effects in correlated systems, is indeed at play in the operation of a new class of electronic memories, the "Mott memories." However, the combined electronic and thermal effects are difficult to disentangle in Mott insulators undergoing such transitions. We report here a comparison between the properties under an electric field of a canonical Mott insulator and a model built on a realistic two-dimensional resistor network able to capture both thermal effects and electronic transitions. This comparison made specifically on the family of narrow gap Mott insulators AM 4 Q 8 , (A = Ga or Ge; M = V, Nb or Ta; and Q = S or Se) unambiguously establishes that the resistive transition experimentally observed under an electric field arises from a purely electronic mechanism.
Physical Review B, 2015
We report the existence of metallic two dimensional electron gases (2DEGs) at the (001) and (101)... more We report the existence of metallic two dimensional electron gases (2DEGs) at the (001) and (101) surfaces of bulk-insulating TiO2 anatase due to local chemical doping by oxygen vacancies in the near-surface region. Using angle-resolved photoemission spectroscopy, we find that the electronic structure at both surfaces is composed of two occupied subbands of dxy orbital character. While the Fermi surface observed at the (001) termination is isotropic, the 2DEG at the (101) termination is anisotropic and shows a charge carrier density three times larger than at the (001) surface. Moreover, we demonstrate that intense UV synchrotron radiation can alter the electronic structure and stoichiometry of the surface up to the complete disappearance of the 2DEG. These results open a route for the nano-engineering of confined electronic states, the control of their metallic or insulating nature, and the tailoring of their microscopic symmetry, using UV illumination at different surfaces of anatase.
Scientific Reports, 2013
Resistive random access memory based on the resistive switching phenomenon is emerging as a stron... more Resistive random access memory based on the resistive switching phenomenon is emerging as a strong candidate for next generation non-volatile memory. So far, the resistive switching effect has been observed in many transition metal oxides, including strongly correlated ones, such as, cuprate superconductors, colossal magnetoresistant manganites and Mott insulators. However, up to now, no clear evidence of the possible relevance of strong correlation effects in the mechanism of resistive switching has been reported. Here, we study Pr 0.7 Ca 0.3 MnO 3 , which shows bipolar resistive switching. Performing micro-spectroscopic studies on its bare surface we are able to track the systematic electronic structure changes in both, the low and high resistance state. We find that a large change in the electronic conductance is due to field-induced oxygen vacancies, which drives a Mott metal-insulator transition at the surface. Our study demonstrates that strong correlation effects may be incorporated to the realm of the emerging oxide electronics.
This work explores a simple aproximation to describe isolated impurity scattering in a strongly c... more This work explores a simple aproximation to describe isolated impurity scattering in a strongly correlated metal. The approximation combines conventional one electron scattering theory and the Dynamic Mean Field Theory to describe strong correlations in the host. It becomes exact in several limits, including those of very weak and very strong impurity potentials. Original electronic structure appears at the impurity site when the impurity potential strength is moderate and the host is close to the Mott transition. Our results may provide useful guidance for interpretation of scanning tunneling microscopy experiments in strongly correlated systems.
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Papers by Marcelo Rozenberg