arXiv subjects

V. G. Karpov

Publications and source records attributed to V. G. Karpov.

At least 19 recordsLinked to original sources

arXiv2020

Adiabatic theory of SET and RESET transitions

We develop a phenomenological theory of pulse induced phase transformations behind the SET (from high to low resistive state) and RESET (backward) processes in nonvolatile memory. We show that in modern era devices, both evolve in the adiabatic regime with energy deposition time much shorter than that of thermalization. They are however different by the operating modes: voltage source driven for SET and current source driven for RESET. The characteristic temperatures and transition rates are expressed through material and process parameters.

cond-mat.mes-hall↗

arXiv2020

Numerical modeling of nonohmic percolation conduction and Poole Frenkel laws

We present a numerical model that simulates the current-voltage (I-V) characteristics of materials that exhibit percolation conduction. The model consists of a two dimensional grid of exponentially different resistors in the presence of an external electric field. We obtained exponentially non-ohmic I-V characteristics validating earlier analytical predictions and consistent with multiple experimental observations of the Poole-Frenkel laws in non-crystalline materials. The exponents are linear in voltage for samples smaller than the correlation length of percolation cluster L, and square root in voltage for samples larger than L.

cond-mat.dis-nn↗

arXiv2020

Percolation with plasticity for neuromorphic systems

We develop a theory of percolation with plasticity systems (PWPs) rendering properties of interest for neuromorphic computing. Unlike the standard percolation between two large electrodes, they have multiple ($N\gg 1$) interfaces and exponentially large number ($N!$) of conductive pathways between them. These pathways consist of non-ohmic random resistors that can undergo bias induced nonvolatile modifications (plasticity). The neuromorphic properties of PWPs include: multi-valued memory, high dimensionality and nonlinearity capable of transforming input data into spatiotemporal patterns, tunably fading memory ensuring outputs that depend more on recent inputs, and no need for massive interconnects. A few conceptual examples of functionality here are random number generation, matrix-vector multiplication, and associative memory. Understanding PWP topology, statistics, and operations opens a field of its own calling upon further theoretical and experimental insights.

cond-mat.dis-nn↗

arXiv2020

Pulse percolation conduction and multi-value memory

We develop a theory of pulse conduction in percolation type of materials such as noncrystalline semiconductors and nano-metal compounds. For short voltage pulses, the corresponding electric currents are inversely proportional to the pulse length and exhibit significant nonohmicity due to strong local fields in resistive regions of the percolation bonds. These fields can trigger local switching events incrementally changing bond resistances in response to pulse trains. Our prediction opens a venue to a class of multi-value nonvolatile memory implementable with a variety of materials.

cond-mat.mes-hall↗

arXiv2019

Qualitative methods in condensed matter physics

Expanding on our former hypothesis that, in the current information age, teaching physics should become more intuition-based and aiming at pattern recognition skills, we present multiple examples of qualitative methods in condensed matter physics. They include the subjects of phonons, thermal and electronic properties of matter, electron-phonon interactions and some properties of semiconductors.

physics.ed-ph↗

arXiv2019

Percolation with plasticity for neuromorphic computing

We introduce the percolation with plasticity (PWP) systems that exhibit neuromorphic functionalities including multi-valued memory, random number generation, matrix-vector multiplication, and associative learning. PWP systems have multiple (N >> 1) interfaces with external circuitry (electrodes) allowing N! >> 1 measurable interelectrode resistances. Due to the underlying material properties, they undergo successive nonvolatile modifications in response to electric pulses. PWP networks offer some advantages over the existing neural network architectures. Overall, random self-tuning PWP systems with high degree of parallelism, multiple inputs and outputs present close similarities to the cortex of mammalian brain. Understanding their topology, electrodynamics, and statistics opens a field of its own calling upon new theoretical and experimental insights.

cond-mat.dis-nn↗

arXiv2019

Physics in the information age: qualitative methods (with examples from quantum mechanics)

The traditional pedagogical paradigm in physics is based on a deductive approach. However, with the recent advances in information technology, we are facing a dramatic increase in the amount of readily available information; hence, the ability to memorize the material and provide rigorous derivations lacks significance. Our success in navigating the current "sea" of information depends increasingly on our skills in pattern recognition and prompt qualitative analysis. Inductive learning (using examples and intuition-based) is most suitable for the development of such skills. This needed change in our pedagogical paradigm remains yet to be addressed in physics curricula. We propose that incorporating inductive elements in teaching -by infusing qualitative methods - will better prepare us to deal with the new information landscape. These methods bring the learning experience closer to the realities of active research. As an example, we are presenting a compendium for teaching qualitative methods in quantum mechanics, a traditionally non-intuitive subject.

physics.ed-ph↗

arXiv2019

The Poole-Frenkel laws and a pathway to multi-valued memory

We revisit the mechanism of Poole-Frenkel non-ohmic conduction in materials of non-volatile memory. Percolation theory is shown to explain both the Poole and Frenkel dependencies corresponding to the cases of respectively small and large samples compared to the correlation radii of their percolation clusters. The applied bias modifies a limited number of microscopic resistances forming the percolation pathways. That understanding opens a pathway to multi-valued non-volatile memory and related neural network applications.

cond-mat.dis-nn↗

arXiv2018

The statistics of tin whisker diameters versus the underlying film grains

We compare the statistics of tin whisker diameters to that of the underlying film grains. Both are well approximated by the lognormal distributions. However, the parameters of those distributions can be rather different, not confirming the assumption that each whisker grows from a single grain. We conclude that several adjacent grains with similar crystal orientations can contribute to a whisker development. Our observations are consistent with the recent theory of multi-filament whisker structure. A modification of the particle size log-normal distribution is developed clarifying the nature of its dispersion.

cond-mat.mtrl-sci↗

arXiv2018

Field induced nucleation in nano-structures

We predict the probability of field induced nucleation (FIN) of conductive filaments across the nano-thin dielectric layers in memory and switching devices. The novelty of our analysis is that it deals with a dielectric layer of thickness below the critical nucleation length. We show how the latter constraint can make FIN a truly threshold phenomenon possible only for voltage (not the field) exceeding a certain critical value that does not depend on the dielectric thickness. Our analysis predicts the possibility of threshold switching without memory under certain thickness dependent voltages. In parallel, the thermal runaway mechanism of electronic switching is described analytically leading to results consistent with the earlier published numerical modeling. Our predictions offer the possibility of experimental verifications deciding between FIN and thermal runaway switching.

cond-mat.mes-hall↗

arXiv2017

Dimensional quantization effects in the thermodynamic of conductive filaments

We consider the physical effects of dimensional quantization in conductive filaments that underlie operations of some modern electronic devices. We show that, as a result of quantization, a sufficiently constricted filament acquires a positive charge. Several applications of this finding include describing the local host material polarization, the stability limit of filament constrictions, equilibrium filament radius, effects of polarity in device switching, and quantization of conductance.

cond-mat.mes-hall↗

arXiv2017

Microscopic structure of metal whiskers

We present TEM images of the interior of metal whiskers (MW) grown on electroplated Sn films. Along with earlier published information, our observations focus on a number of questions, such as why MWs' diameters are in the micron range (significantly exceeding the typical nano-sizes of nuclei in solids), why the diameters remain practically unchanged in the course of MW growth, what is the nature of MW diameter stochasticity, and what is the origin of the well-known striation structure of MW side surfaces. In an attempt to address such questions we performed an in-depth study of MW structure at the nanoscale by detaching a MW from its original film, reducing its size to a thin slice by cutting its sides by a focused ion beam, and performing TEM on that structure. Our observations revealed a rich nontrivial morphology suggesting that MW may consist of many side by side grown filaments. This structure appears to extend to the outside whisker surface and be the reason for the striation. In addition, we put forward a theory where nucleation of multiple thin metal needles results into micron-scale and larger MW diameters. This theory is developed in the average field approximation similar to the roughening transitions of metal surfaces. The theory also predicts MW nucleation barriers and other observed features.

cond-mat.mtrl-sci↗

arXiv2015

Evidence of rapid tin whisker growth under electron irradiation

We have investigated the influence of electric field on tin whisker growth. Sputtered tin samples were exposed to electron radiation, and were subsequently found to have grown whiskers, while sister control samples did not exhibit whisker growth. Statistics on the whisker properties are reported. The results are considered encouraging for substantiating an electrostatic theory of whisker growth, and the technique offers promise for investigating early stages of whisker growth in general and establishing whisker-related accelerated life testing protocols.

cond-mat.mtrl-sci↗

arXiv2015

Field induced nucleation in the presence of a metal electrode

We consider the effect of metal electrodes on the polarizability and nucleation of metal phases responsible for the operations of the emerging solid state memory. Our analysis is based on the image charge approach. We find results for point dipoles in static and oscillatory fields as well as an erect cylindrical nucleus near metal surfaces in resistive switching memories. We predict a large increase in polarizability and nucleation rate due to the metal electrode effects.

cond-mat.mes-hall↗

arXiv2013

Hot spot runaway in thin film photovoltaics and related structures

We show that thin film diode structures, such as photovoltaics and light emitting arrays, can undergo zero threshold localized thermal runaway leading to thermal and electrical nonuniformities spontaneously emerging in originally uniform systems. The linear stability analysis is developed for a system of thermally and electrically coupled two discrete diodes, and for a distributed system. These results are verified with numerical modeling that is not limited to small fluctuations. The discovered instability negatively affects the device performance and reliability. It follows that these problems can be mitigated by properly designing the device geometry and thermal insulation.

cond-mat.mes-hall↗

arXiv2012

Coupled electron--heat transport in nonuniform thin film semiconductor structures

A theory of transverse electron transport coupled with heat transfer in semiconductor thin films is developed conceptually modeling structures of modern electronics. The transverse currents generate Joule heat with positive feedback through thermally activated conductivity. This can lead to instability known as thermal runaway, or hot spot, or reversible thermal breakdown. A theory here is based on the optimum fluctuation method modified to describe saddle stationary points determining the rate of such instabilities and conditions under which they evolve. Depending on the material and system parameters, the instabilities appear in a manner of phase transitions, similar to either nucleation or spinodal decomposition.

cond-mat.dis-nn↗

arXiv2012

Plasmonic mediated nucleation of resonant nano-cavities in metallic layers

We predict plasmonic mediated nucleation of pancake shaped resonant nano-cavities in metallic layers that are penetrable to laser fields. The underlying physics is that the cavity provides a narrow plasmonic resonance that maximizes its polarizability in an external field. The resonance yields a significant energy gain making the formation of such cavities highly favorable. Possible implications include nano-optics and generation of the dielectric bits in conductive films that underlie the existing optical recording phase change technology.

cond-mat.mes-hall↗

arXiv2011

Electric field driven insulator-to-metal phase transitions

We show that strong enough electric fields can trigger nucleation of needle-shaped metallic embryos in insulators, even when the metal phase is energetically unfavorable without the field. This general phenomenon is due to the gigantic induced dipole moments acquired by the embryos which cause sufficient electrostatic energy gain. Nucleation kinetics are exponentially accelerated by the field-induced suppression of nucleation barriers. Our theory opens the venue of field driven material synthesis. In particular, we briefly discuss synthesis of metallic hydrogen at standard pressure.

cond-mat.mtrl-sci↗