Showing 1–38 of 38 results for author: Braun, M

On data-driven parameterizations of multidimensional generalized Langevin dynamics in the presence of a quadratic potential

Authors: Maximilian Braun, Martin Hanke, Niklas Wolf

Abstract: We propose a numerical algorithm to construct a Markov model with an extended list of variables to parameterize the equation of motion of a multidimensional coarse-grained physical system in an external potential, when memory effects are relevant. Our method uses autocorrelation data of the stationary velocities, but it avoids the inverse problem of finding the corresponding memory kernel from the… ▽ More We propose a numerical algorithm to construct a Markov model with an extended list of variables to parameterize the equation of motion of a multidimensional coarse-grained physical system in an external potential, when memory effects are relevant. Our method uses autocorrelation data of the stationary velocities, but it avoids the inverse problem of finding the corresponding memory kernel from these data in a first step. Rather, the data are used to construct a Prony series approximation of the autocorrelation function, and the parameters of this Prony series provide the corresponding Markov model. Numerical results for molecular dynamics data show a good match for parameterized models with five auxiliary variables for a one-dimensional, and twelve auxiliary variables for a two-dimensional system. △ Less

Submitted 6 July, 2026; originally announced July 2026.

MSC Class: 65L09; 65D15; 41A20

Determining extended Markov parameterizations for vector-valued generalized Langevin Equations

Authors: Niklas Bockius, Maximilian Braun, Kay Hofmann, Friederike Schmid, Martin Hanke

Abstract: The generalized Langevin equation is used as a model for various coarse-grained physical processes, e.g., the time evolution of the velocity of a given larger particle in an implicitly represented solvent, when the relevant time scales of the dynamics of the larger particle and the solvent particles are not strictly separated. Since this equation involves an integrated history of past velocities,… ▽ More The generalized Langevin equation is used as a model for various coarse-grained physical processes, e.g., the time evolution of the velocity of a given larger particle in an implicitly represented solvent, when the relevant time scales of the dynamics of the larger particle and the solvent particles are not strictly separated. Since this equation involves an integrated history of past velocities, considerable efforts have been made to approximate this dynamics by data-driven Markov models, where auxiliary variables are used to compensate for the memory term. In recent works we have developed two algorithms which can be used for this purpose, provided the dynamics in question are scalar processes. Here we extend these algorithms to vector-valued processes. As a physical test bed we consider an S-shaped particle sliding on a planar substrate, which gives rise to a truly two-dimensional velocity process. The two algorithms provide Markov approximations of this process with 10-20 auxiliary variables and a very accurate fit of the given autocorrelation data over the entire time interval where these data are non-negligible. △ Less

Submitted 12 November, 2025; originally announced November 2025.

First-principles study of KCoF$_3$: Jahn-Teller effect, dynamical magnetic charges, magnetoelectric multipoles and antimagnetoelectricity

Authors: Bogdan Guster, Maxime Braun, Eric Bousquet

Abstract: We study from \textit{ab~initio} density functional theory calculations the structural and magnetic properties of the crystal KCoF$_3$. We found that the experimentally reported cubic to tetragonal phase transition is due to an electronic first-order Jahn-Teller effect from the R zone boundary point. We also obtain that the magnetic ground state is the G-type antiferromagnetic order, in agreement… ▽ More We study from \textit{ab~initio} density functional theory calculations the structural and magnetic properties of the crystal KCoF$_3$. We found that the experimentally reported cubic to tetragonal phase transition is due to an electronic first-order Jahn-Teller effect from the R zone boundary point. We also obtain that the magnetic ground state is the G-type antiferromagnetic order, in agreement with the R-point Jahn-Teller distortion and that the magnetic moment of the Co atoms contains a strong orbital contribution ($m_L=0.95$ $μ_B$ in the cubic phase and 0.55 $μ_B$ in the tetragonal phase). Furthermore, we compute the dynamical magnetic effective charges and show that it is zero by symmetry for the Co and they can reach a value as large as 200 $10^{-2}μ_{\text{B}}/\text{Å}$ for the apical F anion. This large magnetic effective charge comes from the spin-orbit coupling (50\% of the response is from the orbital moment) contrary to the rare-earth manganites and ferrites with similar order of magnitude but originating from the exchange striction mechanism. The fact that the dynamical magnetic effective charges are non-zero also proves that the tetragonal phase of KCoF$_3$ is antimagnetoelectric with a large magnetic sublattice magnetoelectric response of 210 ps/m per spin-channel. We also discuss the generality of these magnetic effective charges. △ Less

Submitted 22 December, 2025; v1 submitted 1 November, 2024; originally announced November 2024.

Large dynamical magnetic effective charges and anti-magnetoelectricity from spin and orbital origin in multiferroic BiCoO$_3$

Authors: Maxime Braun, Bogdan Guster, Andrea Urru, Houria Kabbour, Eric Bousquet

Abstract: Using first-principles calculations, we explore the magnetoelectric properties of the room-temperature multiferroic crystal BiCoO$_3$. We use both applied magnetic field and finite-difference techniques to show that BiCoO$_3$ is anti-magnetoelectric at the linear level. The calculation of the dynamical effective charges reveals that the total magnetoelectric response is zero due to the compensatin… ▽ More Using first-principles calculations, we explore the magnetoelectric properties of the room-temperature multiferroic crystal BiCoO$_3$. We use both applied magnetic field and finite-difference techniques to show that BiCoO$_3$ is anti-magnetoelectric at the linear level. The calculation of the dynamical effective charges reveals that the total magnetoelectric response is zero due to the compensating non-zero magnetoelectric response of each magnetic sublattice. This calculation also highlights that the the orbital contribution to the response is remarkably larger than the spin one and that each sublattice has a rather large total magnetoelectric response of 85 ps/m. Furthermore, we provide an intuitive recipe to visualize the dynamical magnetic effective charge, allowing to examine its multipolar nature which we confirm by means of ab initio calculations. Given the large value of the local response, we investigate the ferromagnetic phase as well, which gives a giant magnetoelectric response of about 1000 ps/m and coming mainly from the spin contribution this time. Finally, we discuss the possible reasons for such a large magnetoelectric response in BiCoO3 and propose possible strategies to unveil this potentially large response. △ Less

Submitted 10 June, 2024; originally announced June 2024.

Development of an automated millifluidic platform and data-analysis pipeline for rapid electrochemical corrosion measurements: a pH study on Zn-Ni

Authors: Howie Joress, Brian DeCost, Najlaa Hassan, Trevor M. Braun, Justin M. Gorham, Jason Hattrick-Simpers

Abstract: We describe the development of a millifluidic based scanning droplet cell platform for rapid and automated corrosion. This system allows for measurement of corrosion properties (e.g., open circuit potential, corrosion current through Tafel and linear polarization resistance measurements, and cyclic voltammograms) on a localized section of a planar sample. Our system is highly automated and flexibl… ▽ More We describe the development of a millifluidic based scanning droplet cell platform for rapid and automated corrosion. This system allows for measurement of corrosion properties (e.g., open circuit potential, corrosion current through Tafel and linear polarization resistance measurements, and cyclic voltammograms) on a localized section of a planar sample. Our system is highly automated and flexible, allowing for scripted changing and mixing of solutions and point-to-point motion on the sample. We have also created an automated data analysis pipeline. Here we demonstrate this tool by corroding a plate of electroplated Zn$_{85}$Ni$_{15}$ alloy over a range of pH values and correlate our results with XPS measurements and literature. △ Less

Submitted 1 April, 2022; originally announced April 2022.

Jarzynski Equality for Soret Equilibria -- Virtues of Virtual Potentials

Authors: Tobias Thalheim, Marco Braun, Gianmaria Falasco, Klaus Kroy, Frank Cichos

Abstract: The Jarzynski equality relates the free energy difference between two equilibrium states to the fluctuating irreversible work afforded to switch between them. The prescribed fixed temperature for the equilibrium states implicitly constrains the dissipative switching process that can take the system far from equilibrium. Here, we demonstrate theoretically and experimentally that such a relation als… ▽ More The Jarzynski equality relates the free energy difference between two equilibrium states to the fluctuating irreversible work afforded to switch between them. The prescribed fixed temperature for the equilibrium states implicitly constrains the dissipative switching process that can take the system far from equilibrium. Here, we demonstrate theoretically and experimentally that such a relation also holds for the nonisothermal case, where the initial stationary state is not in equilibrium and the switching is effected by dynamically changing temperature gradients instead of a conservative force. Our demonstration employs a single colloidal particle trapped by optically induced thermophoretic drift currents. It relies on identifying suitable equivalents of classical work and heat and our ability to measure their distributions and express them in terms of a virtual potential. △ Less

Submitted 2 September, 2020; originally announced September 2020.

A high-throughput structural and electrochemical study of metallic glass formation in Ni-Ti-Al

Authors: Howie Joress, Brian L. DeCost, Suchismita Sarker, Trevor M. Braun, Sidra Jilani, Ryan Smith, Logan Ward, Kevin J. Laws, Apurva Mehta, Jason Hattrick-Simpers

Abstract: Based on a set of machine learning predictions of glass formation in the Ni-Ti-Al system, we have undertaken a high-throughput experimental study of that system. We utilized rapid synthesis followed by high-throughput structural and electrochemical characterization. Using this dual-modality approach, we are able to better classify the amorphous portion of the library, which we found to be the port… ▽ More Based on a set of machine learning predictions of glass formation in the Ni-Ti-Al system, we have undertaken a high-throughput experimental study of that system. We utilized rapid synthesis followed by high-throughput structural and electrochemical characterization. Using this dual-modality approach, we are able to better classify the amorphous portion of the library, which we found to be the portion with a full-width-half-maximum (FWHM) of 0.42 A$^{-1}$ for the first sharp x-ray diffraction peak. We demonstrate that the FWHM and corrosion resistance are correlated but that, while chemistry still plays a role, a large FWHM is necessary for the best corrosion resistance. △ Less

Submitted 19 December, 2019; originally announced December 2019.

Multivalent Ion-Activated Protein Adsorption Reflecting Bulk Reentrant Behavior

Authors: Madeleine R. Fries, Daniel Stopper, Michal K. Braun, Alexander Hinderhofer, Fajun Zhang, Robert M. J. Jacobs, Maximilian W. A. Skoda, Hendrik Hansen-Goos, Roland Roth, Frank Schreiber

Abstract: Protein adsorption at the solid-liquid interface is an important phenomenon that often can be observed as a first step in biological processes. Despite its inherent importance, still relatively little is known about the underlying microscopic mechanisms. Here, using multivalent ions, we demonstrate the control of the interactions and the corresponding adsorption of net-negatively charged proteins… ▽ More Protein adsorption at the solid-liquid interface is an important phenomenon that often can be observed as a first step in biological processes. Despite its inherent importance, still relatively little is known about the underlying microscopic mechanisms. Here, using multivalent ions, we demonstrate the control of the interactions and the corresponding adsorption of net-negatively charged proteins (bovine serum albumin) at a solid-liquid interface. This is demonstrated by ellipsometry and corroborated by neutron reflectivity and quartz-crystal microbalance experiments. We show that the reentrant condensation observed within the rich bulk phase behavior of the system featuring a nonmonotonic dependence of the second virial cofficient on salt concentration c_s is reflected in an intriguing way in the protein adsorption d(c_s) at the interface. Our findings are successfully described and understood by a model of ion-activated patchy interactions within the framework of classical density functional theory. In addition to the general challenge of connecting bulk and interface behavior, our work has implications for, inter alia, nucleation at interfaces. △ Less

Submitted 11 October, 2017; originally announced October 2017.

Comments: 5 pages, 3 Figures, accepted by Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 119, 228001 (2017)

Ultrafast Carrier Dynamics in VO$_2$ across the Pressure-Induced Insulator-to-Metal Transition

Authors: Johannes M. Braun, Harald Schneider, Manfred Helm, Rafał Mirek, Lynn A. Boatner, Robert E. Marvel, Richard F. Haglund, Alexej Pashkin

Abstract: We utilize near-infrared pump and mid-infrared probe spectroscopy to investigate the ultrafast electronic response of pressurized VO$_2$. Distinct pump-probe signals and a pumping threshold behavior are observed even in the pressure-induced metallic state showing a noticeable amount of localized electronic states. Our results are consistent with a scenario of a bandwidth-controlled Mott-Hubbard tr… ▽ More We utilize near-infrared pump and mid-infrared probe spectroscopy to investigate the ultrafast electronic response of pressurized VO$_2$. Distinct pump-probe signals and a pumping threshold behavior are observed even in the pressure-induced metallic state showing a noticeable amount of localized electronic states. Our results are consistent with a scenario of a bandwidth-controlled Mott-Hubbard transition. △ Less

Submitted 29 August, 2017; originally announced August 2017.

Comments: 10 pages, 9 figures

Photodynamics of quantum emitters in hexagonal boron nitride revealed by low temperature spectroscopy

Authors: Bernd Sontheimer, Merle Braun, Niko Nikolay, Nikola Sadzak, Igor Aharonovich, Oliver Benson

Abstract: Quantum emitters in hexagonal boron nitride (hBN) have recently emerged as promising bright single photon sources. In this letter we investigate in details their optical properties at cryogenic temperatures. In particular, we perform temperature resolved photoluminescence studies and measure photon coherence times from the hBN emitters. The obtained value of 81(1) ps translates to a width of… ▽ More Quantum emitters in hexagonal boron nitride (hBN) have recently emerged as promising bright single photon sources. In this letter we investigate in details their optical properties at cryogenic temperatures. In particular, we perform temperature resolved photoluminescence studies and measure photon coherence times from the hBN emitters. The obtained value of 81(1) ps translates to a width of $\sim$12 GHz which is higher than the Fourier transform limited value of $\sim$32 MHz. To account for the photodynamics of the emitter, we perform ultrafast spectral diffusion measurements that partially account for the coherence times. Our results provide important insight into the relaxation processes in quantum emitters in hBN which is mandatory to evaluate their applicability for quantum information processing. △ Less

Submitted 26 April, 2017; v1 submitted 23 April, 2017; originally announced April 2017.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 96, 121202 (2017)

Friction and Nonlinear Dynamics

Authors: N. Manini, O. M. Braun, E. Tosatti, R. Guerra, A. Vanossi

Abstract: The nonlinear dynamics associated with sliding friction forms a broad interdisciplinary research field that involves complex dynamical processes and patterns covering a broad range of time and length scales. Progress in experimental techniques and computational resources has stimulated the development of more refined and accurate mathematical and numerical models, capable of capturing many of the… ▽ More The nonlinear dynamics associated with sliding friction forms a broad interdisciplinary research field that involves complex dynamical processes and patterns covering a broad range of time and length scales. Progress in experimental techniques and computational resources has stimulated the development of more refined and accurate mathematical and numerical models, capable of capturing many of the essentially nonlinear phenomena involved in friction. △ Less

Submitted 23 May, 2016; originally announced May 2016.

Comments: 41 pages, 13 figures

Journal ref: J. Phys.: Condens. Matter 28, 293001 (2016)

Aftershocks in a frictional earthquake model

Authors: O. M. Braun, E. Tosatti

Abstract: Inspired by spring-block models, we elaborate a "minimal" physical model of earthquakes which reproduces two main empirical seismological laws, the Gutenberg-Richter law and the Omori aftershock law. Our new point is to demonstrate that the simultaneous incorporation of ageing of contacts in the sliding interface and of elasticity of the sliding plates constitute the minimal ingredients to account… ▽ More Inspired by spring-block models, we elaborate a "minimal" physical model of earthquakes which reproduces two main empirical seismological laws, the Gutenberg-Richter law and the Omori aftershock law. Our new point is to demonstrate that the simultaneous incorporation of ageing of contacts in the sliding interface and of elasticity of the sliding plates constitute the minimal ingredients to account for both laws within the same frictional model. △ Less

Submitted 12 May, 2016; originally announced May 2016.

Journal ref: Phys. Rev. E 90, 032403 (2014)

Trapping of Single Nano-Objects in Dynamic Temperature Fields

Authors: Marco Braun, Alois Würger, Frank Cichos

Abstract: In this article we explore the dynamics of a Brownian particle in a feedback-free dynamic thermophoretic trap. The trap contains a focused laser beam heating a circular gold structure locally and creating a repulsive thermal potential for a Brownian particle. In order to confine a particle the heating beam is steered along the circumference of the gold structure leading to a non-trivial motion of… ▽ More In this article we explore the dynamics of a Brownian particle in a feedback-free dynamic thermophoretic trap. The trap contains a focused laser beam heating a circular gold structure locally and creating a repulsive thermal potential for a Brownian particle. In order to confine a particle the heating beam is steered along the circumference of the gold structure leading to a non-trivial motion of the particle. We theoretically find a stability condition by switching to a rotating frame, where the laser beam is at rest. Particle trajectories and stable points are calculated as a function of the laser rotations frequency and are experimentally confirmed. Additionally, the effect of Brownian motion is considered. The present study complements the dynamic thermophoretic trapping with a theoretical basis and will enhance the applicability in micro- and nanofluidic devices. △ Less

Submitted 22 January, 2016; originally announced January 2016.

Comments: 7 pages, 6 figures

Journal ref: Phys.Chem.Chem.Phys. 16, 15207 (2014)

Size scaling of static friction

Authors: O. M. Braun, Nicola Manini, Erio Tosatti

Abstract: Sliding friction across a thin soft lubricant film typically occurs by stick-slip, the lubricant fully solidifying at stick, yielding and flowing at slip. The static friction force per unit area preceding slip is known from molecular dynamics (MD) simulations to decrease with increasing contact area. That makes the large-size fate of stick-slip unclear and unknown; its possible vanishing is import… ▽ More Sliding friction across a thin soft lubricant film typically occurs by stick-slip, the lubricant fully solidifying at stick, yielding and flowing at slip. The static friction force per unit area preceding slip is known from molecular dynamics (MD) simulations to decrease with increasing contact area. That makes the large-size fate of stick-slip unclear and unknown; its possible vanishing is important as it would herald smooth sliding with a dramatic drop of kinetic friction at large size. Here we formulate a scaling law of the static friction force, which for a soft lubricant is predicted to decrease as f_m + Δf /A^gamma for increasing contact area A, with gamma>0. Our main finding is that the value of f_m, controlling the survival of stick-slip at large size, can be evaluated by simulations of comparably small size. MD simulations of soft lubricant sliding are presented, which verify this theory. △ Less

Submitted 18 January, 2013; originally announced January 2013.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. Lett. 110, 085503 (2013)

Crystalline misfit-angle implications for solid sliding

Authors: Nicola Manini, O. M. Braun

Abstract: For the contact of two finite portions of interacting rigid crystalline surfaces, we compute the dependence of the pinning energy barrier on the misfit angle and contact area. The resulting data are used to investigate the distribution of static frictional thresholds for a contact of polycrystal surfaces, as occurs at the touching points of dry or even lubricated friction. The simplicity of the mo… ▽ More For the contact of two finite portions of interacting rigid crystalline surfaces, we compute the dependence of the pinning energy barrier on the misfit angle and contact area. The resulting data are used to investigate the distribution of static frictional thresholds for a contact of polycrystal surfaces, as occurs at the touching points of dry or even lubricated friction. The simplicity of the model allows us to investigate a broad contact-size and angular range, thus obtaining the statistical properties of the energy barriers opposing sliding for a single asperity. These statistical properties are used as the input of a master-equation model to predict the sliding properties of two macroscopic surfaces in contact. The model is consistent with the well-established result that low temperature should generally favor stick-slip motion, while at high temperature sliding should be smooth. △ Less

Submitted 18 July, 2011; v1 submitted 28 January, 2011; originally announced January 2011.

Comments: 11 pages, including 7 figures

Journal ref: Phys. Lett. A 375, 2946 (2011)

Dependence of boundary lubrication on the misfit angle between the sliding surfaces

Authors: O. M. Braun, Nicola Manini

Abstract: Using molecular dynamics based on Langevin equations with a coordinate- and velocity-dependent damping coefficient, we study the frictional properties of a thin layer of "soft" lubricant (where the interaction within the lubricant is weaker than the lubricant-substrate interaction) confined between two solids. At low driving velocities the system demonstrates stick-slip motion. The lubricant may o… ▽ More Using molecular dynamics based on Langevin equations with a coordinate- and velocity-dependent damping coefficient, we study the frictional properties of a thin layer of "soft" lubricant (where the interaction within the lubricant is weaker than the lubricant-substrate interaction) confined between two solids. At low driving velocities the system demonstrates stick-slip motion. The lubricant may or may not be melted during sliding, thus exhibiting either the "liquid sliding" (LS) or the "layer over layer sliding" (LoLS) regimes. The LoLS regime mainly operates at low sliding velocities. We investigate the dependence of friction properties on the misfit angle between the sliding surfaces and calculate the distribution of static frictional thresholds for a contact of polycrystalline surfaces. △ Less

Submitted 29 December, 2010; originally announced December 2010.

Comments: 8 pages, 11 figures

Journal ref: Phys. Rev. E 83, 021601 (2011)

Spin exchange interaction with tunable range between graphene quantum dots

Authors: Matthias Braun, P. R. Struck, Guido Burkard

Abstract: We study the spin exchange between two electrons localized in separate quantum dots in graphene. The electronic states in the conduction band are coupled indirectly by tunneling to a common continuum of delocalized states in the valence band. As a model, we use a two-impurity Anderson Hamiltonian which we subsequently transform into an effective spin Hamiltonian by way of a two-stage Schrieffer-Wo… ▽ More We study the spin exchange between two electrons localized in separate quantum dots in graphene. The electronic states in the conduction band are coupled indirectly by tunneling to a common continuum of delocalized states in the valence band. As a model, we use a two-impurity Anderson Hamiltonian which we subsequently transform into an effective spin Hamiltonian by way of a two-stage Schrieffer-Wolff transformation. We then compare our result to that from a Coqblin-Schrieffer approach as well as to fourth order perturbation theory. △ Less

Submitted 12 November, 2010; originally announced November 2010.

Comments: 8 pages, 3 figures

Journal ref: Phys. Rev. B 84, 115445 (2011)

The role of lubricant molecular shape in microscopic friction

Authors: Oleg M. Braun, Nicola Manini, Erio Tosatti

Abstract: With the help of a simple two-dimensional model we simulate the tribological properties of a thin lubricant film consisting of linear (chain) molecules in the ordinary soft-lubricant regime. We find that friction generally increases with chain length, in agreement with their larger bulk viscosity. When comparing the tribological properties of molecules which stick bodily to the substrates with o… ▽ More With the help of a simple two-dimensional model we simulate the tribological properties of a thin lubricant film consisting of linear (chain) molecules in the ordinary soft-lubricant regime. We find that friction generally increases with chain length, in agreement with their larger bulk viscosity. When comparing the tribological properties of molecules which stick bodily to the substrates with others carrying a single sticking termination, we find that the latter generally yield a larger friction than the former. △ Less

Submitted 10 October, 2008; originally announced October 2008.

Comments: 11 pages, 15 figures, in print in Phys. Rev. B

Journal ref: Phys. Rev. B 78, 195402 (2008)

Molecular rolling friction: the cogwheel model

Authors: O. M. Braun, Erio Tosatti

Abstract: With the help of a two-dimensional model we study rolling lubrication by circular (2D fullerenes) molecules for a wide range of parameters. The conditions under which microscopic rolling friction may be effective are identified and related to the relative ingraining between substrate and molecule, the latter behaving as a nanosized cogwheel. With the help of a two-dimensional model we study rolling lubrication by circular (2D fullerenes) molecules for a wide range of parameters. The conditions under which microscopic rolling friction may be effective are identified and related to the relative ingraining between substrate and molecule, the latter behaving as a nanosized cogwheel. △ Less

Submitted 5 September, 2008; originally announced September 2008.

Comments: 12 figures

Journal ref: J. Phys.: Condens. Matter 20 (2008) 354007

Rack-and-pinion effects in molecular rolling friction

Authors: Oleg M. Braun, Erio Tosatti

Abstract: Rolling lubrication with spherical molecules working as 'nanobearings' has failed experimentally so far, without a full understanding of the physics involved and of the reasons why. Past model simulations and common sense have shown that molecules can only roll when they are not too closely packed to jam. The same type of model simulations now shows in addition that molecular rolling friction ca… ▽ More Rolling lubrication with spherical molecules working as 'nanobearings' has failed experimentally so far, without a full understanding of the physics involved and of the reasons why. Past model simulations and common sense have shown that molecules can only roll when they are not too closely packed to jam. The same type of model simulations now shows in addition that molecular rolling friction can develop deep minima once the molecule's peripheral 'pitch' can match the substrate periodicity, much as ordinary cogwheels do in a rack-and-pinion system. When the pinion-rack matching is bad, the driven molecular rolling becomes discontinuous and noisy, whence energy is dissipated and friction is large. This suggests experiments to be conducted by varying the rack-and-pinion matching. That could be pursued not only by changing molecules and substrates, but also by applying different sliding directions within the same system, or by applying pressure, to change the effective matching. △ Less

Submitted 5 September, 2008; originally announced September 2008.

Comments: 5 figures

Journal ref: Phil. Mag. Lett. 88 (2008) 509

Non-adiabatic two-parameter charge and spin pumping in a quantum dot

Authors: Matthias Braun, Guido Burkard

Abstract: We study DC charge and spin transport through a weakly coupled quantum dot, driven by a non-adiabatic periodic change of system parameters. We generalize the model of Tien and Gordon to simultaneously oscillating voltages and tunnel couplings. When applying our general result to the two-parameter charge pumping in quantum dots, we find interference effects between the oscillations of the voltage… ▽ More We study DC charge and spin transport through a weakly coupled quantum dot, driven by a non-adiabatic periodic change of system parameters. We generalize the model of Tien and Gordon to simultaneously oscillating voltages and tunnel couplings. When applying our general result to the two-parameter charge pumping in quantum dots, we find interference effects between the oscillations of the voltage and tunnel couplings. Furthermore, we discuss the possibility to electrically pump a spin current in presence of a static magnetic field. △ Less

Submitted 31 January, 2008; originally announced January 2008.

Comments: 4.1 pages, 4 figures

Journal ref: Phys. Rev. Lett. 101, 036802 (2008)

Lubricated friction between incommensurate substrates

Authors: Andrea Vanossi, Giuseppe E. Santoro, Nicola Manini, Erio Tosatti, O. M. Braun

Abstract: This paper is part of a study of the frictional dynamics of a confined solid lubricant film - modelled as a one-dimensional chain of interacting particles confined between two ideally incommensurate substrates, one of which is driven relative to the other through an attached spring moving at constant velocity. This model system is characterized by three inherent length scales; depending on the p… ▽ More This paper is part of a study of the frictional dynamics of a confined solid lubricant film - modelled as a one-dimensional chain of interacting particles confined between two ideally incommensurate substrates, one of which is driven relative to the other through an attached spring moving at constant velocity. This model system is characterized by three inherent length scales; depending on the precise choice of incommensurability among them it displays a strikingly different tribological behavior. Contrary to two length-scale systems such as the standard Frenkel-Kontorova (FK) model, for large chain stiffness one finds that here the most favorable (lowest friction) sliding regime is achieved by chain-substrate incommensurabilities belonging to the class of non-quadratic irrational numbers (e.g., the spiral mean). The well-known golden mean (quadratic) incommensurability which slides best in the standard FK model shows instead higher kinetic-friction values. The underlying reason lies in the pinning properties of the lattice of solitons formed by the chain with the substrate having the closest periodicity, with the other slider. △ Less

Submitted 27 September, 2007; originally announced September 2007.

Comments: 14 pagine latex - elsart, including 4 figures, submitted to Tribology International

Journal ref: Tribol. Int. 41, 920 (2008)

Signature of chirality in scanning-probe imaging of charge flow in graphene

Authors: Matthias Braun, Luca Chirolli, Guido Burkard

Abstract: We theoretically propose to directly observe the chiral nature of charge carriers in graphene mono- and bilayers within a controlled scattering experiment. The charge located on a capacitively coupled scanning probe microscope (SPM) tip acts as a scattering center with controllable position on the graphene sheet. Unambiguous features from the chirality of the particles in single and bilayer grap… ▽ More We theoretically propose to directly observe the chiral nature of charge carriers in graphene mono- and bilayers within a controlled scattering experiment. The charge located on a capacitively coupled scanning probe microscope (SPM) tip acts as a scattering center with controllable position on the graphene sheet. Unambiguous features from the chirality of the particles in single and bilayer graphene arise in the ballistic transport in the presence of such a scattering center. To theoretically model the scattering from the smooth potential created by the SPM tip, we develop the scattering formalism within first-order Born approximation. We calculate the current through a device with an SPM tip between two constrictions (quantum point contacts) as a function of the tip position. △ Less

Submitted 7 February, 2008; v1 submitted 21 July, 2007; originally announced July 2007.

Comments: 5 pages, 4 figures, added references, corrected typos

Journal ref: Phys. Rev. B 77, 115433 (2008)

Theory of a Magnetically-Controlled Quantum-Dot Spin Transistor

Authors: Daniel Urban, Matthias Braun, Jürgen König

Abstract: We examine transport through a quantum dot coupled to three ferromagnetic leads in the regime of weak tunnel coupling. A finite source-drain voltage generates a nonequilibrium spin on the otherwise non-magnetic quantum dot. This spin accumulation leads to magnetoresistance. A ferromagnetic but current-free base electrode influences the quantum-dot spin via incoherent spin-flip processes and cohe… ▽ More We examine transport through a quantum dot coupled to three ferromagnetic leads in the regime of weak tunnel coupling. A finite source-drain voltage generates a nonequilibrium spin on the otherwise non-magnetic quantum dot. This spin accumulation leads to magnetoresistance. A ferromagnetic but current-free base electrode influences the quantum-dot spin via incoherent spin-flip processes and coherent spin precession. As the dot spin determines the conductance of the device, this allows for a purely magnetic transistor-like operation. We analyze the effect of both types of processes on the electric current in different geometries. △ Less

Submitted 28 September, 2007; v1 submitted 4 May, 2007; originally announced May 2007.

Comments: 7 pages, 6 figures

Journal ref: Phys. Rev. B 76, 125306 (2007)

Faraday-rotation fluctuation spectroscopy with static and oscillating magnetic fields

Authors: Matthias Braun, Jürgen König

Abstract: By Faraday-rotation fluctuation spectroscopy one measures the spin noise via Faraday-induced fluctuations of the polarization plane of a laser transmitting the sample. In the fist part of this paper, we present a theoretical model of recent experiments on alkali gas vapors and semiconductors, done in the presence of a {\em static} magnetic field. In a static field, the spin noise shows a resonan… ▽ More By Faraday-rotation fluctuation spectroscopy one measures the spin noise via Faraday-induced fluctuations of the polarization plane of a laser transmitting the sample. In the fist part of this paper, we present a theoretical model of recent experiments on alkali gas vapors and semiconductors, done in the presence of a {\em static} magnetic field. In a static field, the spin noise shows a resonance line, revealing the Larmor frequency and the spin coherence time $T_2$ of the electrons. Second, we discuss the possibility to use an {\em oscillating} magnetic field in the Faraday setup. With an oscillating field applied, one can observe multi-photon absorption processes in the spin noise. Furthermore an oscillating field could also help to avoid line broadening due to structural or chemical inhomogeneities in the sample, and thereby increase the precision of the spin-coherence time measurement. △ Less

Submitted 29 June, 2006; v1 submitted 26 January, 2006; originally announced January 2006.

Comments: 5 pages, 7 figures

Journal ref: Phys. Rev. B 75, 085310 (2007)

Frequency-Dependent Current Noise through Quantum-Dot Spin Valves

Authors: Matthias Braun, Jürgen König, Jan Martinek

Abstract: We study frequency-dependent current noise through a single-level quantum dot connected to ferromagnetic leads with non-collinear magnetization. We propose to use the frequency-dependent Fano factor as a tool to detect single-spin dynamics in the quantum dot. Spin precession due to an external magnetic and/or a many-body exchange field affects the Fano factor of the system in two ways. First, th… ▽ More We study frequency-dependent current noise through a single-level quantum dot connected to ferromagnetic leads with non-collinear magnetization. We propose to use the frequency-dependent Fano factor as a tool to detect single-spin dynamics in the quantum dot. Spin precession due to an external magnetic and/or a many-body exchange field affects the Fano factor of the system in two ways. First, the tendency towards spin-selective bunching of the transmitted electrons is suppressed, which gives rise to a reduction of the low-frequency noise. Second, the noise spectrum displays a resonance at the Larmor frequency, whose lineshape depends on the relative angle of the leads' magnetizations. △ Less

Submitted 17 January, 2006; originally announced January 2006.

Comments: 12 pages, 15 figures

Journal ref: Phys. Rev. B 74, 075328 (2006)

Manipulating Single Spins in Quantum Dots Coupled to Ferromagnetic Leads

Authors: Matthias Braun, Jürgen König, Jan Martinek

Abstract: We discuss the possibility to generate, manipulate, and probe single spins in single-level quantum dots coupled to ferromagnetic leads. The spin-polarized currents flowing between dot and leads lead to a non-equilibrium spin accumulation, i.e., a finite polarization of the dot spin. Both the magnitude and the direction of the dot's spin polarization depends on the magnetic properties of leads an… ▽ More We discuss the possibility to generate, manipulate, and probe single spins in single-level quantum dots coupled to ferromagnetic leads. The spin-polarized currents flowing between dot and leads lead to a non-equilibrium spin accumulation, i.e., a finite polarization of the dot spin. Both the magnitude and the direction of the dot's spin polarization depends on the magnetic properties of leads and their coupling to the dot. They can be, furthermore, manipulated by either an externally applied magnetic field or an intrinsically present exchange field that arises due to the tunnel coupling of the strongly-interacting quantum-dot states to spin-polarized leads. The exchange field can be tuned by both the gate and bias voltage, which, therefore, provide convenient handles to manipulate the quantum-dot spin. Since the transmission through the quantum-dot spin valve sensitively depends on the state of the quantum-dot spin, all the dynamics of the latter is reflected in the transport properties of the device. △ Less

Submitted 20 December, 2005; originally announced December 2005.

Comments: 23 pages, 11 figures, to be published in Lecture Notes in Physics

Probing level renormalization by sequential transport through double quantum dots

Authors: Bernhard Wunsch, Matthias Braun, Jürgen König, Daniela Pfannkuche

Abstract: We study electron transport through double quantum dots in series. The tunnel coupling of the discrete dot levels to external leads causes a shift of their energy. This energy renormalization affects the transport characteristics even in the limit of weak dot-lead coupling, when sequential transport dominates. We propose an experimental setup which reveals the renormalization effects in either… ▽ More We study electron transport through double quantum dots in series. The tunnel coupling of the discrete dot levels to external leads causes a shift of their energy. This energy renormalization affects the transport characteristics even in the limit of weak dot-lead coupling, when sequential transport dominates. We propose an experimental setup which reveals the renormalization effects in either the current-voltage characteristics or in the stability diagram. △ Less

Submitted 19 August, 2005; originally announced August 2005.

Journal ref: Phys. Rev. B 72, 205319 (2005)

Spin current through a tunnel junction

Authors: Matthias Braun, Jürgen König, Jan Martinek

Abstract: We derive an expression for the spin-current through a tunnel barrier in terms of many-body Green's functions. The spin current has two contributions. One can be associated with angular-momentum transfer by spin-polarized charge currents crossing the junction. If there are magnetic moments on both sides of the tunnel junction, due to spin accumulation or ferromagnetic ordering, then there is a s… ▽ More We derive an expression for the spin-current through a tunnel barrier in terms of many-body Green's functions. The spin current has two contributions. One can be associated with angular-momentum transfer by spin-polarized charge currents crossing the junction. If there are magnetic moments on both sides of the tunnel junction, due to spin accumulation or ferromagnetic ordering, then there is a second contribution related to the exchange coupling between the moments. △ Less

Submitted 22 November, 2004; originally announced November 2004.

Comments: 5 pages, 1 figure, submitted to Superlattices and Microstructures

Journal ref: Superlattices and Microstructures 37 (2005) p333-336

Hanle Effect in Transport through Quantum Dots Coupled to Ferromagnetic Leads

Authors: Matthias Braun, Jürgen König, Jan Martinek

Abstract: We suggest a series of transport experiments on spin precession in quantum dots coupled to one or two ferromagnetic leads. Dot spin states are created by spin injection and analyzed via the linear conductance through the dot, while an applied magnetic field gives rise to the Hanle effect. Such a Hanle experiment can be used to determine the spin lifetime in the quantum dot, to measure the spin i… ▽ More We suggest a series of transport experiments on spin precession in quantum dots coupled to one or two ferromagnetic leads. Dot spin states are created by spin injection and analyzed via the linear conductance through the dot, while an applied magnetic field gives rise to the Hanle effect. Such a Hanle experiment can be used to determine the spin lifetime in the quantum dot, to measure the spin injection efficiency into the dot, as well as proving the existence of intrinsic spin precession which is driven by the Coulomb interaction. △ Less

Submitted 19 April, 2005; v1 submitted 18 November, 2004; originally announced November 2004.

Comments: 7 pages, 4 figures, minor changes, added references

Journal ref: Europhys. Lett., 72 (2), pp. 294-300 (2005)

Theory of Transport through Quantum-Dot Spin Valves in the Weak-Coupling Regime

Authors: Matthias Braun, Jürgen König, Jan Martinek

Abstract: We develop a theory of electron transport through quantum dots that are weakly coupled to ferromagnetic leads. The theory covers both the linear and nonlinear transport regime, takes non-collinear magnetization of the leads into account, and allows for an externally-applied magnetic field. We derive generalized rate equations for the dot's occupation and accumulated spin and discuss the influenc… ▽ More We develop a theory of electron transport through quantum dots that are weakly coupled to ferromagnetic leads. The theory covers both the linear and nonlinear transport regime, takes non-collinear magnetization of the leads into account, and allows for an externally-applied magnetic field. We derive generalized rate equations for the dot's occupation and accumulated spin and discuss the influence of the dot's spin on the transmission. A negative differential conductance and a nontrivial dependence of the conductance on the angle between the lead magnetizations is predicted. △ Less

Submitted 8 December, 2004; v1 submitted 20 April, 2004; originally announced April 2004.

Comments: 14 pages, 13 figures, updated version as published, Journal reference

Journal ref: Phys. Rev. B 70, 195345 (2004)

Influence of a Uniform Current on Collective Magnetization Dynamics in a Ferromagnetic Metal

Authors: J. Fernandez-Rossier, M. Braun, A. S. Núñez, A. H. MacDonald

Abstract: We discuss the influence of a uniform current, $\vec{j} $, on the magnetization dynamics of a ferromagnetic metal. We find that the magnon energy $ε(\vec{q})$ has a current-induced contribution proportional to $\vec{q}\cdot \vec{\cal J}$, where $\vec{\cal J}$ is the spin-current, and predict that collective dynamics will be more strongly damped at finite ${\vec j}$. We obtain similar results for… ▽ More We discuss the influence of a uniform current, $\vec{j} $, on the magnetization dynamics of a ferromagnetic metal. We find that the magnon energy $ε(\vec{q})$ has a current-induced contribution proportional to $\vec{q}\cdot \vec{\cal J}$, where $\vec{\cal J}$ is the spin-current, and predict that collective dynamics will be more strongly damped at finite ${\vec j}$. We obtain similar results for models with and without local moment participation in the magnetic order. For transition metal ferromagnets, we estimate that the uniform magnetic state will be destabilized for $j \gtrsim 10^{9} {\rm A} {\rm cm}^{-2}$. We discuss the relationship of this effect to the spin-torque effects that alter magnetization dynamics in inhomogeneous magnetic systems. △ Less

Submitted 22 November, 2003; originally announced November 2003.

Comments: 12 pages, 2 figures

Journal ref: Phys. Rev. B69, 174412 (2004)

Friction in a solid lubricant film

Authors: O. M. Braun, M. Peyrard

Abstract: Molecular dynamics study of a thin (one to five layers) lubricant film between two substrates in moving contact are performed using Langevin equations with an external damping coefficient depending on distance and velocity of atoms relative the substrates, motivated by microscopic configurations. They show that the minimal friction coefficient is obtained for the solid-sliding regime. A detailed… ▽ More Molecular dynamics study of a thin (one to five layers) lubricant film between two substrates in moving contact are performed using Langevin equations with an external damping coefficient depending on distance and velocity of atoms relative the substrates, motivated by microscopic configurations. They show that the minimal friction coefficient is obtained for the solid-sliding regime. A detailed analysis of the results, the comparison with other microscopic modeling approaches of friction, and the evaluation of quantities that can be compared to experiments, such as the velocity of the transition from stick-slip to smooth sliding, are used to discuss the relevance of the microscopic simulations of friction. △ Less

Submitted 12 October, 2000; originally announced October 2000.

Comments: 21 pages, 20 Postscript figures, submitted to Phys. Rev. E (2000)

Report number: Preprint of Inst. of Physics NASU No. 39 (2000)

Supersonic and multiple topological excitations in the driven Frenkel-Kontorova model with exponential interaction

Authors: O. M. Braun

Abstract: The criteria for the existence of supersonic and multiple topological excitations (kinks) in the driven Frenkel-Kontorova model (a chain of atoms placed into an external periodic potential) with anharmonic (exponential) interatomic interactions are studied. The criteria for the existence of supersonic and multiple topological excitations (kinks) in the driven Frenkel-Kontorova model (a chain of atoms placed into an external periodic potential) with anharmonic (exponential) interatomic interactions are studied. △ Less

Submitted 8 October, 2000; originally announced October 2000.

Comments: 6 pages, 3 Postscript figures, to be published in Phys. Rev. E (2000)

Report number: Preprint of Inst. of Physics NASU No. 38 (2000)

Role of entropy barriers for diffusion in the periodic potential

Authors: O. M. Braun

Abstract: Diffusion of a particle in the N-dimensional external potential which is periodic in one dimension and unbounded in the other N-1 dimensions is investigated. We find an analytical expression for the overdamped diffusion and study numerically the cases of moderate and low damping. We show that in the underdamped limit, the multi-dimensional effects lead to reduction (comparing with the one-dimens… ▽ More Diffusion of a particle in the N-dimensional external potential which is periodic in one dimension and unbounded in the other N-1 dimensions is investigated. We find an analytical expression for the overdamped diffusion and study numerically the cases of moderate and low damping. We show that in the underdamped limit, the multi-dimensional effects lead to reduction (comparing with the one-dimensional motion) of jump lengths between subsequent trapping of the atom in bottoms of the external periodic potential. As application we consider the diffusion of a dimer adsorbed on the crystal surface. △ Less

Submitted 7 October, 2000; originally announced October 2000.

Comments: 9 pages, 10 Postscript figures, to be published in Phys. Rev. E (2000)

Report number: Preprint of Inst. of Physics NASU No. 37 (2000)

Dynamical transitions in correlated driven diffusion in a periodic potential

Authors: O. M. Braun, T. Dauxois, M. V. Paliy, M. Peyrard

Abstract: The diffusion of a two-dimensional array of particles driven by a constant force in the presence of a periodic external potential exhibits a hierarchy of dynamical phase transitions when the driving force is varied. This behavior can be explained by a simple phenomenological approach which reduces the system of strongly interacting particles to weakly interacting quasi-particles (kinks). The ric… ▽ More The diffusion of a two-dimensional array of particles driven by a constant force in the presence of a periodic external potential exhibits a hierarchy of dynamical phase transitions when the driving force is varied. This behavior can be explained by a simple phenomenological approach which reduces the system of strongly interacting particles to weakly interacting quasi-particles (kinks). The richness of the strongly coupled system is however not lost because, contrary to a single-Brownian particle, the array shows an hysteretic behavior even at non-zero temperature. The present investigation can be viewed as a first step toward understanding nanotribology. △ Less

Submitted 20 January, 1997; originally announced January 1997.

Comments: 4 pages, 3 pictures, revtex to appear in Phys Rev. Lett

Journal ref: Physical Review Letters 78, 1295-1298 (1997)

Mobility and diffusivity in a generalized Frenkel-Kontorova model

Authors: Oleg M. Braun, Thierry~Dauxois, Maxim V. Paliy, Michel Peyrard

Abstract: Molecular dynamics simulations are used to investigate the atomic mobility and diffusivity of a generalized Frenkel-Kontorova model which takes into account anharmonic (exponential) interaction of atoms subjected to a three-dimensional substrate potential periodic in two dimensions and nonconvex (Morse) in the third dimension. The numerical results are explained by a phenomenological theory whic… ▽ More Molecular dynamics simulations are used to investigate the atomic mobility and diffusivity of a generalized Frenkel-Kontorova model which takes into account anharmonic (exponential) interaction of atoms subjected to a three-dimensional substrate potential periodic in two dimensions and nonconvex (Morse) in the third dimension. The numerical results are explained by a phenomenological theory which treats a system of strongly interacting atoms as a system of weakly interacting quasiparticles (kinks). Model parameters are chosen close to those for {\bf K}-{\bf W}(112) adsorption system. △ Less

Submitted 12 April, 1996; originally announced April 1996.

Comments: 27 Revtex pages, 9 figures to appear in Phys. Rev. B

Journal ref: Physical Review B 54, 321-332 (1996)

Solitonic-exchange mechanism of surface~diffusion

Authors: Oleg M. Braun, Thierry Dauxois, Michel Peyrard

Abstract: We study surface diffusion in the framework of a generalized Frenkel-Kontorova model with a nonconvex transverse degree of freedom. The model describes a lattice of atoms with a given concentration interacting by Morse-type forces, the lattice being subjected to a two-dimensional substrate potential which is periodic in one direction and nonconvex (Morse) in the transverse direction. The results… ▽ More We study surface diffusion in the framework of a generalized Frenkel-Kontorova model with a nonconvex transverse degree of freedom. The model describes a lattice of atoms with a given concentration interacting by Morse-type forces, the lattice being subjected to a two-dimensional substrate potential which is periodic in one direction and nonconvex (Morse) in the transverse direction. The results are used to describe the complicated exchange-mediated diffusion mechanism recently observed in MD simulations [J.E. Black and Zeng-Ju Tian, Phys. Rev. Lett. {\bf 71}, 2445-2448(1993)]. △ Less

Submitted 12 April, 1996; originally announced April 1996.

Comments: 22 Revtex pages, 9 figures to appear in Phys. Rev. B

Journal ref: Physical Review B 54, 313-320 (1996)