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Physics Dept.
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Condensed Matter Physics

Overview

Condensed Matter Physics is a vibrant, exciting, and robust sub-discipline of Physics. New discoveries are frequent, many with applications to technology and direct benefit to society. At the University of Massachusetts at Amherst, the Condensed Matter Physics program is thriving, with thirteen faculty members and two faculty additions anticipated within the next year or two. In addition, active emeritus faculty members continue to be associated with the program. There are a number of interconnected research programs which provide graduate students and postdoctoral research associates with opportunities to engage nature at the frontiers condensed matter, including the The Laboratory for Low Temperature Physics, and the Theoretical Quantum Fluids Group.


The experimental opportunities span a broad spectrum of the disciplines with work in porous media, granular systems, multiphase flow phenomena, complex fluids, colloids, polymer physics, nanostructure technology, device physics, microfabrication, superfluidity, superconductivity, network analysis, thin film physics, and the physics of restricted geometries. And, a major new direction for the department, Biological Physics, is being developed with close conections to the Condensed Matter Physics Group.


Theoretical work is often done in association with the experimental program, but there is active independent theoretical work including computational techniques applied to the study of critical points and phase transitions, computational complexity, porous materials, hysteresis in various physical systems, spin polarized quantum systems, and Bose Einstein Condensation.


Listed below are the major research programs in Condensed Matter Physics offered by the department. Additional information may be obtained about individual programs or faculty members by clicking on highlighted key words. Information for those interested in visiting the Condensed Matter Group is given at the end of this page.


Research Programs


The experimental and theoretical programs are focused on six broad directions: (1) Low Temperature Physics including restricted geometries, (2) soft Condensed Matter Physics, including colloids, polymers, porous media and granular materials, (3) nanoscience, (4) Computational Physics, (5) Biological Physics and (6) other aspects of Condensed Matter and Statistical Physics. The major research programs are directed by individual faculty members and these are described in alphabetical order separately for each faculty member for experiment and theory. Several of the faculty are involved in more than one of the broad research areas. The programs described below provide opportunities for graduate student research and are typically funded by the National Science Foundation or other federal agencies or industry.


The programs are grouped into three categories:




Experimental Programs


Donald Candela: (e-mail)
 
            

Professor Candela's research activity is roughly divided between two areas of condensed-matter physics: (a) Quantum fluids and solids at very low temperatures, and (b) Classical statistical systems at ambient temperature.


             

The low-temperature work uses high-field NMR to study 3He fluids as prototypical many-body Fermi systems. The overall goal is to observe the quantum effects that occur in condensed-matter systems at extreme conditions of low temperature and high magnetic field. One direction is the study of these Fermi fluids in random environments such as aerogel. It is carried out at UMass, at typical fields and temperatures of 8 T and 4.5 mK, respectively. A second direction is the study spin transport in Fermi fluids at even more extreme field and temperature conditions (20 T, 1 mK). This is a collaborative project with investigators at the NHMFL High B/T facility in Florida.

          

The ambient-temperature work uses the technique of Pulsed-Field-Gradient NMR (PFG-NMR) to study the small-scale motions in classical statistical systems such as fluid flowing in porous media and granular flows. The PFG-NMR technique is similar to the more familiar technique of magnetic-resonance imaging (MRI), but it can probe motion over much smaller distance and time scales. Present projects are aimed at studying oscillating fluid flows in porous media and the fluctuating velocities in steady and oscillating granular flows.


Anthony Dinsmore: (e-mail)
 

Professor Dinsmore's group studies the statistical mechanics of "soft condensed matter:" colloids, emulsions, lipid membranes, nanoparticle suspensions and other squishy things. Our experiments probe the relationships among inter-particle forces, structure, and dynamics of many-bodied systems -- relationships that are central to condensed-matter physics. Current experiments focus on the kinetics of freezing and melting colloidal crystals, on the distribution of stresses in jammed emulsions, on interactions among particles adsorbed at liquid interfaces, and on interactions among membrane proteins. We study these problems chiefly using optical microscopy in two and three dimensions (the latter using a fast confocal microscope).


Some of the work applies fundamental understanding to evelop new materials. Self-assembled materials can have unique mechanical, optical and electronic properties with applications in nanotechnology and biomedical engineering. Current areas of study include maximizing the scattering of light from randomly packed arrays of strongly-scattering spheres. By harnessing the tendency of small particles to adsorb on liquid interfaces, we also have contributed to the development of nanoscopic membranes composed of functionalized nanoparticles. This approach also lets us fabricate functioning electronic devices (e.g., transistors) via self-assembly of nanoparticles on metallic droplets.


Lori Goldner: (e-mail)

Dr. Goldner's lab specializes in the study of single biological molecules and molecular complexes using optical techniques. In the last decade, the development of techniques to measure and manipulate single organic or biological molecules is facilitating a new understanding of molecular interactions and dynamics in biophysics, cell and molecular biology, and polymer science. In biophysics and molecular biology, the ability to identify, track, and measure the conformation and interaction of e.g., single proteins or RNA molecules, is leading to new insights in protein folding and RNA interactions and functionality, is helping to validate models of protein folding or molecular dynamics, and is leading to new insights in cell signaling and protein expression in living systems. The extension of single molecule techniques to track and measure individual biomolecules in living cells has the potential to revolutionize our understanding of cell signaling. In polymer science, visualizing the motion of individual molecules aids enormously in understanding flow, phase transitions, and other polymer dynamics. Most recently, professor Goldner's group is working towards understanding the physics of biological molecules and molecular complexes in confining or crowded environments. They seek to understand how individual biomolecular components work together to drive empirical behavior of living systems. Towards this end they have recently developed the use of aqueous droplets with volume below 1 fL (diameter smaller than 1 micron) to confine and manipulate single molecules and molecular complexes. These droplets can be mixed and manipulated using optical tweezers to facilitate the assembly of molecular components one-at-a-time.

More information on Prof. Goldner's work can be found here.

 

 


Robert Hallock: (e-mail)


  

The research program directed by Robert Hallock is primarily aimed at the study of liquid helium in restricted geometries, particularly thin helium films. Research is directed in several specific areas: (1) thin films of mixtures of the two forms of helium, 3He and 4He, (2) the propagation of waves known as third sound on helium films on patterned substrates, (3) the hysteretic capillary condensation of helium in porous materials and avalanche behavior in superfluid helium pore draining, (4) helium films on hydrogen surfaces, (5) helium in quasi one-dimensional geometry, and (6) solid helium.


  

The study of superfluid mixture films provides enhanced understanding of the two dimensional behavior of helium. In particular the study of 3He as an impurity in a 4He film leads to enhanced understanding of two dimensional Fermi systems. Such systems hold the potential to be novel superfluids, and a search for such new superfluid behavior is in progress. The study of such films on the weak binding substrate hydrogen is providing insight into superfluidity in films more dillute than a single atomic layer. Third sound propagation in two dimensional patterned environments is being used to study localization phenomena in a context which can't be addressed in other ways.


  

Superfluid helium provides a unique vantage point from which to study novel behavior in constrained three dimensional geometries. One aspect of this is the recent observation of avalanche phenomena when superfluid helium drains from a three dimensional porous material. Such avalanches may be an example of phenomena which can be described by a non-equilibrium Ising model. Helium also provides the opportunity to study one dimensional adsorption. In such studies, buckytube bundles are being used to study the behavior of 3He and 4He in such a highly constrained environment.


Recent experiments in the ara of solid helium have been interpreted as showing evidence for "supersolid" behavior. The subject area is controversial and unsettled. The study of solid helium will show whether or not it is possible to pass 4He atoms through solid helium and should add considerably to our understanding of solid helium.


  

A list of recent publications (which may not be quite up to date), which have resulted from work carried out by Professor Hallock's research group is available. A list of current and former graduate students who have worked in the group (and their thesis topics) is also available, as is a list of current and former Postdoctoral Research Associates.


Narayanan Menon: (e-mail)


  

Professor Menon's research area is experimental studies of disordered condensed matter. A particular interest is the dynamics of classical fluids in circumstances where particles are constrained to move collectively rather than as independent entities. Our understanding of this middle ground between the ordered solid, and random, gas-like, systems is poorly developed. Spatial structure here often reveals little about dynamics: a snapshot of molecules in a glass, or of sand grains in an intermittently jamming flow, appear no different from a freely-flowing liquid. A major component of the experimental effort addresses the long-standing question regarding the origin of slow dynamics in supercooled liquids using a variety of wide-frequency susceptibility measurements.


Another research interest is in the dynamics of granular fluids, composed of macroscopic particles (such as sand) driven into flow by external forces. This is an area of broad relevance even beyond statistical physics and dynamical systems, in that granular systems are important in a number of geophysical, industrial, and other human contexts. A basic unanswered question in this field is whether granular flow (such as in an hour-glass) can be described by a continuum approach on the lines of fluid mechanics for conventional fluids. Laser light scattering techniques, video microscopy and direct stress measurements are used to glean detailed information regarding the microscopic dynamics of grains in flow in an effort to establish key elements of statistical and continuum approaches to describing these systems.


  

Prof. Menon's group's web page provides more details about his work.


Jennifer Ross: (e-mail)

   

The Ross Lab studies the physics of cytoskeletal filaments called microtubules. Microtubules are the structural elements of the cell that support the cell's structure and create the forces required for cell division. Microtubules are also the tracks for motor proteins called kinesin and dynein-dynactin. Understanding the intrinsic properties of these cellular structures shed light on their biological role in the cell.


In order to study microtubules and motors, the Ross Lab uses advanced optical microscopy to observe these biomolecules one-at-a-time. Single molecule experiments are performed using total internal reflection fluorescence microscopy and optical trapping to measure the motion and forces exerted by these molecules.


Professor Ross' web page has more information about her research program.

 

 


Mark Tuominen: (e-mail)

Professor Tuominen's research work is centered on the science and technology of quantum nanostructures. This work involves the fabrication of new nanometer-scale devices and materials systems, together with experiments that probe the interesting electronic and magnetic behavior these tiny devices exhibit. One project involves the physics of single-electron devices which utilize of mesoscopic superconductors to investigate macroscopic quantum behavior in a tunable way. The experiments investigate the intrinsic noise of these devices (which are extremely sensitive charge detectors) and their interaction with the electrodynamic environment. A different project involves the investigation of electron transport through single-atoms and single-molecules. Unlike the case of macroscopic wires, it is observed that the transport through a single atom is quantum-mechanically coherent, exhibiting a quantized conductance. Another major research project involves the formation of ultrasmall nanostructures by utilizing polymeric self-assembly to template the desired materials and devices. For example, this technique permits the investigation of the "giant" magnetoresistive properties of magnetic-multilayer nanowires with diameters in the 5-100 nm range (pictured above in a four-contact measurement configuration).


             

The research laboratory includes facilities for both fabricating and testing the nanostructure devices described above. This includes: a scanning-electron microscope for lithography and imaging, a microelectronic cleanroom for sample processing, high-vacuum thin-film deposition equipment, pulsed electrochemical deposition equipment, STM/AFM, a He3/He4 dilution refrigerator, a 3He refrigerator, a Quantum Design PPMS system, and various instrumentation for low-noise electrical characterization of nanostructure samples.



Theoretical Programs


Egor Babaev: (e-mail)

Professor Babaev's current research interests are connected with quantum fluids, gases and solids. In particular he is working on the problem of possibilities of novel types of quantum ordered states in condensed matter (going beyond the usual paradigm where these sates fall into categories of quantum fluids, gases, solids and superconductors). In connection with it, he is examining possible ordered states in the quantum domain of projected metallic state of hydrogen at ultrahigh compression, and predicted there possible formation of two type of novel quantum fluids: metallic superfluid and superconducting superfluid. His other interests include multicomponent Bose-Einstein condensates, dipolar condensates, multicomponent superconductors, neutron stars interior. In these systems, he has found several new solutions for topological defects, such as knotted solitons (with Faddeev and Niemi) and discussed how collective quantum behaviour there manifests itself in various exotic rotational and magnetic responses. He has also studied various phase transitions between the states with different symmetries in multicomponent systems.

Professor Babaev's web page can be found has further information. More information about quantum fluids research at UMass Amherst can be found here.

 

 


Benny Davidovich: (e-mail)

Professor Davidovitchís current research interests are primarily in nonequilibrium phenomena in condensed matter physics, in particular pattern formation in dissipative systems and nonlinear dynamics of soft materials. Despite significant progress in the last decades, our current understanding of the behavior of macroscopic systems away from their thermodynamic equilibrium is still lacking. Whereas the study of systems at thermodynamic equilibrium is generally pursued by constructing appropriate free energy and searching for its minima, developing concepts and tools that can be widely used to characterize nonequilibrium states is still an ongoing endeavor.

Prof. Davidovitch studies several problems in nonequilibrium dynamics. One such problem is shear banding, a phenomenon common to flows in various types of complex fluids, induced by shear force. Inspired by an observation of oscillatory shear-banded profile in partially-ordered colloidal suspensions, Prof. Davidovitch and collaborators have recently suggested that shear banding could be attributed to coexistence of two linearly-responding phases rather than to a nonlinear rheology, as is typically assumed. Current work in this direction is focused on developing simple theoretical models whose analysis will enable distinction between universal and system-specific aspects of shear banding. Another problem is the spontaneous formation of nano-scale patterns on solid surfaces by ion bombarded. In a recent study, Prof. Davidovitch and collaborators have shown that observed pattern properties indicate on the relevance of nonlocal processes, on top of local mechanisms (such as surface diffusion or erosive surface response to single impact event), currently believed to underlie a continuum-level theory of the phenomenon. Current work on this project is aimed at developing a detailed nonlinear theory which will correctly relate the large variety of observed surface patterns to tunable system parameters (e.g. beam angle, ion type and energy), thus helping to make a ion-sputtering an efficient tool for nano-pattering purposes. Another problem currently studied is the influence of geometric fluctuations of charged interfaces and fronts in plasma or electrolyte on the screening of electrostatic potential. Prof. Davidovitch and his collaborators have recently found that such fluctuations can potentially have unexpected long-range effect on the screened potential, and they currently explore the relevance of this result under various physical conditions.

 

 


Jonathan Machta: (e-mail)


Professor Machta's research interests are in theoretical and computational aspects of statistical physics and condensed matter physics with and emphasis on (1) the development and application of new cluster algorithms for simulating equilibrium phase transitions and (2) applications of computational complexity theory to problems in statistical physics. He has also worked on phase transitions and transport in porous media, superfluidity, random and self-avoiding walks and nonlinear dynamics.


             

Cluster Monte Carlo techniques were introduced a decade ago and have revolutionized computational statistical physics because they are much more efficient than conventional Monte Carlo methods. Prof. Machta and his collaborators recently introduced several new kinds of cluster algorithms and applied them to a range of problems in statistical physics. One of these is the 'invaded cluster' algorithm which simulates critical points without prior knowledge of the critical temperature and with almost no critical slowing. A second method is the 'two-replica' cluster algorithm which is useful for simulating phase transitions in systems without internal symmetries.


             

Professor Machta's second area of current interest is the interface between theoretical computer science and statistical physics. Over the past several years, he and his collaborators have analyzed a number of models in statistical physics from the standpoint of computational complexity theory. Complexity theory provides a robust set of computational models and a hierarchy of complexity classes defined with respect to these models. He has been able to classify a number of physically motivated models within this complexity hierarchy. This work helps guide algorithm development. A second, less obvious motivation, is that the computational complexity of simulating a system is a way of characterizing the system itself. Indeed, a case can be made that a useful way to define 'physical complexity' is via computational complexity.

A selected list of Professor Machta's recent publications is available.


William Mullin: (e-mail)


            


Professor Mullin's research interests are in quantum fluids and solids. In particular he and his collaborators have studied transport and equilibrium properties of fluid Fermi and Bose systems, including pure liquid 3He, 3He gas, dilute solutions of 3He in liquid 4He, and trapped alkali Bose gases. Transport in these systems is interesting because of the strong dependence that coefficients such as spin diffusion, thermal conductivity, and viscosity have on the degree of polarization. Recent UMass/University of Florida experiments (Candela et al) observed a giant enhancement of viscosity in dilute solutions at very high polarization and low temperature confirming theoretical analysis provided by Mullin for these experiments.


The discovery in 1995 of Bose-Einstein condensation (BEC) in alkali gases held in magnetic or optical traps caused great excitement among researchers in the area of quantum systems. In these gases particles avalanche into the lowest quantum state below a certain transition temperature, which can be as low as nanokelvin for a few thousand of particles. LaloŽ and Mullin (LM) have been studying what happens in these systems when two condensates, having opposite spin are mixed. The measurement of transverse spin causes the appearance of a phase angle that is analogous to a hidden variable considered in local realistic alternatives to quantum mechanics. On the other hand, LM show that Bellís Theorem, characterizing such local realistic theories and normally studied for two-particle systems, is violated in the measurements of a arbitrarily large number of boson spins.M


Neutral fermions, in particular helium atoms, flowing in nanotubes present a rich ground for new research. Prof. Mullin and collaborators have found that flow of such particles through nanotubes might be used as an additional cooling mechanism in a dilution refrigerator. Such systems may also reveal phenomena, such as quantized conductance, localization, universal conductance fluctuations, etc. normally studied in electronic nanostructures.





             

Professor Mullin's web page provides access to his publications. Professor Mullin retired in 2000 but continues as a member of the Condensed Matter group with a very active current research program in spite of emeritus faculty status.


Nikolai Prokof'ev: (e-mail)


             


Professor Prokof'ev's research program includes physics of dissipative quantum systems and Monte Carlo simulations. He was also interested in the properties of strongly correlated electrons, conductance and metal-insulator phase transitions in one-dimensional systems, and quantum magnetism.


             

A few years ago Prof. Prokof'ev and his collaborators realized that quantum Monte Carlo simulations can be performed directly in a continuous parameter space, thus avoiding artificial discretization of continuous variables and systematic errors (it could be the imaginary time continuum for lattice models or the space-time continuum for polaron-like models). In a separate development, a new idea for performing Monte Carlo updates using artificially created and evolving objects in configuration space was implemented, which allowed calculations of the Green's functions as easily as, say, the total energy in previous schemes. The new algorithm was applied recently to a number of cases, which include (i) disordered 1D Hubbard model, (ii) quantum spin chains in a magnetic field; (iii) smearing of the Coulomb staircase in quantum dots; (iv) the Frohlich-Feynman polaron problem, and (v) quasi-condensation in the interacting 2D Bose gas. The current research interestes of Prof. Prokof'ev in this area are concentrated on the critical temperature shift due to interactions in the 3D Bose gas, hole dynamics in 2D magnetic systems, and spin glass algorithms.


             

In an attempt to understand decoherence mechanisms in nanoscale magnetic particles and superconducting devices Prof. Prokof'ev in collaboration with Philip Stamp found that at low temperatures conventional dissipative couplings usually described by the oscillator bath model (phonons, electrons, magnons, photons, etc.) give way to the spin bath environment formed by nuclear spins and paramagnetic impurities. The first work dealt with the topological aspects of the coupling of large magnetic molecules to this "spin bath" (the environmental spins add a Haldane/Berry phase to each path involved in the dynamics of the nanomagnet - this adds a "random phase" to the action, and can destroy phase interference even when no energy is exchanged between the system and the spin bath). More recently the problem of "central spin" quantum dynamics has been solved in a large number of relevant experimental cases. It seems that the spin bath environment is probably the most serious obstacle in the way of making superconducting qubits functional.


             

Prof. Prokof'ev frequently collaborates with Prof. Svistunov.


Christian Santangelo: (e-mail)

  

Professor Santangelo's current research interests are in theoretical soft condensed matter physics, and especially the role of geometry in self-assembly and liquid crystalline phases. Self-assembly shows great promise for forming precise target structures on nanoscopic or microscopic scales which can then, in turn, be used in a variety of applications. Nevertheless, it has proven quite a challenge to connect the interactions of macromolecules to the large-scale structure of their assemblies. With collaborators, he is developing techniques to model self-assembly in colloids interacting through a soft corona and the formation of undulated and network phases of wormlike micelles in diblock copolymers. His recent work has focused on geometry and topological defects in smectic liquid crystals, materials with one-dimensional periodic order. In particular, he has studied geometrically-induced order in two dimensional smectics on curved surfaces, which has an analogy to gravitational lensing. These curved smectics are a toy model for the more complicated geometrical frustration that occurs in three dimensional smectics. Other work includes the assembly of aqueous charged systems in regimes in which ionic correlations become important.


Professor Santangelo's web page has more information.

 


Boris Svistunov: (e-mail)

    

Prof. Svistunov's research activities concentrate in the areas of Bose-Einstein condensation, superfluidity and related phenomena in quantum gases, liquids, solids, and ultra-cold atomic systems in optical lattices. He is also heavily involved in developing and applying high-performance Monte Carlo techniques in quantum and classical statistical physics.


             

In collaboration with Yuri Kagan and Georgy Shlyapnikov he predicted the so-called m!-effect---a generic effect of suppressing m-body inelastic processes in the Bose-Einstein condensate (BEC), as compared to the normal state. Observation of this effect was one of the first direct experimental proofs of the BEC formation in ultra-cold atomic gases. In this collaboration Prof. Svistunov has proposed two non-trivial relaxation scenarios: kinetics of Bose-Einstein condensation (with numeric simulation carried out with Natalia Berloff) and cascade decay of superfluid turbulence (vortex line tangle) in the T=0 limit.


             

Prof. Svistunov has developed (in collaboration with Nikolay Prokof'ev and Igor Tupitsyn) two novel Monte Carlo approaches: Diagrammatic Monte Carlo and Worm Algorithm. The diagrammatic Monte Carlo applies to the problems where the quantities of interest are expressed in terms of positive definite convergent diagrammatic series. A textbook example is the Frohlich polaron model, which has been successfully studied with this approach. The Worm Algorithm is a generic powerful scheme of simulating quantum and classical discrete systems in the case when the effective configuration space can be represented in the form of closed paths. Among the most important problems solved with this algorithm in the nearest past is the problem of critical temperature and fluctuation region in weakly interacting 3D and 2D Bose gases, direct simulation of ultra-cold atoms in 3D optical lattice, superfluid-insulator transition in the commensurate system of lattice bosons with disorder, counterflow superfluidity of two-component bosonic systems in a lattice (predicted in collaboration with Anatoly Kuklov), weak first-order quantum phase transitions.


             

The two hottest problems being solved at the moment are (i) the problem of BCS-BEC crossover in the system of fermions with resonant attractive interaction: by determinant diagrammatic Monte Carlo with worm-type updates, in collaboration with Nikolay Prokof'ev, Evgeni Burovski, and Matthias Troyer; and (ii) supersolidity (=superfluidity in the solid phase) of He-4: by the recently developed (with Massimo Boninsegni and Nikolay Prokof'ev) worm algorithm for continuous-space systems.



Adjunct Faculty Programs


Murugappan Muthukumar (Wilmer D. Barrett Distinguished Professor of Polymer Science and Engineering; Adjunct Professor of Physics): (e-mail)


  

Professor Muthukumar's theoretical research program has three main centers of activity in the area of the statistical mechanics of polymers. (1) Phase Transitions in Polymers: Under study is the role of density fluctuations in phase separation of polymer blends and solutions, order-disorder transitions in block copolymers and liquid crystalline systems. The kinetics of these ordering processes are also studied with and without shear flows. (2) Polyelectrolyte Dynamics: Under investigation are the dynamics of polyelectrolytes and other water-soluble polymers in the context of viscosity, diffusion, and electrophoresis. Also under study are models of the ways polymer molecules move in tangled environments wiht a focus on the physics of separation methods. Professor Muthukumar uses Monte Carlo and related simulation techniques, various experimental methods, and analytical theory. (3) Dynamics of Self-Assembly and Pattern Recognition: Here the emphasis is on the kinetics of self-assembly in model random copolymers, phospholipids and biological polymers such as proteins, polynucleotides, and carbohydrates. Recognition of patterns on membranes by polymers with prescribed sequences is investigated using computer modeling and theoretical methods.



Visits to the Group

The Condensed Matter Group welcomes visitors at any time. Most of the Condensed Matter Program is housed in Hasbrouck Laboratory which is barely visible at the extreme left of this photograph of the campus pond. Anyone interested in visiting the Condensed Matter Group should feel free to contact directly the faculty member most relevant to them by use of the e-mail connections above listed beside each faculty member's name. Directions to the campus are available as are campus maps. The University of Massachusetts is located in Amherst, at the northern end of the Pioneer Valley (the Pioneer Valley site is a well done web site that also connects to pages for many, but not all, of the local towns) in west-central Massachusetts. The area is blessed with a wonderful quality of life with many outdoor activities, five area colleges, a free bus system, and a remarkable variety of cultural events.



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