Venkat Chandrasekhar - US grants

9313726 Chandrasekhar This is an experimental research program focused on low temperature electron transport in nanofabricated systems in which strong electrostatic interactions between electrons lead to Coulomb blockade and single electron charging phenomena. The experiments are designed to increase fundamental understanding of the microscopic mechanism of electron transport in this regime, with emphasis on modifications introduced by quantum effects. The structures consist of short insulating wires placed between metallic contact electrodes. The goal is to understand the origin of the single electron charging effects that have been observed in this novel system. With an eye to potential device applications, different methods of increasing the temperature range over which the Coulomb blockade effects can be observed will be investigated by exploiting some unique properties of insulators. The project involves graduate students in research that is at the cutting edge of nanolithographic fabrication techniques. %%% This experimental project deals with the motion of individual electrons among ultrasmall structures that are fabricated by tracing patterns only a few atoms in width using a finely focused electron beam. The resulting structures show remarkable responses at low temperatures to applied electric and magnetic fields. The response arises from quantum effects that are only poorly understood in the structures under study. The ability of detection motion of a single electron has fundamental interest and is technologically significant as a route to the smallest microelectronics device currently envisioned. The project will employ insulating nanostructures which have potential for allowing operation of the microdevices at higher temperatures. ***

9313726 Chandrasekhar This is an experimental research program focused on low temperature electron transport in nanofabricated systems in which strong electrostatic interactions between electrons lead to Coulomb blockade and single electron charging phenomena. The experiments are designed to increase fundamental understanding of the microscopic mechanism of electron transport in this regime, with emphasis on modifications introduced by quantum effects. The structures consist of short insulating wires placed between metallic contact electrodes. The goal is to understand the origin of the single electron charging effects that have been observed in this novel system. With an eye to potential device applications, different methods of increasing the temperature range over which the Coulomb blockade effects can be observed will be investigated by exploiting some unique properties of insulators. The project involves graduate students in research that is at the cutting edge of nanolithographic fabrication techniques. %%% This experimental project deals with the motion of individual electrons among ultrasmall structures that are fabricated by tracing patterns only a few atoms in width using a finely focused electron beam. The resulting structures show remarkable responses at low temperatures to applied electric and magnetic fields. The response arises from quantum effects that are only poorly understood in the structures under study. The ability of detection motion of a single electron has fundamental interest and is technologically significant as a route to the smallest microelectronics device currently envisioned. The project will employ insulating nanostructures which have potential for allowing operation of the microdevices at higher temperatures. ***

9801982 Chandrasekhar This experimental condensed matter physics project addresses the fabrication and characterization of normal metal (N) - superconductors (S) structures whose dimensions are comparable to the fundamental coherence lengths in such materials. The electrical conductance and other properties of these nanostructures are predicted, and observed, to display non-classical and surprising behavior. This experimental project will extend our knowledge of the superconducting proximity effect by conducting the first measurements of the thermoelectric and magnetic response of NS structures. In the first part of the project, the energy and length dependence of the electrical conductance of normal wires in contact with a superconductor will be investigated in order to verify fundamental predictions of the theory of NS devices. The second part of the project is devoted to measuring the thermopower of so-called Andreev interferometers, which are loops in which one arm is fabricated from a superconductor and the other from a normal metal. In the last part of the project, the magnetic response of isolated Andreev interferometers will be measured using dc SQUID magnetometers. These last two properties have not been extensively studied, and their measurement should further our understanding of the correlations induced in the normal metal by its proximity to a superconductor. The samples for these experiments will be patterned and fabricated using electron-beam lithography. Their low temperature transport properties will be investigated by linear and nonlinear multiprobe differential resistance measurements as a function of temperature and magnetic field. The low temperature magnetic properties will be measured using ultra-sensitive dc SQUID magnetometers. Post-doctoral associates and graduate students trained on this project will be exposed to a wide variety of experimen tal techniques which will be useful in future careers in either industry or academia. %%% This experimental condensed matter physics project addresses the fabrication and characterization of ultra-small normal metal (N) - superconductors (S) structures. Superconductors have unusual electrical and magnetic properties at low temperatures, the best known being their ability to carry an electrical current without resistance. These properties form the basis for a number of practical devices such as very high field magnets. When a superconductor is placed in good contact with a conventional normal metal such as copper or gold, some of the superconducting properties "leak" a very small distance into the normal metal. With modern lithographic techniques borrowed from the semiconductor industry, it is now possible to probe the properties of normal metals on these very small size scales. The aim of this project is to investigate the electrical, thermoelectric and magnetic properties of such small normal metals placed in close proximity to a superconductor. In the first part of this project, the electrical properties of normal metal wires in proximity to a superconductor will be studied to verify and further our current theoretical understanding of the nature of the superconductivity induced in the normal metal. In the second and third parts of the project, the consequences of this induced superconductivity on the thermoelectric and magnetic properties of the normal metal wires will be explored. These last two properties have not been yet been studied experimentally, and these measurements will help us gain further insight into the interaction between superconductors and normal metals on very small size scales. The samples for these experiments will be fabricated using advanced electron-beam lithography techniques, and measured using sophisticated resistance and magnetic sensors a t temperatures near absolute zero. Post-doctoral associates and graduate students trained on this project will be exposed to a number of experimental techniques which will be useful in future careers in either industry or academia. ***

It is proposed to conduct local spectroscopic and potentiometric measurements on submicron scale devices to investigate some of the most interesting issues in mesoscopic physics. The tools that will be developed will enable a host of new experimental investigations that would not be possible by any other means.

The probe to be used in these studies is scanning tunneling microscopy (STM), one that has been used extensively in recent years to investigate structures on the nanometer scale. The requirements of the proposed experiments necessitate the assembly of a number of experimental techniques in one experimental tool, which has not been done before. After assembly of the instrument, the experiments to be performed involve imaging the magnetic vortices created in a superconducting film by an external magnetic field, and by the magnetic field emanating from ferromagnetic particles embedded in the film. The project will entail a close collaboration with Professor Herve' Courtois' group at the CRTBT in Grenoble, France.

Ferromagnets are becoming increasingly important as components of a wide variety of potentially useful devices. These experiments seek to answer a number of fundamental questions regarding the properties of ferromagnets on the nanometer scale, and their interactions with other materials such as superconductors. Given the increasing importance of scanning probe microscopies in investigating and characterizing the properties of nanoscale systems, the students involved in this project will be well-trained in techniques that will be useful in their future careers, whether those careers are in industry, academia, or government laboratories.

0210449
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This proposal was received in response to the Nanoscale Science and Engineering Initiative, Program Solicitation NSF 01-157, in the NIRT category. The proposal focuses on understanding intrinsic phenomena governing spin transport at the nanoscale, and the development of new methods for its manipulation for future spin-controlled, magneto-electronic, devices. It addresses one of the most exciting aspects of current research on next-generation electronic devices: the manipulation of spin, rather than only electrical charge. The advantages of these magnetoelectronic devices include nonvolatility, faster switching in static memory elements, and higher density due to a simpler device structure. These issues become even more important as technology drives device sizes toward the nanoscale, where new fundamental physical effects emerge that alter spin transport, as well as high-frequency dynamics and switching times.
An understanding of these issues at the nanoscale requires single-crystal magnetic heterostructures with atomically-sharp interfaces, patterned to nanometer dimensions. This proposal probes nanoscale spin transport phenomena in epitaxial magnetic oxide nanostructures grown with atomic-layer control, whose magnetic, electronic, and interfacial properties are tuned at will. Layers with defined electronic, magnetic, and morphological characteristics positioned with atomic-layer control in epitaxial systems are used to address crucial fundamental questions in magnetic nanostructures.
This research program consists of 1) design, growth, and characterization of epitaxial magnetic oxide heterostructures with atomic layer control by pulsed laser deposition with in-situ real-time structural analysis 2) high-resolution and analytical TEM to determine atomic structure and electronic properties of the interfaces; 3) nanoscale patterning of novel magnetic heterostructures below 50 nm; 4) scanning probe measurements of topography and local electronic properties; 5) education and outreach efforts with a focus on introducing young people to modern, multidisciplinary science and technology, using the research direction as a vehicle.
The multidisciplinary, multiuniversity/industry team consists of members working in Materials Science, Physics, Electrical Engineering, and device development. Research, education, and outreach all follow the theme of nanoscale structures, novel phenomena, and spin transport control. This work will build a scientific foundation for the understanding of new phenomena in nanoscale spin-controlled devices. The PIs industrial and multidisciplinary interactions will be very beneficial in advancing research as well as in educating students. This study will also provide fundamental guidelines in the atomic-scale control of nanoscale systems such as ferroelectrics and oxide-semiconductor integration that are important for next-generation technology.

This individual investigator award supports a project that will investigate the electrical and magnetic properties of metallic heterostructures fabricated from superconducting (S), normal metal (N), and ferromagnetic (F) elements. The goal is to explore the nature of the interaction between ferromagnetism and superconductivity in FS structures, and to investigate the influence of long-range phase coherence on the thermal transport in NS heterostructures. Electrical transport properties of FS heterostructures, will be used to attempt to verify the existence of long-range superconducting correlations in a diffusive ferromagnet placed in contact with a superconductor. In addition, effects of finite spin polarization in the ferromagnet on the transport properties of FS structures will be studied. Using a novel local thermometry technique, effects in the thermopower of NS devices, as well the thermal conductance of NS structures, will be explored. Samples will be fabricated by electron-beam lithography and measured at millikelvin temperatures using various low temperature cryostats, including a dilution refrigerator and a 3He refrigerator. The students and post-docs trained on this project will gain experience with both microfabrication and low temperature techniques. Thus becoming well-prepared for careers in industry, academia or government laboratories.

Superconductors are materials that show a number of unusual electrical and magnetic properties at low temperatures, the best known property being their ability to carry an electrical current without resistance. These properties form the basis for a number of useful applications, such as very high field magnets. When a superconductor is placed in good contact with a conventional normal metal such as copper or gold, or a ferromagnet like iron or nickel, the interaction between the two different elements results in new effects with potential device applications. The goal of this project is to investigate the electrical and thermal transport properties of normal metals and ferromagnets placed in close proximity to a superconductor. Electrical properties of structures incorporating ferromagnets and superconductors will be studied to investigate the interplay between magnetism and superconductivity in very small devices. In addition, heat transport in a normal metal placed in contact with a superconductor will be explored. These measurements will help us gain further insight into the interaction between superconductors, ferromagnets and normal metals on very small size scales. The samples for these experiments will be fabricated using advanced electron-beam lithography techniques, and measured using sophisticated techniques at temperatures near absolute zero. Post-doctoral associates and graduate students trained on this project will be exposed to a number of experimental techniques which will be useful in future careers in either industry or academia.

PROPOSAL NO: 0403590
INSTITUTION: Northwestern University
PRINCIPAL INVESTIGATOR: Ismail , Yehea
TITLE: NER: Exploring Possibilities for Carbon Nanotube as Circuits and
Interconnects


Abstract:
The scaling of CMOS technologies is showing worrisome trends in terms of noise immunity, power consumption, reliability issues, and performance bottle necks. Major challenges are facing VLSI technologies in trying to maintain the current exponential growth in speed and density of devices. These physical challenges are resulting into an exponentially growing effort to maintain Moore's law. Hence, completely different alternative ways may be soon required to keep the hundreds of billions of dollars VLSI industry alive. Since their introduction, carbon nanotubes have been envisioned as the next leap in electronic circuits due to many attractive features such as their ability to act as both transistors and interconnect as well as photo devices. This multifunctional behavior is very desirable in building a single monolithic technology based only on carbon nanotubes that can perform every function available with silicon and GaAs technologies. Also, the speed and power consumption of these devices are expected to be immensely superior to CMOS technologies. In addition, there may be noise and coupling advantages when using metallic carbon nanotubes as interconnect relative to the typical metals used presently.

The proposed research will focus on the design, simulation, and testing of SWNT circuits and interconnects as well as extracting circuit and system level characteristics. The specific topics that the PIs and their groups plan to conduct under this project are: Accurate models and simulations of simple SWNT circuits using different logic styles. Comparison of different logic styles used for SWNTs and consequently a possible outcome of the best suitable style for SWNT circuits. Establishment of testing and fabrication techniques that are cost-effective, simple, and replicable. Modeling of metallic SWNT interconnects. Modeling of SWNT-metal based hybrid interconnects.

*****NON-TECHNICAL ABSTRACT******
The increasing miniaturization of electronic circuitry that allows the development of faster computers and slimmer cell phones is presently facing two questions: what impediments are there to making today's technology even smaller? and can fundamentally new types of devices, based on quantum effects, be manufactured? This individual investigator award supports a project that will address both of these questions. One of the biggest obstacles to shrinking current devices is that as computer chips get smaller and smaller they heat up more and more. In addressing this problem the proposed research will attempt to understand the basic physics behind heat conduction for wires that are on the nanometer scale, which appears to differ from heat conduction in larger wires. This research will advance the drive towards fundamentally new types of devices by looking at the interplay between different materials on size scales small enough that the quantum interactions between these materials yield new physical effects that could be harnessed for future technologies. In addition to training graduate students and post-docs, this project will train undergraduates and high school students in basic techniques for making objects on the micro- and nanoscales.

*****TECHNICAL ABSTRACT******
This individual investigator award supports a project exploring the electrical and thermal properties of metallic, mesoscopic heterostructures composed of superconducting (S), normal-metal (N), and ferromagnetic (F) materials. The electrical transport experiments will examine a broad array of coherent processes caused by proximity effects between the materials. These experiments will include investigations into triplet superconductivity in FSF devices, non-local Andreev reflection in NSN and FSF devices, and tunable pi-junctions in SNS devices. The thermal transport experiments will pursue prior observations of anomalous behavior in nanoscale structures, including possible deviations from the Wiedemann-Franz law in normal-metal wires and unexplained symmetries in the thermopower of Andreev interferometers. While the most critical fabrication steps and milliKelvin temperature measurements of these devices will be undertaken by graduate students and post-docs, some nano- and microfabrication steps will be performed by undergraduate and local high school students, training them in the basics of e-beam and photolithography.

0960120
Chandrasekhar
Northwestern U.

Technical Summary:
This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).

This project is for the development of a facility to enable low-temperature, time-, frequency- and spatially-resolved radiofrequency and microwave measurements for a wide variety of experiments in condensed matter and materials physics at Northwestern University. Four sets of experiments will be attempted during the project period: investigation of cross-correlation noise in mesoscopic devices; magnetization dynamics of nanoscale ferromagnets; time-of-flight measurements in Luttinger liquids in semiconductor devices; and persistent currents in normal metals. However, the usefulness of this new facility will extend far beyond these initial experiments, involving other groups at Northwestern, and enabling research groups at Northwestern to actively incorporate high-frequency techniques in their future experimental research plans. It will also allow undergraduate and graduate students and post-docs at Northwestern to gain familiarity with radio and microwave frequency techniques, a skill that is becoming increasing important in both academia and industry.


Non-Technical Abstract:
This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).

The increasing miniaturization of electronic circuitry that allows the development of fast computers and slim cell phones is also accompanied by a desire to make these devices work even smaller and faster. Reducing the size of device elements to the scale of nanometers--one billionth the size of a meter--presents new experimental challenges in trying to study their behavior at time scales of a nanosecond or shorter, time scales that are increasingly important for cutting-edge electronic devices. This project is devoted to developing instrumentation to study the behavior of nanoscale materials at frequencies starting at a few gigahertz, the frequencies at which the current fastest desktop computers operate, to a few tens of gigahertz. In addition to providing insight into better ways to make the next generation of nanoscale, high-frequency devices, the instrumentation that will be developed will enable experiments exploring fundamental quantum phenomena at high frequency, and in the process train the next generation of scientists and engineers in these essential techniques.

****NON-TECHNICAL ABSTRACT****

According to quantum mechanics, small objects such as electrons behave like waves in addition to behaving like discrete particles. As waves, electrons show many interesting effects. An example is the interference of waves: think of the patterns of waves on the surface of a small pond when a stone is thrown in it. Quantum waves also encode the results of various interactions and correlations between particles, and devices that take advantage of these effects are the fundamental building blocks of many modern electronic devices. The effects of interference and correlations between electrons due to their wave nature can be seen by measuring the electric currents that are generated by them. However, these fragile interference and correlation effects are easily destroyed, and extremely low temperatures (close to absolute zero) and very sensitive measurement techniques are required to observe them. This project will explore correlations between electrons in metals that are induced by their interactions with a superconductor, which is a material that can carry an electrical current without resistance at very low temperatures. The experimental techniques that will be used include fabrication of very small devices that include metallic and superconducting parts, and measurements of the currents and voltages through these devices, as well as the electrical noise generated by these currents and voltages at very low temperatures. A major component of this project is the involvement of undergraduate and graduate students in research, who will learn sophisticated techniques of nanofabrication and device measurements. This training will enable the students to have careers in academia or high-technology industries.


****TECHNICAL ABSTRACT****

This project will explore quantum correlations induced in normal metals and ferromagnets due to their interactions with superconductors. In particular, the primary focus of this project will be on non-local correlations introduced in spatially separated normal metals and ferromagnets induced by crossed Andreev reflection (CAR) and elastic co-tunneling (EC). CAR and EC will be probed using nonlocal electrical transport measurements as well as noise measurements. The noise measurements will also be used to probe for spin entanglement of quasiparticles using samples with ferromagnetic elements. A second set of measurements will focus on the conditions under which one can observe superconducting correlations over long length scales in systems with finite spin polarization. While superconducting correlations have been observed in ferromagnets, the length scale over which these correlations extend has been extremely short. The new material systems that will be investigated may extend this length scale, and lead to the possibility of new superconducting devices in addition to furthering our understanding of exotic forms of superconductivity in ferromagnets. Integral to the project is the training of graduate and undergraduate students in nanofabrication and sophisticated measurement techniques.