Joyeeta Nag - Academia.edu (original) (raw)

Papers by Joyeeta Nag

Research paper thumbnail of Non-congruence of thermally driven structural and electronic transitions in VO2

Journal of Applied Physics, 2012

Coupled structural and electronic phase transitions underlie the multifunctional properties of st... more Coupled structural and electronic phase transitions underlie the multifunctional properties of strongly-correlated materials. For example, colossal magnetoresistance 1,2 in manganites involves phase transition from paramagnetic insulator to ferromagnetic metal linked to a structural Jahn-Teller distortion 3. Vanadium dioxide (VO 2) likewise exhibits an insulator-to-metal transition (IMT) at ~67 o C with abrupt changes in transport and optical properties and coupled to a structural phase transition (SPT) from monoclinic to tetragonal 4. The IMT and SPT hystereses are signatures of first-order phase transition tracking the nucleation to stabilization of a new phase. Here we have for the first time measured independently the IMT and SPT hystereses in epitaxial VO 2 films, and shown that the hystereses are not congruent. From the measured volume fractions of the two phases in the region of strong correlation, we have computed the evolving dielectric function under an effective-medium approximation. But the computed dielectric functions could not reproduce the measured IMT, implying that there is a strongly correlated metallic phase that is not in the stable rutile structure, consistent with Qazilbash et al 5. Search for a corresponding macroscopic structural intermediate also yielded negative result. The complex physics of VO 2 phase transition has long been debated 6-10. Unlike other strongly correlated materials 11 exhibiting IMT, such as V 2 O 3 12,13 , where phase transitions are satisfactorily explained by the Mott mechanism alone, the IMT in VO 2 is complicated by accompanying spin-Peierls instability 14 that leads via strong electron-phonon coupling

Research paper thumbnail of Ultralow thermal conductivity via topological network control of vibrational localization in amorphous chalcogenides

Amorphous chalcogenide alloys are key materials for data storage and energy scavenging applicatio... more Amorphous chalcogenide alloys are key materials for data storage and energy scavenging applications due to their large non-linearities in optical and electrical properties as well as low vibrational thermal conductivities. Here, we report on a mechanism to suppress the thermal transport in a representative amorphous chalcogenide system, silicon telluride (SiTe), by nearly an order of magnitude via systematically tailoring the cross-linking network among the atoms. As such, we experimentally demonstrate that in fully dense amorphous SiTe the thermal conductivity can be reduced to as low as 0.1 ± 0.01 W/m/K for high tellurium content with a density nearly twice that of amorphous silicon. Using ab-initio simulations integrated with lattice dynamics, we attribute the ultralow thermal conductivity of SiTe to the suppressed contribution of extended modes of vibration, namely propagons and diffusons. This leads to a large shift in the mobility edge - a factor of five - towards lower freque...

Research paper thumbnail of Tuning network topology and vibrational mode localization to achieve ultralow thermal conductivity in amorphous chalcogenides

Nature Communications, 2021

Amorphous chalcogenide alloys are key materials for data storage and energy scavenging applicatio... more Amorphous chalcogenide alloys are key materials for data storage and energy scavenging applications due to their large non-linearities in optical and electrical properties as well as low vibrational thermal conductivities. Here, we report on a mechanism to suppress the thermal transport in a representative amorphous chalcogenide system, silicon telluride (SiTe), by nearly an order of magnitude via systematically tailoring the cross-linking network among the atoms. As such, we experimentally demonstrate that in fully dense amorphous SiTe the thermal conductivity can be reduced to as low as 0.10 ± 0.01 W m−1 K−1 for high tellurium content with a density nearly twice that of amorphous silicon. Using ab-initio simulations integrated with lattice dynamics, we attribute the ultralow thermal conductivity of SiTe to the suppressed contribution of extended modes of vibration, namely propagons and diffusons. This leads to a large shift in the mobility edge - a factor of five - towards lower f...

Research paper thumbnail of Interface controlled thermal resistances of ultra-thin chalcogenide-based phase change memory devices

Nature Communications, 2021

Phase change memory (PCM) is a rapidly growing technology that not only offers advancements in st... more Phase change memory (PCM) is a rapidly growing technology that not only offers advancements in storage-class memories but also enables in-memory data processing to overcome the von Neumann bottleneck. In PCMs, data storage is driven by thermal excitation. However, there is limited research regarding PCM thermal properties at length scales close to the memory cell dimensions. Our work presents a new paradigm to manage thermal transport in memory cells by manipulating the interfacial thermal resistance between the phase change unit and the electrodes without incorporating additional insulating layers. Experimental measurements show a substantial change in interfacial thermal resistance as GST transitions from cubic to hexagonal crystal structure, resulting in a factor of 4 reduction in the effective thermal conductivity. Simulations reveal that interfacial resistance between PCM and its adjacent layer can reduce the reset current for 20 and 120 nm diameter devices by up to ~ 40% and ~...

Research paper thumbnail of 3Di DC-DC Buck Micro Converter with TSVs, Grind Side Inductors, and Deep Trench Decoupling Capacitors in 32nm SOI CMOS

2016 IEEE 66th Electronic Components and Technology Conference (ECTC), 2016

High performance processors and ASICs typically require multiple voltages and multi-domain voltag... more High performance processors and ASICs typically require multiple voltages and multi-domain voltage controls across the die. Conventional approaches distribute the voltage regulation elements between the processor, the package laminate, and the printed circuit board. We propose an alternative approach where the voltage regulator is embodied in a 3D configuration such that the inductor, capacitor and the switches are formed on a separate silicon chip sandwiched between the processor and the laminate. Due to the close proximity of regulator to the processor, this approach can enable granular voltage domains, while minimizing disruptions to the processor layout. We describe a 4-f DC-DC buck converter fabricated on 32nm SOI wafers using TSVs to connect the switches on the front-side of the wafer to the inductors on the grind-side. The process builds on a 32nm SOI CMOS flow, adding deep trench (DT) capacitors and TSV's. Down conversion from a standard I/O voltage under various load conditions was evaluated, and an efficiency of 77% was achieved.

Research paper thumbnail of Ultrafast Phase Transition via Catastrophic Phonon Collapse Driven by Plasmonic Hot-Electron Injection

Nano Letters, 2014

Ultrafast photoinduced phase transitions could revolutionize data-storage and telecommunications ... more Ultrafast photoinduced phase transitions could revolutionize data-storage and telecommunications technologies by modulating signals in integrated nanocircuits at terahertz speeds. In quantum phase-changing materials (PCMs), microscopic charge, lattice, and orbital degrees of freedom interact cooperatively to modify macroscopic electrical and optical properties. Although these interactions are well documented for bulk single crystals and thin films, little is known about the ultrafast dynamics of nanostructured PCMs when interfaced to another class of materials as in this case to active plasmonic elements. Here, we demonstrate how a mesh of gold nanoparticles, acting as a plasmonic photocathode, induces an ultrafast phase transition in nanostructured vanadium dioxide (VO 2) when illuminated by a spectrally resonant femtosecond laser pulse. Hot electrons created by optical excitation of the surface-plasmon resonance in the gold nanomesh are injected ballistically across the Au/VO 2 interface to induce a subpicosecond phase transformation in VO 2. Density functional calculations show that a critical density of injected electrons leads to a catastrophic collapse of the 6 THz phonon mode, which has been linked in different experiments to VO 2 phase transition. The demonstration of subpicosecond phase transformations that are triggered by optically induced electron injection opens the possibility of designing hybrid nanostructures with unique nonequilibrium properties as a critical step for all-optical nanophotonic devices with optimizable switching thresholds.

Research paper thumbnail of Ultrafast Phase Transition in Vanadium Dioxide Driven by Hot-Electron Injection

EPJ Web of Conferences, 2013

We present a novel all-optical method of triggering the phase transition in vanadium dioxide by m... more We present a novel all-optical method of triggering the phase transition in vanadium dioxide by means of ballistic electrons injected across the interface between a mesh of Au nanoparticles coveringd VO 2 nanoislands. By performing non-degenerate pump-probe transmission spectroscopy on this hybrid plasmonic/phase-changing nanostructure, structural and electronic dynamics can be retrieved and compared.

Research paper thumbnail of Ultrafast changes in lattice symmetry probed by coherent phonons

Nature Communications, 2012

The electronic and structural properties of a material are strongly determined by its symmetry. C... more The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that

Research paper thumbnail of Non-congruence of thermally driven structural and electronic transitions in VO2

Journal of Applied Physics, 2012

Coupled structural and electronic phase transitions underlie the multifunctional properties of st... more Coupled structural and electronic phase transitions underlie the multifunctional properties of strongly-correlated materials. For example, colossal magnetoresistance 1,2 in manganites involves phase transition from paramagnetic insulator to ferromagnetic metal linked to a structural Jahn-Teller distortion 3. Vanadium dioxide (VO 2) likewise exhibits an insulator-to-metal transition (IMT) at ~67 o C with abrupt changes in transport and optical properties and coupled to a structural phase transition (SPT) from monoclinic to tetragonal 4. The IMT and SPT hystereses are signatures of first-order phase transition tracking the nucleation to stabilization of a new phase. Here we have for the first time measured independently the IMT and SPT hystereses in epitaxial VO 2 films, and shown that the hystereses are not congruent. From the measured volume fractions of the two phases in the region of strong correlation, we have computed the evolving dielectric function under an effective-medium approximation. But the computed dielectric functions could not reproduce the measured IMT, implying that there is a strongly correlated metallic phase that is not in the stable rutile structure, consistent with Qazilbash et al 5. Search for a corresponding macroscopic structural intermediate also yielded negative result. The complex physics of VO 2 phase transition has long been debated 6-10. Unlike other strongly correlated materials 11 exhibiting IMT, such as V 2 O 3 12,13 , where phase transitions are satisfactorily explained by the Mott mechanism alone, the IMT in VO 2 is complicated by accompanying spin-Peierls instability 14 that leads via strong electron-phonon coupling

Research paper thumbnail of Ultralow thermal conductivity via topological network control of vibrational localization in amorphous chalcogenides

Amorphous chalcogenide alloys are key materials for data storage and energy scavenging applicatio... more Amorphous chalcogenide alloys are key materials for data storage and energy scavenging applications due to their large non-linearities in optical and electrical properties as well as low vibrational thermal conductivities. Here, we report on a mechanism to suppress the thermal transport in a representative amorphous chalcogenide system, silicon telluride (SiTe), by nearly an order of magnitude via systematically tailoring the cross-linking network among the atoms. As such, we experimentally demonstrate that in fully dense amorphous SiTe the thermal conductivity can be reduced to as low as 0.1 ± 0.01 W/m/K for high tellurium content with a density nearly twice that of amorphous silicon. Using ab-initio simulations integrated with lattice dynamics, we attribute the ultralow thermal conductivity of SiTe to the suppressed contribution of extended modes of vibration, namely propagons and diffusons. This leads to a large shift in the mobility edge - a factor of five - towards lower freque...

Research paper thumbnail of Tuning network topology and vibrational mode localization to achieve ultralow thermal conductivity in amorphous chalcogenides

Nature Communications, 2021

Amorphous chalcogenide alloys are key materials for data storage and energy scavenging applicatio... more Amorphous chalcogenide alloys are key materials for data storage and energy scavenging applications due to their large non-linearities in optical and electrical properties as well as low vibrational thermal conductivities. Here, we report on a mechanism to suppress the thermal transport in a representative amorphous chalcogenide system, silicon telluride (SiTe), by nearly an order of magnitude via systematically tailoring the cross-linking network among the atoms. As such, we experimentally demonstrate that in fully dense amorphous SiTe the thermal conductivity can be reduced to as low as 0.10 ± 0.01 W m−1 K−1 for high tellurium content with a density nearly twice that of amorphous silicon. Using ab-initio simulations integrated with lattice dynamics, we attribute the ultralow thermal conductivity of SiTe to the suppressed contribution of extended modes of vibration, namely propagons and diffusons. This leads to a large shift in the mobility edge - a factor of five - towards lower f...

Research paper thumbnail of Interface controlled thermal resistances of ultra-thin chalcogenide-based phase change memory devices

Nature Communications, 2021

Phase change memory (PCM) is a rapidly growing technology that not only offers advancements in st... more Phase change memory (PCM) is a rapidly growing technology that not only offers advancements in storage-class memories but also enables in-memory data processing to overcome the von Neumann bottleneck. In PCMs, data storage is driven by thermal excitation. However, there is limited research regarding PCM thermal properties at length scales close to the memory cell dimensions. Our work presents a new paradigm to manage thermal transport in memory cells by manipulating the interfacial thermal resistance between the phase change unit and the electrodes without incorporating additional insulating layers. Experimental measurements show a substantial change in interfacial thermal resistance as GST transitions from cubic to hexagonal crystal structure, resulting in a factor of 4 reduction in the effective thermal conductivity. Simulations reveal that interfacial resistance between PCM and its adjacent layer can reduce the reset current for 20 and 120 nm diameter devices by up to ~ 40% and ~...

Research paper thumbnail of 3Di DC-DC Buck Micro Converter with TSVs, Grind Side Inductors, and Deep Trench Decoupling Capacitors in 32nm SOI CMOS

2016 IEEE 66th Electronic Components and Technology Conference (ECTC), 2016

High performance processors and ASICs typically require multiple voltages and multi-domain voltag... more High performance processors and ASICs typically require multiple voltages and multi-domain voltage controls across the die. Conventional approaches distribute the voltage regulation elements between the processor, the package laminate, and the printed circuit board. We propose an alternative approach where the voltage regulator is embodied in a 3D configuration such that the inductor, capacitor and the switches are formed on a separate silicon chip sandwiched between the processor and the laminate. Due to the close proximity of regulator to the processor, this approach can enable granular voltage domains, while minimizing disruptions to the processor layout. We describe a 4-f DC-DC buck converter fabricated on 32nm SOI wafers using TSVs to connect the switches on the front-side of the wafer to the inductors on the grind-side. The process builds on a 32nm SOI CMOS flow, adding deep trench (DT) capacitors and TSV's. Down conversion from a standard I/O voltage under various load conditions was evaluated, and an efficiency of 77% was achieved.

Research paper thumbnail of Ultrafast Phase Transition via Catastrophic Phonon Collapse Driven by Plasmonic Hot-Electron Injection

Nano Letters, 2014

Ultrafast photoinduced phase transitions could revolutionize data-storage and telecommunications ... more Ultrafast photoinduced phase transitions could revolutionize data-storage and telecommunications technologies by modulating signals in integrated nanocircuits at terahertz speeds. In quantum phase-changing materials (PCMs), microscopic charge, lattice, and orbital degrees of freedom interact cooperatively to modify macroscopic electrical and optical properties. Although these interactions are well documented for bulk single crystals and thin films, little is known about the ultrafast dynamics of nanostructured PCMs when interfaced to another class of materials as in this case to active plasmonic elements. Here, we demonstrate how a mesh of gold nanoparticles, acting as a plasmonic photocathode, induces an ultrafast phase transition in nanostructured vanadium dioxide (VO 2) when illuminated by a spectrally resonant femtosecond laser pulse. Hot electrons created by optical excitation of the surface-plasmon resonance in the gold nanomesh are injected ballistically across the Au/VO 2 interface to induce a subpicosecond phase transformation in VO 2. Density functional calculations show that a critical density of injected electrons leads to a catastrophic collapse of the 6 THz phonon mode, which has been linked in different experiments to VO 2 phase transition. The demonstration of subpicosecond phase transformations that are triggered by optically induced electron injection opens the possibility of designing hybrid nanostructures with unique nonequilibrium properties as a critical step for all-optical nanophotonic devices with optimizable switching thresholds.

Research paper thumbnail of Ultrafast Phase Transition in Vanadium Dioxide Driven by Hot-Electron Injection

EPJ Web of Conferences, 2013

We present a novel all-optical method of triggering the phase transition in vanadium dioxide by m... more We present a novel all-optical method of triggering the phase transition in vanadium dioxide by means of ballistic electrons injected across the interface between a mesh of Au nanoparticles coveringd VO 2 nanoislands. By performing non-degenerate pump-probe transmission spectroscopy on this hybrid plasmonic/phase-changing nanostructure, structural and electronic dynamics can be retrieved and compared.

Research paper thumbnail of Ultrafast changes in lattice symmetry probed by coherent phonons

Nature Communications, 2012

The electronic and structural properties of a material are strongly determined by its symmetry. C... more The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that