Md Shafayat Hossain | Princeton University (original) (raw)

Papers by Md Shafayat Hossain

Proceedings of the National Academy of Sciences, 2020

What are the ground states of an interacting, low-density electron system? In the absence of diso... more What are the ground states of an interacting, low-density electron system? In the absence of disorder, it has long been expected that as the electron density is lowered, the exchange energy gained by aligning the electron spins should exceed the enhancement in the kinetic (Fermi) energy, leading to a (Bloch) ferromagnetic transition. At even lower densities, another transition to a (Wigner) solid, an ordered array of electrons, should occur. Experimental access to these regimes, however, has been limited because of the absence of a material platform that supports an electron system with very high quality (low disorder) and low density simultaneously. Here we explore the ground states of interacting electrons in an exceptionally clean, two-dimensional electron system confined to a modulation-doped AlAs quantum well. The large electron effective mass in this system allows us to reach very large values of the interaction parameter rs, defined as the ratio of the Coulomb to Fermi energies. As we lower the electron density via gate bias, we find a sequence of phases, qualitatively consistent with the above scenario: a paramagnetic phase at large densities, a spontaneous transition to a ferromagnetic state when rs surpasses 35, and then a phase with strongly nonlinear current-voltage characteristics, suggestive of a pinned Wigner solid, when rs exceeds ≃38. However, our sample makes a transition to an insulating state at rs≃27, preceding the onset of the spontaneous ferromagnetism, implying that besides interaction, the role of disorder must also be taken into account in understanding the different phases of a realistic dilute electron system.

Nature Physics, 2020

In 1929, Felix Bloch suggested that the paramagnetic Fermi sea of electrons should make a spontan... more In 1929, Felix Bloch suggested that the paramagnetic Fermi sea of electrons should make a spontaneous transition to a fully magnetized state at very low densities, because the exchange energy gained by aligning the spins exceeds the enhancement in the kinetic energy1. However, experimental realizations of this effect have been hard to implement. Here, we report the observation of an abrupt, interaction-driven transition to full magnetization, highly reminiscent of Bloch ferromagnetism. Our platform utilizes the two-dimensional Fermi sea of composite fermions near half-filling of the lowest Landau level. We measure the Fermi wavevector—which directly provides the spin polarization—and observe a sudden transition from a partially spin-polarized to a fully spin-polarized ground state as we lower the density of the composite fermions. Our theoretical calculations that take Landau level mixing into account provide a semi-quantitative account of this phenomenon.

Phys. Rev. Lett., 2020

A fundamental concept in physics is the Fermi surface, the constant-energy surface in momentum sp... more A fundamental concept in physics is the Fermi surface, the constant-energy surface in momentum space encompassing all the occupied quantum states at absolute zero temperature. In 1960, Lut-tinger postulated that the area enclosed by the Fermi surface should remain unaffected even when electron-electron interaction is turned on, so long as the interaction does not cause a phase transition. Understanding what determines the Fermi surface size is a crucial and yet unsolved problem in strongly interacting systems such as high-Tc superconductors. Here we present a precise test of the Luttinger theorem for a two-dimensional Fermi liquid system where the exotic quasi-particles themselves emerge from the strong interaction, namely for the Fermi sea of composite fermions (CFs). Via direct, geometric resonance measurements of the CFs' Fermi wavevector down to very low electron densities, we show that the Luttinger theorem is obeyed over a significant range of interaction strengths, in the sense that the Fermi sea area is determined by the density of the minority carriers in the lowest Landau level. Our data also address the ongoing debates on whether or not CFs obey particle-hole symmetry, and if they are Dirac particles. We find that particle-hole symmetry is obeyed, but the measured Fermi sea area differs quantitatively from that predicted by the Dirac model for CFs. Composite fermions (CFs) are emergent quasi-particles of a strongly interacting, two-dimensional electron system (2DES) at high perpendicular magnetic fields when the electrons' kinetic energy is quenched into a set of quantized energy levels, the so-called Landau levels (LLs) [1-3]. In the lowest LL, the electrons have no kinetic energy and the system is a prime example of a flat band system where interaction dominates the physics. When the lowest LL is half-filled, i.e., the LL filling factor (ν) equals 1/2, the interacting electrons each pair with an even number of flux quanta and form flux-electron CFs that condense into a metallic phase with a well-defined Fermi sea [Fig. 1(a)] [1-3]. Thus the CF Fermi sea is born out of strong interaction within a flatband system, and provides an ideal platform to test the Luttinger theorem [4], a major theorem in many-body physics which postulates that the Fermi sea and its area should be resilient against interaction. Here we investigate the validity of the Luttinger theorem and its link [5, 6] to particle-hole symmetry [Fig. 1(b)], in a nearly half-filled, flatband system of interacting CFs. According to the CF theory [1-3], the flux-electron CFs ignore the large, external magnetic field (B) and only experience an effective magnetic field B * = B − B ν=1/2 , where B ν=1/2 is the field at ν = 1/2, ν = hn/eB is the LL filling factor, and n is the 2DES density. Near ν = 1/2 CFs execute cyclotron motion in a small B * , similarly to their electron counterparts near B = 0 [2, 3, 7-11].

Phys. Rev. B, 2019

Two-dimensional interacting electrons exposed to strong perpendicular magnetic fields generate em... more Two-dimensional interacting electrons exposed to strong perpendicular magnetic fields generate emergent, exotic quasiparticles phenomenologically distinct from electrons. Specifically, electrons bind with an even number of flux quanta, and transform into composite fermions (CFs). Besides providing an intuitive explanation for the fractional quantum Hall states, CFs also possess Fermi-liquid-like properties, including a well-defined Fermi sea, at and near even-denominator Landau level filling factors such as ν = 1/2 or 1/4. Here, we directly probe the Fermi sea of the rarely studied four-flux CFs near ν = 1/4 via geometric resonance experiments. The data reveal some unique characteristics. Unlike in the case of two-flux CFs, the magnetic field positions of the geometric resonance resistance minima for ν < 1/4 and ν > 1/4 are symmetric with respect to the position of ν = 1/4. However, when an in-plane magnetic field is applied, the minima positions become asymmetric, implying a mysterious asymmetry in the CF Fermi sea anisotropy for ν < 1/4 and ν > 1/4. This asymmetry, which is in stark contrast to the two-flux CFs, suggests that the four-flux CFs on the two sides of ν = 1/4 have very different effective masses, possibly because of the proximity of the Wigner crystal formation at small ν. Ultra-low-disorder two-dimensional electron systems (2DESs) subjected to a perpendicular magnetic field (B ⊥) give rise to a plethora of quantum many-body phases of matter. Many of these phases can be understood based on composite fermions, quasiparticles comprised of an electron and an even number of flux quanta [1-3]. Near Landau level (LL) filling factor ν = 1/2, e.g., an electron merges with two flux quanta to form a two-flux composite fermion (2 CF). While the electron system is highly interacting and is in a high B ⊥ , the 2 CFs behave as essentially non-interacting particles and only feel an effective magnetic field B * = B − B ν=1/2 , where B ν=1/2 is the field at ν = 1/2. Importantly, these 2 CFs occupy a Fermi sea at ν = 1/2 and can execute cyclotron motion near ν = 1/2 at small B * , similar to their fermion counterparts near B = 0 [3]. With

International Conference on Informatics, Electronics & Vision (ICIEV), 2012

This paper illustrates a secure and reliable system for remotely controlling electrical appliance... more This paper illustrates a secure and reliable system for remotely controlling electrical appliances by Short Messaging Service (SMS) and/or internet. The system requires serial interfacing of two micro-controllers with SIEMENS AX75 cell phone and a personal computer respectively. A microcontroller unit was used to merge the independent subsystems for SMS and internet based control. The micro-controllers were programmed in C and a webpage was created using PHP for internet based controlling. The proposed parallel controlling system incorporates user verification and registration, password protection and RF communication between central controller and the remote receiver segments. The overall scheme offers advantages over so far reported rigid control systems, which involve high cost GSM or GPRS modems. Also the proposed scheme can serve the purpose of appliance controlling in a large scale.

IEEE International Conference on Electron Devices and Solid State Circuit (EDSSC), 2012

The 3-D poisson's equation with eight boundary conditions is solved analytically and an analytica... more The 3-D poisson's equation with eight boundary conditions is solved analytically and an analytical model of potential profile and threshold voltage for rectangular gate-all-around III-V nanowire MOSFET device is developed with quantum correction. Dependence of threshold voltage on channel width, oxide thickness, gate-length, doping and channel material composition are determined from the developed model with experimental and ATLAS verification. The model agrees well with experimental and simulation results and offers an insight to the device performance.

IEEE International Conference of Electron Devices and Solid-state Circuits, 2013

Carbon nanotube (CNT) shows near-ballistic transport and therefore has become a topic of extensiv... more Carbon nanotube (CNT) shows near-ballistic transport and therefore has become a topic of extensive research. Cylindrical CNT field effect transistor (CNTFET) shows better performance than double gate CNTFETs which is simulated here self-consistently using tight-binding approximation exploiting the coaxial symmetry taking intrinsic (10, 0) zigzag nanotube as channel material, doped and gate-overlapped source/drain . The effect of temperature on transport properties is examined in ballistic regime. The analysis reveals that drain current is almost independent of temperature, where the sub-threshold swing decreases significantly with temperature and transconductance falls down at lower temperature for sub-0V gate bias.

IEEE International Conference on Electron Devices and Solid State Circuit (EDSSC), 2012

This paper presents the ballisitic current limit and gate leakage due to direct tunneling of a Re... more This paper presents the ballisitic current limit and gate leakage due to direct tunneling of a Rectangular Gate-all-around InGaAs Nanowire Transistor and their variation with fin width, oxide thickness and In compostion in InGaAs. Ballistic current is found to be higher (1.5×10 11 Am -2 ) for about 20nm fin width, sub-5nm oxide thickness and In-rich InGaAs channel. On the other hand, gate leakage is prominent for sub-4nm oxide thickness, larger fin width and In-rich InGaAs.

7th International Conference on Electrical and Computer Engineering, 2012

This paper presents the characterization of interface trap charge for 30 nm In 0.53 Ga 0.47 As ch... more This paper presents the characterization of interface trap charge for 30 nm In 0.53 Ga 0.47 As channel Gate-all-around field effect transistor (GAAFET) using ALD Al 2 0 3 as the oxide. The interface trap charge density (D it ) is extracted from CV model through self consistent iterations. The CV model is formulated by self consistent Schrödinger-Poisson solver. Wave function penetration effect has been considered while solving the Schrodinger equation. The result gives an intuition about the D it profile. The difference between the initial assumed CV and the final CV demonstrates the effect of D it on the CV profile of the device. We repeated the same for In .65 Ga .35 As and In .75 Ga .25 As and then did a comparative study of the devices. The donor-like traps dominate the D it profile for higher mole-fraction of In in InGaAs which in our case is In 0.75 Ga 0.25 As. The stronger inversion of In-rich channels and hence, better transport characteristics is evident from these results.

7th International Conference on Electrical and Computer Engineering, 2012

This paper presents quantum definition based threshold voltage calculation of Gate-All-Around InG... more This paper presents quantum definition based threshold voltage calculation of Gate-All-Around InGaAs nanowire transistor. Though similar determination was previously established for TG FinFETs in recent literature, application of this method on Gate-All-Around Nanowire Transistor is yet to be done. A self-consistent solver, which takes wave function penetration and other quantum mechanical effects into account, has been used here to establish the capacitance-voltage characteristics that have been used for threshold voltage calculation. Using the extracted threshold voltages, effect of channel width and channel material composition variation on threshold has been studied and a modification of classical analytical formula is proposed based on a fitting parameter.

International Conference on Electrical and Computer Engineering, 2014

Since the fabrication of first III-V Gate-All-Around (GAA) MOSFET it is under extensive research,... more Since the fabrication of first III-V Gate-All-Around (GAA) MOSFET it is under extensive research, as it is one of the potential candidates to replace the state of art tri-gate FinFETs, to continue progressive scaling. In this work, transport characterization of experimentally demonstrated gate-all-around (GAA) In x Ga 1-x As nanowire MOSFET in near-ballistic regime is performed using 3D self-consistent Schrödinger-Poisson solver based on Uncoupled Mode Space approach, taking wave function penetration and other quantum mechanical effects into account. The effects of channel length variation on transport characteristics are also examined.

International Conference on Informatics, Electronics & Vision (ICIEV), 2012

Bangladesh, not being bestowed with fossil fuels is compelled to rely on renewable energy sources... more Bangladesh, not being bestowed with fossil fuels is compelled to rely on renewable energy sources. Despite having potentiality as sustainable energy sources, due to the lack of proper planning, biogas and biomass contribution to master plan by Bangladesh government of having 5% of total generation from renewable energy sources will be insufficient. Even IDCOL, the leading contributor in this field is struggling to keep up with their envisioned target of 37,269 Biogas plants within Dec. 2012 as their trend of installment estimate only 29,320 plants at end of year 2012. By 2015, if their current scheme sustains, only 3268 cubic meter per year for biogas will be produced and 14.5 million tons of coal equivalent for biomass generation will be insufficient comparing to demand of 66.46 million tons of coal equivalent. Trend analysis by means of least mean square algorithm is presented to show the demand-supply discrepancy up to 2015.

2nd International Conference on Electronic Design (ICED), 2014

Gate-all-around structure with III-V channel material shows improved channel performance with hig... more Gate-all-around structure with III-V channel material shows improved channel performance with high carrier mobility and less short channel effect and therefore is being studied rigorously for next generation transistors. We propose an analytical model to calculate gate capacitance and drain current of gate-all-around (GAA) nanowire MOSFET, a prospective device to replace the state-of-art FinFET in near future as per ITRS roadmap. The gate capacitance in strong inversion region is modeled incorporating quantum mechanical effects which are verified against the results obtained from self-consistent simulation of Schrödinger-Poisson equation appeared in recent literature. This model can also be extended for calculating gate capacitance in strong inversion region of different Multi-gate MOSFETs. A Spice compatible analytic model for drain current is also proposed which shows excellent agreement with the reported results of experimentally demonstrated In 0.53 Ga 0.47 As (2×10 16 /cm 3 ) GAA MOSFET. Using the proposed formula for gate capacitance in strong inversion region and drain current together with semi-numerical ballistic MOSFET model, the performance of In 0.53 Ga 0.47 As (2×10 16 /cm 3 ) GAA MOSFET is examined. This device is found suitable for ultra-high performance application with very high intrinsic cut-off frequency resulting from very low gate delay and very high on current and gate capacitance. The proposed analytical model of gate capacitance utilizes a modified form of co-axial cable capacitance along with the quantum capacitance limit to form a computationally efficient formula that is in well agreement with the results appeared in recent literature. On the other hand, Landauer-Buttiker formula and compact model for drain current of planar bulk-MOSFET are utilized to form the model for analytic drain current that shows excellent agreement with the experimental results appeared in the literature in recent past. The proposed model can be used for Spice modeling and circuit simulation of In 0.53 Ga 0.47 As GAA MOSFET. Moreover, this model is flexible and can be modified for other high performance multi-gate nano-devices.

3rd International Conference on the Developments in Renewable Energy Technology (ICDRET), 2014

Limited fossil fuels will soon not be enough to meet up the energy-shortage in Bangladesh and the... more Limited fossil fuels will soon not be enough to meet up the energy-shortage in Bangladesh and therefore enhanced attention must be given to utilize renewable energy sources. Solar and wind are two prominent renewable energy sources in Bangladesh. We propose an optimum decentralized solar PV-wind-biomass-fossil fuel based hybrid power system to meet up the energy-demand of a typical community in an environment-friendly way. The potential of the decentralized hybrid system in Bangladesh is estimated utilizing Geospatial toolkit, NASA SSE solar radiation data and HOMER optimization software. Financial viability of the system for Bangladesh is also assessed utilizing a proposed decentralized hybrid system using HOMER for Rangpur which has unique high solar (4.75 kWh/m2/day) and high wind (over 2m/s wind-speed throughout the year with 250W/m2 power density) supply in Bangladesh. The annual electricity generation of the proposed system is almost 21.55 MWh and its cost of energy is less in Rangpur (minimum 0.246 $/kWh) relative to other areas due to its rich green energy resources. The proposed system, simulated for four different fuel-combinations, is most cost-effective but most detrimental to the environment if the generator is run with natural gas. On the other hand, Co-firing with biomass results in doubling the cost but reduces pollutant-generation to almost 50%.

UKSim 14th International Conference on Computer Modelling and Simulation, 2012

This paper suggests two alternative schemes to avoid the use of hydraulic and pneumatic horns res... more This paper suggests two alternative schemes to avoid the use of hydraulic and pneumatic horns responsible for severe sound pollution. Cost effective signaling systems using radio frequency (RF) and infra-red (IR) have been developed and working circuitry has been implemented to replace the conventional horns. The greater impact of these signaling schemes on the environment has been discussed-especially in case of reducing sound pollution. Use of global positioning system (GPS) and cellular data network has been avoided intentionally to ensure simplicity, cost-effectiveness and independent operation. Special emphasis has been put to ensure compatibility with any types of vehicles and thereby avoid factory level modification. These systems are successfully tested by establishing communication between two cars. IR communication has been found faster and more reliable at a smaller distance than RF communication. Both of these communication systems can be a means of cost-effective reduction of sound pollution.

UKSim 14th International Conference on Computer Modelling and Simulation, 2012

The y4 copter is intended to be a UAV (Unmanned Aerial Vehicle) with autonomous subsystems. Autom... more The y4 copter is intended to be a UAV (Unmanned Aerial Vehicle) with autonomous subsystems. Automation is implemented in case of maintaining the desired roll, pitch and yaw. An improved algorithm that uses Quaternion is implemented to determine Euler angles avoiding Gimbal lock. It has a self-stabilization system achieved using Proportional Integral Derivative (PID) control system. A custom remote is built to communicate with the y4 copter using XBee modules which offer long range and security. Secured data transmission between the controller and the copter makes it suitable for military applications. Perhaps the most significant purpose of this y4 copter is to exercise a low cost and efficient engineering design with total automation considering all possible aerodynamics.

ECS Transactions, 2013

Carbon NanoTube (CNT) channel promises near ballistic transport and is examined extensively for p... more Carbon NanoTube (CNT) channel promises near ballistic transport and is examined extensively for potential application in next-generation Nano-scale transistor. Cylindrical gate-all-around CNT field effect transistor (CNTFET) with semiconducting CNT S/D contacts and Schottky barrier CNTFET (SBCNTFET) are modeled numerically based on self-consistent solution of Poisson-Schrödinger equation with open boundary conditions within Non-Equilibrium Green’s Function formalism considering coaxial symmetry. The effect of dielectric constant (κ) and physical thickness of gate dielectric on transport characteristics of CNTFET and SBCNTFET are examined in this work. Gate capacitance, carrier injection velocity, current and therefore speed increases significantly with the inclusion of high κ dielectric and decrease in thickness. SBCNTFET also shows better performance with thin high κ dielectric layer. High trans-conductance and much lower Sub-threshold swing is visible with the inclusion of high κ gate stack.

ECS Journal of Solid State Science and Technology, 2013

Due to ultra-long mean free path of Carbon nanotube (CNT), CNT field effect transistor (CNTFET) i... more Due to ultra-long mean free path of Carbon nanotube (CNT), CNT field effect transistor (CNTFET) is supposed to operate in ballistic regime and therefore CNTEFT is under extensive research for its application in high performance devices replacing conventional silicon technology. Surrounding gate topology which is better performing than back-gated structure is considered for physical self-consistent modeling under NEGF framework using tight binding approximation. A detailed study of transport in ballistic regime reveals that equally doped source/drain (S/D) and channel with no potential gradient, short S/D about 5 nm, gate length close to 10 nm and (13, 0) zigzag CNT channel is optimum for designing high performance CNTFET.

ECS Transactions, 2013

This work demonstrates a self-consistent 2-D numerical model for calculating the charge profile a... more This work demonstrates a self-consistent 2-D numerical model for calculating the charge profile and gate capacitance and therefore obtaining C-V characteristics of a gate-all-around III-V nanowire transistor with a high mobility In0.53Ga0.47As channel and atomic layer deposited Al2O3/20nm WN gate stacks which has recently been demonstrated experimentally. Finite element method is used to solve Poisson’s equation and Schrödinger’s equation in a coupled manner taking wave function penetration, energy level splitting and other quantum effects into account while calculating the charge profile and gate capacitance for different gate bias. The functional dependence of C-V characteristics on different physical/process parameters i.e. alloy composition, oxide thickness, fin-width, doping density are explored as well.

International Journal of Numerical Modelling: Electronic Networks, Devices and Fields, 2015

High‐κ gate‐all‐around structure counters the Short Channel Effect (SCEs) mostly providing excell... more High‐κ gate‐all‐around structure counters the Short Channel Effect (SCEs) mostly providing excellent off‐state performance, whereas high mobility III–V channel ensures better on‐state performance, rendering III–V nanowire GAAFET a potential candidate for replacing the current FinFETs in microchips. In this paper, a 2D simulator for the III–V GAAFET based on self‐consistent solution of Schrodinger–Poisson equation is proposed. Using this simulator, capacitance–voltage profile and threshold voltage are characterized, which reveal that gate dielectric constant (κ) and oxide thickness do not affect threshold voltage significantly at lower channel doping. Moreover, change in alloy composition of InxGa1‐xAs, channel doping, and cross‐sectional area has trivial effects on the inversion capacitance although threshold voltage can be shifted by the former two. Although, channel material also affects the threshold voltage, most sharp change in threshold voltage is observed with change in fin width of the channel (0.005 V/nm for above 10 nm fin width and 0.064 V/nm for sub‐10 nm fin width). Simulation suggests that for lower channel doping below 1023 m−3, fin width variation affects the threshold voltage most. Whereas when the doping is higher than 1023 m−3, both the thickness and dielectric constant of the oxide material have strong effects on threshold voltage (0.05 V/nm oxide thickness and 0.01 V/per unit change in κ)