author(s) : shwetha g. bhat and dr. p.s. anil kumar 3o4s) : shwetha g. bhat and dr. p.s. anil kumar...
TRANSCRIPT
Author(s) : Shwetha G. Bhat and Dr. P.S. Anil Kumar
Affiliation(s) : Department of Physics, Indian Institute of Science, Bangalore
– 560012, INDIA
Email of Presenter : [email protected]
Title : Study on Fe3O4/MgO/GaAs systems for spin injection
experiments
Abstract:
It is seen that the insertion of insulator in between the ferromagnet and
semiconductor always increases the efficiency of spin injection of ferromagnet into
semiconductor eliminating the problem of resistance mismatch between the two.
Fe3O4 is believed to be a half metallic ferrimagnet, and hence it can be expected to
have a very high spin polarization. MgO being a very good insulator with the
selective Δ-like energy bands has a very good lattice match with Fe3O4 and GaAs.
The growth of oriented films is confirmed by x-ray diffraction measurement. The
magnetization measurement of Fe3O4 shows a clear Verwey transition around 120K
as well as a clear M-H loop at room temperature. Devices of Fe3O4/MgO/GaAs
(doped) were fabricated to carry out the electrical Hanle measurements with the 3-
terminal non-linear (NL) geometry. From the I-V measurements it is seen that the
junction behaves purely as a tunnel junction. The transport measurements clearly
indicate the spin injection from Fe3O4 to GaAs through MgO as a function of the
tunnel current. Depending the nature of current either injection or accumulation
current, the shape of the plot of NL voltage vs. magnetic field is accordingly modified.
If the insulator thickness is reduced, then the NL voltage showed no dependency of
magnetic field, with I-V characteristic being Schottky natured.
Author(s) : Deep Narayan Biswas, Partha Sarathi Mandal, Shyma R.
Varier, Nishaina Sahadev and Kalobaran Maiti
Affiliation(s) : TATA INSTITUTE OF FUNDAMENTAL RESEARCH
Email of Presenter : [email protected]
Title : Evidence of unusual spin polarization of the surface states of
W(110) surface.
Abstract:
We studied the surface electronic structure of W(110) surface employing spin and
angle resolved photoemission spectroscopy. Experimental results exhibit highly
dispersive linear bands corresponding to the surface states and signature of Dirac
cones. Spin resolved spectra exhibit unusual spin character of the energy.
Author(s) : Debmalya Chakraborty and Amit Ghosal
Affiliation(s) : Indian Institute of Science Education and Research, Kolkata
Email of Presenter : [email protected]
Title : Impurity Effects on Strongly Correlated d-wave
Supercondutors
Abstract:
The effect of impurities on strongly correlated High Tc Superconductors (HTSC) is
investigated in the framework of disordered Hubbard Model. The strong electronic
correlation, inherent to the undoped parent HTSC compounds (Mott insulator), is
treated using Gutzwiller approximation leading to removal of double occupancy. The
resulting t-J like model with all terms up to order t2/U, but with inhomogenously
renormalized parameters, is treated within Bogoliubov-de Gennes mean field theory
that captures the physics of spatial inhomogeneity in all the order parameters. Our
results for single particle density of states (DOS) as well as two particle correlation
functions, such as, Off-diagonal Long Range order (ODLRO), show that the strong
correlations make the superconductor robust to impurities than the equivalent
uncorrelated case. This robustness is primarily due to a complex interplay of spatial
organization of different orders in the background of a significantly smaller ‘effective’
disorder. Our finding is justified through detailed analysis of DOS, ODLRO and
spatial profiles of different order parameters. It will be demonstrated that the effect of
a single impurity plays a crucial role in explaining our qualitative findings.
Author(s) : Narjes Gorjizadeh, Su Ying Quek
Affiliation(s) : Institute of High Performance Computing (IHPC)
Email of Presenter : [email protected]
Title : Interface Effects on Tunneling Magnetoresistance in Organic
Spintronics: Can Amine-Au link groups with Au/Fe Leads give
Reproducible Magnetoresistance?
Abstract:
Organics are promising alternatives to conventional semiconductors for spintronics
applications. Besides low cost and mechanical flexibility, the weak spin-orbit
interaction in organic systems can give rise to longer spin-coherence time and
distances compared to inorganic semiconductors.1 Chemical functionalization can
also lead to magnetic centers within the molecules, with the potential for interesting
spintronic effects. However, organic spintronics still lacks a benchmark system with
reproducible magnetoresistance (TMR), TMR being highly sensitive to interface
effects. A key breakthrough in molecular electronics was the discovery of amine-Au
and other link groups that give reproducible conductance.2 Here, we use first
principles calculations to explore if the same amine-Au link groups can give
reproducible magnetoresistance with Au-covered Fe leads. We show that the TMR
can be larger and less sensitive to contact geometry than in previously studied thiol-
Ni junctions. However, the TMR is in general very sensitive to symmetry and to the
number of Au layers in the junction, due to complications from interface and quantum
well states. Our results pave the way for understanding how to achieve reproducible
TMR in organic spintronics, and sheds light on origins of TMR in experiments where
the organic is separated from magnetic electrodes by a non-magnetic spacer.
References
1. Rocha et al, Nature Materials 4, 335 (2005)
2. Venkataraman, et al, Nano Lett. 6, 458 (2006)
Author(s) : Priti Gupta*, A. A. Rahman, Nirupam Hatui, Mahesh Gokhale
Mandar M. Deshmukh and Arnab Bhattacharya
Affiliation(s) : Department of Condensed Matter Physics and Materials
Science, Tata Institute of Fundamental Research, Mumbai,
India
Email of Presenter : [email protected]
Title : Group III-Nitride quantum wells grown on CVD graphene
Abstract:
Graphene films are stable at high-temperature, flexible, optically transparent,
and have good electrical and thermal conductivities. Chemical vapour deposition
(CVD) can be used to synthesize large area films of graphene, which when used as
a substrate for III-N growth may make it possible to realize novel devices [1]. We
have recently reported the MOVPE growth of III-N films on graphene allowing a facile
method to make free-standing nitride layers [2]. In this work we report the synthesis
and optical characterization of semi-polar InGaN/GaN and GaN/AlGaN quantum well
structures grown on a graphene layer.
CVD-grown graphene layers on Cu were transferred onto SiO2-coated silicon
substrates. The III-N layers were deposited by a low pressure MOVPE using
standard precursors. We have grown various AlGaN alloys on AlN buffer layers, and
optimized the MOVPE deposition parameters such as temperature, V/III ratio and
pressure to obtain, under appropriate conditions, (10-11) oriented semi-polar layers.
Various multi-quantum well stacks of GaN(QW)/AlGaN(barrier) and InGaN
(QW)/GaN (barrier) were grown. These layers, though polycrystalline, are of high
quality and show strong room temperature photoluminescence (PL) (Fig. 1) and
should pave the way to develop optoelectronic nitride devices on graphene.
Fig. 1: Room/low temperature PL from a 20-period GaN/AlGaN multiquantum
well stack grown on graphene
3. References
[1] K. Chung, C-H. Lee and G-C. Yi, Science 330, (2010) 655;
[2] P. Gupta, A.A. Rahman, et al., “MOVPE growth of (Al, Ga)N on graphene”, Paper
Fr. A2-1, Proc. Intl. conference on MOVPE, ICMOVPE-16, May 20-25,2012, Busan
Korea
Author(s) : Tran Thi Thu Hanh, Yoshinari Takimoto, and Osamu Sugino
Affiliation(s) : The Institute for Solid State Physics, The University of Tokyo,
Kashiwa, Chiba 277-8581, Japan
Email of Presenter : [email protected]
Title : Ab initio Modeling of the Hydrogen Adsorption on Pt(111)
Surface
Abstract:
The hydrogen adsorption on the Pt(111) electrode surface has been intensively
investigated. To gain insight into detailed atomistic picture on the equilibrium
coverage and structure, we have constructed a lattice gas model by determining the
on-site energy and the interaction parameters using the first principles total-energy
calculation. Therein atop, fcc, hcp and bridge sites are covered by hydrogen atoms
under various coverage conditions (0ML < Θ ≤ 1ML) and the total-energy
calculations are done for the (1x1), (2x2) and (3x3) cells. The total-energies of (3x3)
cell, corrected by the zero-point energy (ZPE), are found well fitted to the lattice gas
model. With this model, the Monte Carlo (MC) simulation has been performed. The
first-principles calculation combined with MC simulation successfully explains the
interaction of H atoms on the Pt(111) surface. We are calculating to identify the
active site of the hydrogen adsorption on the Pt(111) surface, which has been
hitherto conceptually discussed but has not been shown by an atomistic simulation.
Author(s) : Manish Kumar, Jyoti Thakur, Mukhtiyar Singh, Hardev Singh
Affiliation(s) : Department of Physics, Kurukshetra University, Kurukshetra-
136119 INDIA
Email of Presenter : [email protected]
Title : ab-initio investigation of Half metallicity in ordered and
disordered CoFeMnSi quaternary Heusler alloy
Abstract:
An ab-initio investigations of electronic and magnetic properties of ordered
and disordered CoFeMnSi quaternary Heusler alloy (HA) has been made using full
potential linearized augmented plane wave (FPLAPW) method based on density
functional theory (DFT). The generalized gradient approximation (GGA) governs the
exchange-correlation (XC) potentials in the present calculations. We observe that the
increasing Mn concentration in Co1-xFeMn1+xSi (x = 0, 0.25, 0.50 and 0.75) lowers
the total magnetic moment due to antiparallel alignment of extra Mn-atom to original
Mn-atom. Further, ordered CoFeMnSi Heusler alloy is a true half-metallic
ferrromagnet with integer magnetic moment of 4.00 μB per formula unit and mainly
localized at Mn site. The magnetic moment decreases with increase of Mn-disorder
in CoFeMnSi. The variation of half metallicity with increasing Mn-disorder is also
analyzed. This type of quaternary Heusler alloys can be proved an ideal candidate
for spin valves and magnetic tunnel junction applications.
Author(s) : A.R. Kumarasinghe1 *, F. Bondino2, R.N. Wijesinghe1, S.
George1, K.M. Nalin de Silva1, P. R. Kidambi3, S. Hoffman3,
P. Hiralal3 and Gehan A J Amaratunga1,3
Affiliation(s) : 1Sri Lanka Institute of Nanotechnology (SLINTEC), Zone 1,
EPZ, Biyagama, Colombo, Sri Lanka, 2Beamline BACH, IOM
CNR, Laboratorio Nazionale TASC, Area Science Park,
Bassovizza Edificio MM - S.S. 14 Km. 163,5, I-34149,
Basovizza (TS), Trieste, Italy, 3Centre of Advanced Photonics
and Electronics, Department of Engineering, University of
Cambridge, 9 J.J. Thomson Avenue, Cambridge, CB 3 0 FA,
UK,
Email of Presenter : [email protected], [email protected],
Title : C-K-edge NEXAFS, core level photoemission and Raman
spectroscopy of multilayer graphene oxide membrane and
natural graphite from vein
Abstract:
Synchrotron radiation (SR) based core level photoemission spectroscopy, C and O
K-edge X-ray absorption spectroscopy (NEXAFS) and micro Raman spectroscopy
are used to investigate the structural and electronic properties of graphene oxide
(GO) multilayer membrane produced using natural graphite from vein. The multilayer
membrane was chosen because of its easy fabrication procedure and its foreseen
wide range of applicability. Electronic properties of GO membrane prepared using
infrequent form of precursor graphite are reported for the first time here. In the C 1s
region of the multilayer GO, distinctly separated peaks are observed at higher
binding energy (BE) region, which are attributed to the higher order hybridized
carbons presence in the membrane. The broad full-width half maximum (FWHM) of
the C 1s main peak at a BE of 284.7 eV indicates that other forms of carbons such
as vacancies and hydrogenated species (C-H) are also present. The C K-edge
NEXAFS spectrum of GO becomes featureless in the photon energy range
295-311eV where strong absorption and number of electronic transitions are
observed for the precursor vein graphite. The suppression of the features (i.e.,
featurelessness) at higher photon energies shows that the membrane behaves as a
material composed of strongly decoupled layers. Neither hydrogenated nor
oxygenated carbon species are observed with precursor graphite according to the C
1s core level spectra. The C K-edge NEXAFS for the precursor graphite reveals that
the distribution of the density of states in unoccupied orbitals of this new type of
precursor graphite is similar to that of man-made highly oriented pyrolytic graphite
(HOPG). This observation is further verified by micro Raman spectroscopy where a
very small D peak at a wavenumber of 1355 cm-1 and strong G peak at a
wavenumber of 1575 cm-1 are observed. The D/G ratio, which is ~ 0.0337, is more
comparable to HOPG than commercial flake graphite. A new peak is observed at
photon energy of 287 eV in the C K-edge NEXAFS spectrum of the precursor
graphite which may have resulted in from the molecular nature of sp2 bonded carbon
suggesting that there are isolated pentagonal defects in the graphene plane of the
precursor graphite which give a C60 like curvature and pseudo molecular signatures.
Key words: electronic properties, graphite oxide membrane, self-assembled layers,
synchrotron radiation, C K-edge NEXAFS, core level photoemission.
*Corresponding author. Email: [email protected],
[email protected], Tel: 0094-11-4650507, Fax: 0094-11-4741995.
Permanent Institute: Faculty of Science and Technology, UWU, 90000, Sri Lanka.
Author(s) : Suchun Li1,2,3, Chee Kwan Gan3, Young-Woo Son4, Yuan Ping
Feng1,2, and Su Ying Quek3
Affiliation(s) : 1NUS Graduate School for Integrative Sciences and
Engineering, National University of Singapore, Singapore
2Department of Physics, National University of Singapore,
Singapore
3Institute of High Performance Computing, A*STAR, Singapore
4Korea Institute for Advanced Study, Seoul 130-722, South
Korea
Email of Presenter : [email protected]
Title : Length-Independent Transmission Peaks in Nanostructured
AGNR-Junctions
Abstract:
The prospect of all-carbon nanoelectronics has motivated significant interest in the
transport of electrons through graphene nanoribbon (GNR) junctions. Bottom-up
synthesis approaches now offer atomic control over the widths of armchair edge
GNRs (AGNR) [1] while top-down lithographic methods offer possibilities of creating
nanostructured GNR junctions [2]. In this work, we explore, using a first principles
scattering state approach [3], electron transport through nanostructured AGNR
junctions, in which the central scattering region consists of an AGNR connected
seamlessly to wider AGNRs (which make up the two electrodes of the junction). At
the interface between the central AGNR segment and the wider AGNR electrodes,
the armchair edges of the narrower and wider AGNRs are joined by either zigzag
edges or armchair edges (the two most stable edge configurations). We predict that
when both interfaces have zigzag edges, transmission near the Fermi level is
dominated by resonant transmission peaks whose energies are, contrary to
expectation, independent of the length of the central AGNR region. We show that
these peaks arise from interactions between the zigzag edges at the two interfaces,
and discuss why the energies of these peaks are length-independent. We also
predict that these peaks are useful in producing negative difference resistance.
[1] S. Linden et al., Phys Rev Lett 108, 216801 (2012)
[2] L. Tapaszto et al., Nature Nanotechnology, 3, 397 (2008); X. Wang et al., Nature
Chemistry 2, 661 (2010)
[3] H. J. Choi et al., Phys Rev B, 76, 155420 (2007)
Author(s) : Watchara Liewrian
Affiliation(s) : Department of Physics, Faculty of Science,
King Mongkut's University of Technology Thonburi (KMUTT)
126 Pracha U-thit Rd.,Bangmod Tung-Kru,Bangkok 10140,
Thailand
Email of Presenter : [email protected]
Title : Interplay between the exchange interaction and the orbital
effect of perpendicular magnetic field in graphene junction:
Spin-polarized transport
Abstract:
We theoretically investigate the effect of magnetic vector potential and exchange
interactions on the spin-polarized transport properties of 6massless Dirac particles in
graphene junction. By combining the orbital effect and the Zeeman interaction of
magnetic fields in graphene junction, it is found that the junction mimics behavior of
half-metallic tunneling junction, in which it acts as a metal to particles of one spin
orientation but as an insulator or a semiconductor to those of the opposite
orientation. The idea of the half-metallic tunneling junction can provide a source of
∼100% spin-polarized current. Adjustment of the position of the Fermi level in
ferromagnetic layer by placing a gate voltage on top of the ferromagnetic layer shows
that reverse of the orientation of the completely spin-polarized current passing
through the junction is controlled by adjusting the gate voltage. These interesting
characteristics should lead to a practical gate voltage controlled spin filtering and
spin-polarized switching devices.
Author(s) : Shiuan-Fan Liou
Affiliation(s) :
Email of Presenter : [email protected]
Title : Edge Excitations of Bose-Einstein Condensate on Honeycomb
Lattice
Abstract:
Edge excitations of bosonic condensates on honeycomb lattice are investigated.
By considering weakly interacting bosons in the tight-binding limit with on-site and
nearest-neighboring interactions respectively, it is found that edge phonons generally
emerge as excitations of condensates either inside the gap between bulk acoustic
spectrum and optical spectrum or attached as an edge acoustic mode to the bulk
acoustic spectrum. Origins of edge phonons are analyzed. In particular, it is found
that even though excitations are governed by a non-Hermitian Hamiltonian, edge
phonons inside the bulk phonon spectrum gap have the same topological origin as
that of the BDI class in the classification of topological insulators. Possible
extensions to other lattices are briefly discussed.
Author(s) : Ya-Lin Lo
Affiliation(s) : Department of Physics National Taiwan University
Email of Presenter : [email protected]
Title : Length- and temperature-dependent crossover of charge
transport across molecular junctions
Abstract:
We study the electronic transport in a molecular junction, in which each unit is
coupled to a local phonon bath, using the non-equilibrium Green's function method.
We observe the conductance oscillates with the molecular chain length and the
oscillation period in odd-numbered chains depends strongly on the applied bias. This
oscillatory behavior is smeared out at the bias voltage near the phonon energy. For
the phonon-free case, we find a crossover from tunneling to thermally activated
transport as the length of the molecule increases. In the presence of electron-phonon
interaction, the transport is thermally driven and a crossover from the thermally
suppressed to assisted conduction is observed.
[1] Ya-lin Lo, Shih-Jye Sun, Ying-Jer Kao, PRB 84 075106(2011)
Author(s) : Xin Luo and Su Ying Quek
Affiliation(s) : Institute of High Performance Computing, Singapore
Email of Presenter : [email protected]
Title : Theoretical Study of High-frequency Raman Modes in Bulk and
Few Trilayer MoS2
Abstract:
MoS2, a prototypical transition metal dichalcogenide semiconductor, has attracted
great interest due to its unique physical properties and potential industry applications,
especially in low dimensions. MoS2 consists of weakly interacting trilayers (TLs;
three atomic planes in 1 TL). Raman spectroscopy has been proposed as a means
to count the number of TLs in thin films, because Raman frequencies are sensitive to
thickness. However, controversial experimental results have been reported for the
evolution trend of E12g mode with thickness [1,2]. Here, we combine the first-
principles density functional perturbation theory calculations and a force constants
model to elucidate the effect of decreasing thickness on Raman frequencies in MoS2,
and compare our results with experiment. Based on our force constants model, we
find that if the force constants are kept the same from bulk to 2D, the predicted
frequencies of both A1g and E12g modes decrease with decreasing thickness.
However, when the effect of larger force constants at the surface is taken into
account, this surface effect tends to harden the frequencies, causing an anomalous
blue shift in the E12g mode, in agreement with density functional calculations and the
experiment by Lee et al. [1]. In contrast, the frequency of the A1g mode still
decreases with decreasing thickness, in excellent agreement with experiment. We
further predict that accumulated strain can explain the red shift in the E12g mode that
was observed in the experiment by Matte et al. [2], where a bent TL is shown in their
TEM image.
[1] Lee, C. et al.ACS Nano 4, 2695 (2010)
[2] Matte, H.S.S.R. et al. Angew. Chem. 122, 4153 (2010)
Author(s) : Harsan Haijiao Ma(马海蛟)
Affiliation(s) : 1NUSNNI-NanoCore, National University of Singapore,
Singapore 117411, Singapore
2Department of Physics, National University of Singapore,
Singapore 117542, Singapore
Email of Presenter : [email protected]
Title : LaAlO3/SrTiO3 superlattices on SrTiO3(110)
Abstract:
Two dimensional electron gas(2DEG) emerging at the interface of
LaAlO3/SrTiO3(001) was attributed to electronic reconstruction at the interface
because of polar catastrophe. Further more, unexpected 2EDEG at the interface of
LaAlO3/SrTiO3(110) and LaAlO3/SrTiO3(111) even amorphous LaAlO3 on
SrTiO3(001) showing very interesting behavior such as conducting anistropy has
being explored which is difficult to understand by polar catastrophe model. Here we
report transport results in LaAlO3/SrTiO3 superlattices on SrTiO3(110) substrate.
Resistance vs temperature(RT) measurement in different configurations and along
different directions show very different features and Angular Dependence
Magnetoresistance(ADMR) measurement and magnetioresistance measurement
also show very interesting behavior. Signatures of SrTiO3(001) structure phase
transitions were observed in the RT behavior of 2DEG. 2θ and 4θ components
apeared in ADMR of 2DEG in LaAlO3/SrTiO3 superlattices on SrTiO3(110) and show
different properties for different directions.
Author(s) : Carina B. Maliakkal, Azizur A. Rahman, John P. Mathew,
Nirupam Hatui, Priti Gupta, Ritam Sinha, T. S. Abhilash,
Mandar M. Deshmukh and Arnab Bhattacharya
Affiliation(s) : Department of Condensed Matter Physics and Materials
Science, Tata Institute of Fundamental Research, Homi
Bhabha Road, Mumbai 400005.
Email of Presenter : [email protected]
Title : Synthesis and Characterization of GaN Nanowires
Abstract:
Semiconductor nanowires are being intensely researched as building blocks for
nanoscale electronic and photonic device applications. GaN based materials are
ideal candidates for optoelectronic devices in the near UV and visible spectral region.
In this work we have fabricated GaN nanowires using metalorganic chemical vapor
deposition via different routes. Non catalytic and selective area growth techniques
resulted only in short stubby hexagonal islands or pyramidal features. Gold catalyst
mediated growths demonstrated thin nanowires under certain growth conditions.
Under similar conditions longer wires were obtained on r, c and m plane sapphire
substrates by using a nickel catalyst formed in-situ from the decomposition of nickel
nitrate (Fig. 1). Scanning electron microscopy, energy dispersive X-ray spectroscopy,
transmission electron microscopy, photoluminescence and cathodo-luminescense
were used to characterise the samples. Electrical contacts to the GaN nanowires
were made using electron beam lithography. Electrical properties of the as grown
wires were studied using field effect transistor geometry.
Figure 1: GaN nanowires grown using Ni
catalyst
Author(s) : S. Mathew,1,2, K. Gopinadhan,1,2 T. K. Chan,3,6 X. J. Yu,1,4 D.
Zhan,5 L. Cao,6 A. Rusydi,1,4,6 M. B. H. Breese,3,4,6
S. Dhar,1,2 Z. X. Shen,5
T. Venkatesan,1,2 and John TL Thong,1
Affiliation(s) :
1,2NUSNNI-NanoCore, National University of Singapore 117576
2Department of Electrical and Computer Engineering,
National University of Singapore, Singapore, 117576
3Center for Ion Beam Applications (CIBA), Department of Physics,
National University of Singapore, Singapore 117542
4Singapore Synchrotron Light Source (SSLS),
National University of Singapore, Singapore 117603
5Division of Physics and Applied Physics, School of Physical and
Mathematical Sciences,
Nanyang Technological University, Singapore 637371
6Department of Physics, National University of Singapore,
Singapore 117542
Email of Presenter : [email protected]
Title : Magnetism in MoS2 induced by proton irradiation
Abstract:
Molybdenum disulphide, a diamagnetic layered dichalcogenide solid, is found to
show magnetic ordering at room temperature when exposed to a 2 MeV proton
beam. The temperature dependence of magnetization displays ferrimagnetic
behavior with a Curie temperature of 895 K. A disorder mode corresponding to a
zone-edge phonon and a Mo valence higher than +4, have been detected in the
irradiated samples using Raman and X-ray photoelectron spectroscopy,
respectively. The possible origins of long-range magnetic ordering in irradiated MoS2
samples will be discussed.
Author(s) : Vineeth Mohanan Parakkat, P.S.Anil Kumar
Affiliation(s) : Department of Physics, Indian Institute of Science Bangalore
Email of Presenter : [email protected]
Title : Investigation of Electron Magnon Interaction in Ferromagnetic
nanostructures
Abstract:
Magnon contribution to the resistance of film and nanowires of Permalloy is
investigated by magnetotransport measurements. The temperature dependence of
high field MR (upto 5T) of Permalloy films indicate the magnon contribution to
resistivity. By patterning films into nanowires we are able to observe and distinguish
Anisotropic-Magnetoresistance (AMR) and Magnon Magnetoresistance (MMR)
contributions by varying their width thereby tuning their shape anisotropy. Nanowires
of thickness 20nm and varying widths down to 160nm were prepared by e-beam
lithography and Pulsed Laser Deposition. A linear non-saturating longitudinal MR
observed in high field regime for NiFe nanowires could never be explained using
AMR but only MMR can account for it. MMR follows as MMR s)B, where M is
magnetization along the easy axis, Ms saturation magnetization, B magnetic field
sat
saturation. A cross over in magnetoresistance at low field from AMR-dominant to
MMR-dominant one was observed as wire width reduces since the AMR value is
getting reduced from 1.2% for 650nm wide wire to 0.26% for 160nm, however the
contributions from the electron-magnon interactions still persist. The MMR proves to
be an excellent way of determining M/Ms, understanding the magnetization reversal
mechanisms in nanostructures, domain wall dynamics and also fundamental
problems like electron magnon interactions.
Author(s):
Thanh Thi Kim Nguyen
Affiliation(s):
Department of Physics, University of Cincinnati, Cincinnati,
Ohio 45221, USA.
Institute of Physics and Electronics, Vietnam Academy of
Science and Technology, Hanoi 10000, Vietnam.
Email of Presenter: [email protected]
Title: Dynamic response of a single-electron transistor in the ac
Kondo regime
Abstract:
A single-electron transistor (SET) in a magnetic field irradiated with microwaves is
studied theoretically in non-equilibrium Kondo regime. The two fold effect of
frequency- -microwaves is considered as follows: the voltage oscillates with
number). We describe the system by the Kondo model at a specific point in the
Toulouse limit. A non-perturbative technique is proposed, namely, the non-equilibrium
the microwave irradiation is considered affecting only the voltage, one sees the Kondo
satellites as stated in the previous studies. Moreover, the features of the differential
conductance and magnetic susceptibility of a SET become richer when the Kondo
couplings are considered oscillating on time. We obtain the satellite peak splitting. It
explains the possibilities one can find in experimental results that the distance
between peaks, which appear in the differential conductance - magnetic amplitude
characteristics G(H) or in the differential conductance - dc voltage characteristics
G(Vdc), can be smaller than
Author(s) : Satyapal S. Rathore and Satish Vitta
Affiliation(s) : Department of Metallurgical Engineering and Materials
Science, IIT Bombay, Mumbai – 400 076; India
Email of Presenter : [email protected]
Title : ‘Glassy’ behavior in multiferroic Ba3NbFe3Si2O14
Abstract:
‘Geometrically frustrated’ systems in which arrangement of magnetic spins and
interaction between them lead to unconventional ground states has attracted intense
research activity as many novel magneto-electric multiferroics belong to this class. A
recent addition to this family is Fe – Langasite, Ba3NbFe3Si2O14 (BNFSO) with
frustrated triangular (a-b plane) and helical (c-axis) arrangement of Fe3+ spins. In the
present work a detailed investigation of the magnetic and dielectric response of
BNFSO is performed in order to understand the correlation between its magnetic and
electric phase transitions.
The structural characterization performed by X-ray powder diffraction at room
temperature and the subsequent Rietveld refinement reveals that polycrystalline
BNFSO crystallize in a hexagonal P321 structure. The BNFSO orders
antiferromagnetically with a Nèel transition temperature, TN ~ 26 K, in good
agreement with single crystal studies. The temperature dependence of ac magnetic
susceptibility in the frequency range 10 Hz to 1 KHz shows two clear peaks. A
frequency independent peak at TN~26 K signaling a transition to first order
antiferromagnetic ordered state. And a high temperature frequency dependent spin-
glass like behavior with activation energy of ~ 68 meV. The dielectric response (10
Hz to 1 MHz) is in agreement with magnetic studies and also shows relaxor-like
frequency dispersion above TN with activation energy 71 meV. However, unlike
magnetic transition the dielectric transition near TN was observed to be of dispersive
in nature with activation energy of 30 meV. The high and low temperature ‘glassy’
magnetic and dielectric behaviors above TN were observed for first time in BNFSO
and are attributed to the onset and completion of magnetic ordering respectively.
These studies clearly show that nature of magnetic and dielectric transitions at TN are
different for single crystal and polycrystalline Ba3NbFe3Si2O14.
Author(s) : Nishaina Sahadev, Deep Narayan Biswas, Sangeeta Thakur,
Khadiza Ali, Geetha Balakrisnan, kalobaran Maiti
Affiliation(s) : TATA INSTITUTE OF FUNDAMENTAL RESEARCH
Email of Presenter : [email protected]
Title : Evidence of Bulk nature of Kondo effect and different surface
potentials in CeB6
Abstract:
We have studied the electronic structure of CeB6 using photoemission spectroscopy.
We made surface and bulk sensitive measurements. Experimental results show that
the Kondo effect is a bulk phenomenon. Also the Medelung potential at the surface is
different from the bulk.
Author(s) : Nihit Saigal and Sandip Ghosh
Affiliation(s) : Tata Institute of Fundamental Research, Mumbai, India-
400005
Email of Presenter : [email protected]
Title : Reflectance Contrast Spectroscopy for Distinguishing Between
Monolayer and Bilayer Graphene
Abstract:
The authors report a reflectance contrast spectroscopy study to distinguish between
monolayer and bilayer graphene on silicon dioxide covered silicon substrate. They
describe a setup built for reflectance measurements with high spatial resolution,
which uses a combination of a pin-hole and a microscope with a high magnification
objective lens. The measured reflectance contrast spectra are subsequently
compared with simulations in order to identify mono/bilayer graphene.
Author(s) : Anjan Soumyanarayanan1,2, Michael M. Yee1, Yang He1,
Jasper van Wezel3,4, D.J. Rahn5, Kai Rossnagel5, Eric W.
Hudson6, Michael R. Norman3, and Jennifer E. Hoffman1
Affiliation(s) : 1 Harvard University, 2 Massachusetts Institute of Technology, 3
Argonne National Laboratory, 4 University of Bristol, 5 Kiel
University, 6 Pennsylvania State University
Email of Presenter : [email protected]
Title : Quantum Phase Transition from Triangular to Stripe Charge
Order in NbSe2
Abstract:
The competition between proximate electronic phases produces a complex
phenomenology in strongly correlated systems. In particular, fluctuations with charge
and spin density waves, and their effects on superconductivity, have been difficult to
study in the presence of chemical disorder. Here we image a previously unknown
unidirectional (stripe) charge density wave (CDW) smoothly interfacing with the
familiar triangular CDW on the surface of the stoichiometric superconductor NbSe2,
with wavelengths differing by 15%. Our low temperature measurements rule out
thermal fluctuations, and point to local strain as the tuning parameter for this
quantum phase transition. We use this discovery to resolve two longstanding
debates about the anomalous spectroscopic gap and the role of Fermi surface
nesting in the CDW phase of NbSe2. Our results highlight the importance of local
strain in governing phase transitions and competing phenomena, and suggest a new
direction of inquiry for resolving similarly longstanding debates in cuprate
superconductors and other strongly correlated materials.
Author(s) : Phitsini Suvarnaphaet
Affiliation(s) : Department of Physics, Faculty of Science, Mahidol University,
272 Rama 6 Rd., Ratchathewi, Bangkok 10400 Thailand
Email of Presenter : [email protected], [email protected]
Title : A Chemical Vapor Sensor from Graphene/Nanographite
Assembly
Abstract:
Graphene and nanographite, a low dimensional assembly of sp2 hybridized carbon
atoms in a honeycomb packing, have attracted tremendous attention in various fields
due to their superior properties. We have investigated a chemical vapor sensor that
is easily integrated, low-cost, disposable and highly sensitive by using a mixture of
graphene and nanographite as a sensitive area. The assembly of
graphene/nanographite film was deposited on the gold-coated interdigitated
microelectrodes (IDEs). The homogeneity and the presence of ohmic behavior of
sensitive area from graphene/nanographite assembly were confirmed by van der
Pauw technique. The sensor sensitivity was carried out using four-point probe
method at room temperature for detecting common chemicals in laboratory, including
ammonia, ethanol and acetone. The results showed that the electrical resistance was
changed significantly as a function of time exposure to chemical vapor. The
resistance change may be due to either the adsorption of chemical molecules on the
sensitive area or the charge transfer phenomenon between chemical molecules and
the sensitive area. The information from this demonstration could be very useful for
future development of low-cost and disposable chemical sensor based on graphene
and nanographite.
Author(s) : Nguyen Thi Thuy and Tran Minh Tien
Affiliation(s) : Institute of Physics, Vietnam Academy of Science and
Technology, Ha Noi, Vietnam
Email of Presenter : [email protected]
Title : Topological insulating phase in the Haldane - Falicov - Kimball
model
Abstract:
Effects of electron correlations on a topological insulating phase are
studied within a combination of the Haldane and the Falicov - Kimball
model. The Haldane model provides the existence of topological
insulating phase, while electron correlations are the essence of the
Falicov - Kimball model. A mean field approximation is adopted to
investigate the combined model. There is a competition between the trivial
topological charge-ordered phase and topological one. A phase diagram
is also presented.
Author(s) : Keola Wierschem and Pinaki Sengupta
Affiliation(s) : Nanyang Technological University
Email of Presenter : [email protected]
Title : Spin supersolid phase of the extended Shastry-Sutherland
model
Abstract: The Shastry-Sutherland model is a quantum lattice model of
antiferromagnetic Heisenberg spins arranged in a frustrated geometry composed of a
basic square lattice with additional orthogonal connections on every other square
plaquette. We refer to the corresponding bonds as J1 and J2, respectively. In the
limit of J1 << J2, the orthogonal J2 bonds dominate, leading to an exact ground state
of spin singlet dimers on each J2 bond. Here we show that for a generalized Shastry-
Sutherland model with anisotropic exchange and ferromagnetic transverse
interactions, the addition of crosswise connections on the remaining square
plaquettes can lead to the formation of a magnetic field induced spin supersolid
phase. Using quantum Monte Carlo techniques, we demonstrate the coexistence of
diagonal and off-diagonal order in this phase, and study the nearby magnetic phase
diagram. We also discuss the possible implications for several rare-earth tetraboride
compounds whose low temperature magnetic properties are captured by the
extended Shastry-Sutherland model considered here.
Author(s) : Jhih-Shih You1,2, Hao Lee1,2, Shiang Fang1,2, Miguel A.
Cazalilla3,4,5, and Daw-Wei Wang1,2
Affiliation(s) : 1.Physics Department and Frontier Research Center on
Fundamental and Applied Sciences of Matter, National Tsing-Hua
University, Hsinchu, Taiwan
2.Physics Division, National Center for Theoretical Sciences,
Hsinchu, Taiwan
3.Centro de Fsica de Materiales CSIC-UPV/EHU, Paseo Manuel de
Lardizabal 5, E-20018 San Sebastian, Spain
4.DIPC, Paseo Manuel de Lardizabal 4, E-20018 San Sebastian,
Spain
5.Graphene Research Centre National University of Singapore, 6
Science Drive 2, Singapore 117546
Email of Presenter : [email protected]
Title : Tuning the Kosterlitz-Thouless transition to zero temperature in
anisotropic boson systems
Abstract:
We study the two-dimensional Bose-Hubbard model with anisotropic hopping.
Focusing on the effects of anisotropy on superfluid properties such as the helicity
modulus and the normal-to-superfluid [Berezinskii-Kosterlitz-Thouless (BKT)]
transition temperature, two different approaches are compared: large-scale quantum
Monte Carlo simulations and the self-consistent harmonic approximation (SCHA).
For the latter, two different formulations are considered, one applying near the
isotropic limit and the other applying in the extremely anisotropic limit. Thus we find
that the SCHA provides a reasonable description of superfluid properties of this
system provided the appropriate type of formulation is employed. The accuracy of
the SCHA in the extremely anisotropic limit, where the BKT transition temperature is
tuned to zero (i.e., at a quantum critical point) and therefore quantum fluctuations
play a dominant role, is particularly striking.