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Topological Data Analysis combined with Machine Learning for Predicting Permeability of Porous Media
Authors:
Ebru Dagdelen,
Catherin Neena Lalu,
Aakash Karlekar,
Manav Arora,
Matthew Illingworth,
Jonathan Jaquette,
Linda Cummings,
Lou Kondic
Abstract:
Flow in porous media is difficult to address using standard analytical or numerical methods due to its complexity. However, since synthetic representations of porous media are easy to produce and data from physical experiments are becoming more widely available, the problem is well-suited to studies that include machine learning (ML) techniques. We discuss a number of features that can be extracte…
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Flow in porous media is difficult to address using standard analytical or numerical methods due to its complexity. However, since synthetic representations of porous media are easy to produce and data from physical experiments are becoming more widely available, the problem is well-suited to studies that include machine learning (ML) techniques. We discuss a number of features that can be extracted from such data, and their utility as input variables into a standard ML algorithm. These features include structural measures describing the geometry of the porous media, topological measures describing the connectivity, and network measures obtained by modeling the porous media as simplified pore networks. These features enable the prediction of the permeability of the considered (synthetic) porous materials using ML techniques that also leverage the separately computed exact permeability (ground truth). Comparing results obtained using different input variables helps develop a better understanding of the utility of various measures for predicting permeability based on the porous media structure. We show, in particular, that topological data analysis (TDA) provides a useful set of features that can be easily combined with ML to yield meaningful results.
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Submitted 30 July, 2026; v1 submitted 17 May, 2026;
originally announced May 2026.
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Hofstadter butterflies in magnetically modulated graphene bilayer: an algebraic approach
Authors:
Manisha Arora,
Rashi Sachdeva,
Sankalpa Ghosh
Abstract:
It has been shown that Bernal stacked bilayer graphene (BLG) in a uniform magnetic field demonstrates integer quantum Hall effect with a zero Landau-level anomaly \cite{Geimbilayer}. In this article we consider such system in a two dimensional periodic magnetic modulation with square lattice symmetry. It is shown algebraically that the resulting Hofstadter spectrum can be expressed in terms of the…
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It has been shown that Bernal stacked bilayer graphene (BLG) in a uniform magnetic field demonstrates integer quantum Hall effect with a zero Landau-level anomaly \cite{Geimbilayer}. In this article we consider such system in a two dimensional periodic magnetic modulation with square lattice symmetry. It is shown algebraically that the resulting Hofstadter spectrum can be expressed in terms of the corresponding spectrum of monolayer graphene in a similar magnetic modulation. In the weak-field limit, using the tight-binding model, we also derive the Harper-Hofstadter equation for such BLG system in a periodic magnetic modulation. We further demonstrate the topological quantisation of Hall conductivity in such system and point out that the quantised Hall plateaus are equally spaced for all quantum numbers for the quantised Hall conductivity.
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Submitted 12 May, 2022;
originally announced May 2022.
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Symmetry-dependent ultrafast manipulation of nanoscale magnetic domains
Authors:
Nanna Zhou Hagström,
Rahul Jangid,
Meera,
Diego Turenne,
Jeffrey Brock,
Erik S. Lamb,
Boyan Stoychev,
Justine Schlappa,
Natalia Gerasimova,
Benjamin Van Kuiken,
Rafael Gort,
Laurent Mercadier,
Loïc Le Guyader,
Andrey Samartsev,
Andreas Scherz,
Giuseppe Mercurio,
Hermann A. Dürr,
Alexander H. Reid,
Monika Arora,
Hans T. Nembach,
Justin M. Shaw,
Emmanuelle Jal,
Eric E. Fullerton,
Mark W. Keller,
Roopali Kukreja
, et al. (3 additional authors not shown)
Abstract:
Symmetry is a powerful concept in physics, but its applicability to far-from-equilibrium states is still being understood. Recent attention has focused on how far-from-equilibrium states lead to spontaneous symmetry breaking. Conversely, ultrafast optical pumping can be used to drastically change the energy landscape and quench the magnetic order parameter in magnetic systems. Here, we find a dist…
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Symmetry is a powerful concept in physics, but its applicability to far-from-equilibrium states is still being understood. Recent attention has focused on how far-from-equilibrium states lead to spontaneous symmetry breaking. Conversely, ultrafast optical pumping can be used to drastically change the energy landscape and quench the magnetic order parameter in magnetic systems. Here, we find a distinct symmetry-dependent ultrafast behaviour by use of ultrafast x-ray scattering from magnetic patterns with varying degrees of isotropic and anisotropic symmetry. After pumping with an optical laser, the scattered intensity reveals a radial shift exclusive to the isotropic component and exhibits a faster recovery time from quenching for the anisotropic component. These features arise even when both symmetry components are concurrently measured, suggesting a correspondence between the excitation and the magnetic order symmetry. Our results underline the importance of symmetry as a critical variable to manipulate the magnetic order in the ultrafast regime.
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Submitted 17 December, 2021;
originally announced December 2021.
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Near-Unity Spin Hall Ratio in Ni$_x$Cu$_{1-x}$ Alloys
Authors:
Mark W. Keller,
Katy S. Gerace,
Monika Arora,
Erna Krisztina Delczeg-Czirjak,
Justin M. Shaw,
T. J. Silva
Abstract:
We report a large spin Hall effect in the 3$d$ transition metal alloy Ni$_x$Cu$_{1-x}$ for $x\in\left\{ 0.3,0.75\right\} $, detected via the ferromagnetic resonance of a Permalloy (Py = Ni$_{80}$Fe$_{20}$) film deposited in a bilayer with the alloy. A thickness series at $x$ = 0.6, for which the alloy is paramagnetic at room temperature, allows us to determine the spin Hall ratio…
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We report a large spin Hall effect in the 3$d$ transition metal alloy Ni$_x$Cu$_{1-x}$ for $x\in\left\{ 0.3,0.75\right\} $, detected via the ferromagnetic resonance of a Permalloy (Py = Ni$_{80}$Fe$_{20}$) film deposited in a bilayer with the alloy. A thickness series at $x$ = 0.6, for which the alloy is paramagnetic at room temperature, allows us to determine the spin Hall ratio $θ_{\rm{SH}}\approx1$, spin diffusion length $λ_{\rm{s}}$, spin mixing conductance $G_{\uparrow\downarrow}$, and damping $α_{\rm{SML}}$ due to spin memory loss . We compare our results with similar experiments on Py/Pt bilayers measured using the same method. Ab initio band structure calculations with disorder and spin-orbit coupling suggest an intrinsic spin Hall effect in Ni$_x$Cu$_{1-x}$ alloys, although the experiments here cannot distinguish between extrinsic and intrinsic mechanisms.
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Submitted 10 April, 2019; v1 submitted 3 March, 2019;
originally announced March 2019.
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A magnetic Hofstadter butterfly and its topologically quantized Hall conductance
Authors:
Manisha Arora,
Sankalpa Ghosh
Abstract:
The energy spectrum of massless Dirac fermions in graphene under two dimensional periodic magnetic modulation having square lattice symmetry is calculated. We show that the translation symmetry of the problem is similar to that of the Hofstadter or TKNN problem and in the weak field limit the tight binding energy eigenvalue equation is indeed given by Harper Hofstadter hamiltonian. We show that du…
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The energy spectrum of massless Dirac fermions in graphene under two dimensional periodic magnetic modulation having square lattice symmetry is calculated. We show that the translation symmetry of the problem is similar to that of the Hofstadter or TKNN problem and in the weak field limit the tight binding energy eigenvalue equation is indeed given by Harper Hofstadter hamiltonian. We show that due to its magnetic translational symmetry the Hall conductivity can be identified as a topological invariant and hence quantized. We thus extend the idea of Quantum Hall Effect to magnetically modulated two dimensional electron system. Finally we indicate possible experimental systems where this may be verified.
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Submitted 2 August, 2018;
originally announced August 2018.
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Roughness-induced domain structure in perpendicular Co/Ni multilayers
Authors:
N. R. Lee-Hone,
R. Thanhoffer,
V. Neu,
R. Schäfer,
M. Arora,
R. Hübner,
D. Suess,
D. M. Broun,
E. Girt
Abstract:
We investigate the correlation between roughness, remanence and coercivity in Co/Ni films grown on Cu seed layers of varying thickness. Increasing the Cu seed layer thickness of Ta/Cu/8x[Co/Ni] thin films increases the roughness of the films. In-plane magnetization loops show that both the remanance and coercivity increase with increasing seed layer roughness. Polar Kerr microscopy and magnetic fo…
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We investigate the correlation between roughness, remanence and coercivity in Co/Ni films grown on Cu seed layers of varying thickness. Increasing the Cu seed layer thickness of Ta/Cu/8x[Co/Ni] thin films increases the roughness of the films. In-plane magnetization loops show that both the remanance and coercivity increase with increasing seed layer roughness. Polar Kerr microscopy and magnetic force microscopy reveal that the domain density also increases with roughness. Finite element micromagnetic simulations performed on structures with periodically modulated surfaces provide further insight. They confirm the connection between domain density and roughness, and identify the microsocpic structure of the domain walls as the source of the increased remanence in rough films. The simulations predict that the character of the domain walls changes from Bloch-like in smooth films to Néel-like for rougher films.
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Submitted 29 May, 2017; v1 submitted 14 December, 2016;
originally announced December 2016.
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Molecular structure, vibrational, photophysical and nonlinear optical properties of L-threoninium picrate: A first-principles study
Authors:
S. AlFaify,
Mohd. Shkir,
M. Arora,
Ahmad Irfan,
H. Algarni,
Haider Abbas,
Shabbir Muhammad,
Abdullah G. Al-Sehemi
Abstract:
In this work, different computational methods such as HF, B3LYP, range separated functionals (CAM-B3LYP and LC-BLYP) with 6-31G* basis set were applied to investigate the electronic, spectroscopic and nonlinear optical properties of L-threoninium picrate (LTHP) molecule for the first time. The calculated values of IR and Raman vibrational frequencies were found to be in a good agreement with the e…
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In this work, different computational methods such as HF, B3LYP, range separated functionals (CAM-B3LYP and LC-BLYP) with 6-31G* basis set were applied to investigate the electronic, spectroscopic and nonlinear optical properties of L-threoninium picrate (LTHP) molecule for the first time. The calculated values of IR and Raman vibrational frequencies were found to be in a good agreement with the experimental results. Time dependent density functional theory has been applied to calculate the electronic and photophysical properties such as excitation energy, dipole moment and frontier molecular orbital (FMO) energies of LTHP molecule. The excitation energy value calculated by CAM-B3LYP is found to be at 351 nm which is in close agreement with the experimental values. The total/partial DOS (T/PDOS) was determined using GGA/BLYP. The total dipole moment (μtot), static total and anisotropy of polarizability (αtot, Δα) and static first hyperpolarizability (\b{eta}0, \b{eta}tot) values were calculated and compared with the reference compound. The μtot and \b{eta}tot are found to be 3 and 51 time higher than urea molecule respectively. The FMOs, molecular electrostatic potential (MEP), global reactivity descriptors were also calculated and discussed. All these results suggest that the L-threoninium picrate would be a good candidate for optoelectronic device applications.
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Submitted 19 December, 2016; v1 submitted 4 September, 2015;
originally announced September 2015.
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Non-intrinsic superconductivity in InN epilayers: role of Indium Oxide
Authors:
Abdul Kadir,
Sourin Mukhopadhyay,
Tapas Ganguli,
Charudatta Galande,
M. R. Gokhale,
B. M. Arora,
Pratap Raychaudhuri,
Arnab Bhattacharya
Abstract:
In recent years there have been reports of anomalous electrical resistivity and the presence of superconductivity in semiconducting InN layers. By a careful correlation of the temperature dependence of resistivity and magnetic susceptibility with structural information from highresolution x-ray diffraction measurements we show that superconductivity is not intrinsic to InN and is seen only in sa…
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In recent years there have been reports of anomalous electrical resistivity and the presence of superconductivity in semiconducting InN layers. By a careful correlation of the temperature dependence of resistivity and magnetic susceptibility with structural information from highresolution x-ray diffraction measurements we show that superconductivity is not intrinsic to InN and is seen only in samples that show traces of oxygen impurity. We hence believe that InN is not intrinsically a superconducting semiconductor.
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Submitted 14 February, 2008;
originally announced February 2008.
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Determination of InN-GaN heterostructure band offsets from internal photoemission measurements
Authors:
Zahid Hasan Mahmood,
A. P. Shah,
Abdul Kadir,
M. R. Gokhale,
Sandip Ghosh,
Arnab Bhattacharya,
B. M. Arora
Abstract:
Band discontinuities at the InN-GaN heterointerface are experimentally determined from internal photoemission spectroscopy measurements on n+ InN on GaN epilayers. The photocurrent shows two threshold energies, one at 1.624 eV and the other at 2.527 eV. From these we obtain the valence band offset 0.85 eV and the conduction band offset 1.82 eV.
Band discontinuities at the InN-GaN heterointerface are experimentally determined from internal photoemission spectroscopy measurements on n+ InN on GaN epilayers. The photocurrent shows two threshold energies, one at 1.624 eV and the other at 2.527 eV. From these we obtain the valence band offset 0.85 eV and the conduction band offset 1.82 eV.
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Submitted 5 June, 2007;
originally announced June 2007.
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The role of hydrostatic stress in determining the bandgap of InN epilayers
Authors:
Abdul Kadir,
Tapas Ganguli,
Ravi Kumar,
M. R. Gokhale,
A. P. Shah,
Sandip Ghosh,
B. M. Arora,
Arnab Bhattacharya
Abstract:
We establish a correlation between the internal stress in InN epilayers and their optical properties such as the measured absorption band edge and photoluminescence emission wavelength. By a careful evaluation of the lattice constants of InN epilayers grown on c-plane sapphire substrates under various conditions by metalorganic vapor phase epitaxy we find that the films are under primarily hydro…
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We establish a correlation between the internal stress in InN epilayers and their optical properties such as the measured absorption band edge and photoluminescence emission wavelength. By a careful evaluation of the lattice constants of InN epilayers grown on c-plane sapphire substrates under various conditions by metalorganic vapor phase epitaxy we find that the films are under primarily hydrostatic stress. This results in a shift in the band edge to higher energy. The effect is significant, and may be responsible for some of the variations in InN bandgap reported in the literature.
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Submitted 19 May, 2007;
originally announced May 2007.
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Influence of growth parameters on structural properties and bandgap of InN epilayers deposited in a showerhead MOVPE system
Authors:
Abdul Kadir,
Tapas Ganguli,
Ravi Kumar,
M. R. Gokhale,
A. P. Shah,
B. M. Arora,
Arnab Bhattacharya
Abstract:
From a detailed analysis of InN epilayers deposited in a close-coupled showerhead metalorganic vapour phase epitaxy (MOVPE) system under various conditions we investigate the effect of growth parameters on the lattice constants of the InN layer. The layers are under significant internal hydrostatic stress which influences the optical properties. Samples typically fall into two broad categories o…
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From a detailed analysis of InN epilayers deposited in a close-coupled showerhead metalorganic vapour phase epitaxy (MOVPE) system under various conditions we investigate the effect of growth parameters on the lattice constants of the InN layer. The layers are under significant internal hydrostatic stress which influences the optical properties. Samples typically fall into two broad categories of stress, with resultant luminescence emission around 0.8eV and 1.1eV. We can correlate the internal stress in the layer and the value of the optical absorption edge, and the PL emission wavelength.
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Submitted 1 May, 2007;
originally announced May 2007.
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Alloy disorder effects on the room temperature optical properties of GaInNAs quantum wells
Authors:
Bhavtosh Bansal,
Abdul Kadir,
Arnab Bhattacharya,
B. M. Arora,
Rajaram Bhat
Abstract:
The effect of alloy disorder on the optical density of states and the average room temperature carrier statistics in GaInNAs quantum wells is discussed. A red shift between the peak of the room temperature photoluminescence and the surface photovoltage spectra, that systematically increases with the nitrogen content within the quantum wells is observed. The relationship between this Stokes' shif…
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The effect of alloy disorder on the optical density of states and the average room temperature carrier statistics in GaInNAs quantum wells is discussed. A red shift between the peak of the room temperature photoluminescence and the surface photovoltage spectra, that systematically increases with the nitrogen content within the quantum wells is observed. The relationship between this Stokes' shift and the absorption linewidth in different samples suggests that the photoexcited carriers undergo a continuous transition, from being in quasi-thermal equilibrium with the lattice to being completely trapped by the quantum dot-like potential fluctuations, as the nitrogen fraction in the alloy is increased. The values of the 'electron temperature' inferred from the photoluminescence spectra are found to be consistent with this interpretation.
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Submitted 11 November, 2005;
originally announced November 2005.
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Growth kinetics effects on self-assembled InAs/InP quantum dots
Authors:
Bhavtosh Bansal,
M. R. Gokhale,
Arnab Bhattacharya,
B. M. Arora
Abstract:
A systematic manipulation of the morphology and the optical emission properties of MOVPE grown ensembles of InAs/InP quantum dots is demonstrated by changing the growth kinetics parameters. Under non-equilibrium conditions of a comparatively higher growth rate and low growth temperature, the quantum dot density, their average size and hence the peak emission wavelength can be tuned by changing e…
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A systematic manipulation of the morphology and the optical emission properties of MOVPE grown ensembles of InAs/InP quantum dots is demonstrated by changing the growth kinetics parameters. Under non-equilibrium conditions of a comparatively higher growth rate and low growth temperature, the quantum dot density, their average size and hence the peak emission wavelength can be tuned by changing efficiency of the surface diffusion (determined by the growth temperature) relative to the growth flux. We further observe that the distribution of quantum dot heights, for samples grown under varying conditions, if normalized to the mean height, can be nearly collapsed onto a single Gaussian curve.
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Submitted 24 June, 2005;
originally announced June 2005.
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Another origin of bimodal size distribution in InAs self-assembled quantum dots
Authors:
Bhavtosh Bansal,
M. R. Gokhale,
Arnab Bhattacharya,
B. M. Arora
Abstract:
The evolution of InAs quantum dots grown on InP substrates by metal-organic vapour phase epitaxy is studied as a function of InAs coverage. Under specific growth conditions, the onset of the two- to three-dimensional transition is seen to proceed via two distinct pathways: through (i) an abrupt appearance of quantum dots as expected in the usual Stranski-Krastanov growth picture and (ii) a conti…
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The evolution of InAs quantum dots grown on InP substrates by metal-organic vapour phase epitaxy is studied as a function of InAs coverage. Under specific growth conditions, the onset of the two- to three-dimensional transition is seen to proceed via two distinct pathways: through (i) an abrupt appearance of quantum dots as expected in the usual Stranski-Krastanov growth picture and (ii) a continuous evolution of small surface features into well developed quantum dots. The average size of the features in both these families increases with coverage, leading to a bimodal distribution in dot sizes at an intermediate stage of growth evolution that eventually becomes a unimodal distribution as more material is deposited. Complementary information obtained from independent measurements of photoluminescence spectra and surface morphology is correlated and is found to be consistent with the picture of growth proposed.
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Submitted 11 May, 2005;
originally announced May 2005.
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Probing electrodynamical properties of the edge states in a quantum Hall system by surface photovoltage spectroscopy
Authors:
B. Karmakar,
G. H. Dohler,
B. M. Arora
Abstract:
An importent question regarding the dissipation-less current carried by the edge states in a quantum Hall system is understanding the results of the electrodynamical interaction among the mobile electrons in the quantum mechanical limit under a magnetic field B. The interaction affects the transport parameters, the transverse electric field and the electron velocity. We have developed a new surf…
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An importent question regarding the dissipation-less current carried by the edge states in a quantum Hall system is understanding the results of the electrodynamical interaction among the mobile electrons in the quantum mechanical limit under a magnetic field B. The interaction affects the transport parameters, the transverse electric field and the electron velocity. We have developed a new surface photovoltage spectroscopic technique to measure the parameters from the transition energies between the electron and heavy hole edge states. We observe that the measured electron velocity and transverse (Hall) electric field increase as B^{1/2} and B^{3/2} respectively.
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Submitted 11 May, 2005;
originally announced May 2005.
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The effects of macroscopic inhomogeneities on the magneto transport properties of the electron gas in two dimensions
Authors:
B. Karmakar,
M. R. Gokhale,
A. P. Shah,
B. M. Arora,
D. T. N. de Lang,
A. de Visser,
L. A. Ponomarenko,
A. M. M. Pruisken
Abstract:
In experiments on electron transport the macroscopic inhomogeneities in the sample play a fundamental role. In this paper and a subsequent one we introduce and develop a general formalism that captures the principal features of sample inhomogeneities (density gradients, contact misalignments) in the magneto resistance data taken from low mobility heterostructures. We present detailed assessments…
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In experiments on electron transport the macroscopic inhomogeneities in the sample play a fundamental role. In this paper and a subsequent one we introduce and develop a general formalism that captures the principal features of sample inhomogeneities (density gradients, contact misalignments) in the magneto resistance data taken from low mobility heterostructures. We present detailed assessments and experimental investigations of the different regimes of physical interest, notably the regime of semiclassical transport at weak magnetic fields, the plateau-plateau transitions as well as the plateau-insulator transition that generally occurs at much stronger values of the external field only.
It is shown that the semiclassical regime at weak fields plays an integral role in the general understanding of the experiments on the quantum Hall regime. The results of this paper clearly indicate that the plateau-plateau transitions, unlike the the plateau-insulator transition, are fundamentally affected by the presence of sample inhomogeneities. We propose a universal scaling result for the magneto resistance parameters. This result facilitates, amongst many other things, a detailed understanding of the difficulties associated with the experimental methodology of H.P. Wei et.al in extracting the quantum critical behavior of the electron gas from the transport measurements conducted on the plateau-plateau transitions.
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Submitted 30 September, 2003;
originally announced September 2003.
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Observation of Type I/II Transition in GaAs/InGaP Heterostructure by C-V Profiling
Authors:
Shouvik Datta,
M. R. Gokhle,
A. P. Shah,
T. K. Sharma,
B. M. Arora
Abstract:
It is known in the literature that InGaP alloy synthesized within a certain range of growth temperature (520 - 720 degree Centrigrade) shows Cu-Pt crystal ordering. This ordering reduces the band gap energy . It was predicted that the ordering induced modification of energy levels of InGaP lowers its conduction band edge, which could change a type I band alignment at a GaAs/InGaP heterojunction…
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It is known in the literature that InGaP alloy synthesized within a certain range of growth temperature (520 - 720 degree Centrigrade) shows Cu-Pt crystal ordering. This ordering reduces the band gap energy . It was predicted that the ordering induced modification of energy levels of InGaP lowers its conduction band edge, which could change a type I band alignment at a GaAs/InGaP heterojunction to type II . Here we report the first observation of this ordering related type I/II transition for a GaAs/InGaP heterojunction by C-V profiling experiments done at room temperature .
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Submitted 6 March, 1999;
originally announced March 1999.