

Students and collaborators:
 Graduate students:
 20242025: Ph.D. Arnab Adhikari (with Robert Raussendorf).
 Fall 2023: (visiting) M.Sc., Luis Walther  "Renormalized phonon spectra".
 20192023: Ph.D., Stepan Fomichev  "Electronphonon coupling in insulators: beyond the Migdal limit".
 2019: (visiting) Ph.D., FangHan (Edward) Lam "Bipolaronic superconductivity in system with strong Peierls electronphonon coupling".
 20172022: Ph.D., Oliver Yan Chuen Yam (with G. Sawatzky)  "Peierls polarons in complex lattices"
 20172019: M.Sc., Stepan Fomichev  "Effects of the lattice distortion on the magnetic order in rareearth nickelates"; thesis pdf
 20162018: M.Sc., Tao Fang (with G. Xia)  "Fabrication and Raman study of twisted stacked few layer black phosphorus"; thesis pdf
 20162018: M.Sc., Nathan Cheng (with D. Manske)  "Higgs spectroscopy of superconductors : a new method to identify the superconducting gap symmetry"; thesis pdf
 2016: (visiting) Ph.D., Krzysztof Bieniasz (MPIUBC program, with A. Oles)  spin and orbital polarons in KCuF_{3}.
 2014: (visiting) Ph.D., Krzysztof Bieniasz (MPIUBC program, with A. Oles)  orbital polarons in KCuF_{3}.
 20132018: Ph.D., John Sous
(with R. Krems)  "Peierls bipolarons and localization in solidstate and molecular systems"; thesis pdf
 20132015: M.Sc., Alfred Cheung  "Large electropositive cations as surfactants for the growth of polar epitaxial films"; thesis pdf
 20122016: Ph.D., Mirko Moeller  "Temperature driven spectral weight transfer in doped magnetic insulators"; thesis pdf
 20112015: Ph. D., Clemens Adolphs  "Extensions beyond standard models"; thesis pdf
 20112012: (visiting) M.Sc., Mirko Moeller  "Spin polarons and bipolarons on 1D ferromagnetic lattices";
 20102014: Ph.D., Hadi Ebrahimnejad  "Variational studies of dressed quasiparticles' properties and their interactions with external potentials"; thesis pdf
 20102012: M. Sc., MattheW Badali (with Joerg Rottler)  "Nonlocal dielectric response of water"; thesis pdf
 20062011: Ph.D., Dominic Marchand (with Philip Stamp)  "Polaron physics beyond the Holstein model"; thesis pdf
 20062011: Ph.D., Jinshan Wu  "Quantum transport through open systems"; thesis pdf
 20052009: Ph.D., Glen L. Goodvin  "Study of polaron properties using the MA approximation"; thesis pdf
 20052006: (visiting) Ph.D., Benjamat Srisongmuang  transport in superconductor  semiconductor junctions
 20042006: M. Sc., Jinshan Wu  "Nonequilibrium evolution of quantum systems connected to multiple baths"; thesis pdf
 20042006: M. Sc., Kelly Cheung  "Waveguides for spinpolarized currents in dms  nanomagnet hybrids"; thesis pdf
 20022004: M. Sc., Adel Kassaian, "Magnetic susceptibility of diluted magnetic semiconductors"; thesis pdf
 Undergraduate students:
 20232023: ManYat (Jack) Chu, "Timeresolved ARPES of excitons in C60 films."
 summer 2023: Oliver Tong, USRANSERC, "Spectroscopies of few molecule ensembles."
 summer 2022: XiaoXiao (Alice) Xiong, WLIURA, "Spectroscopies ofpolarized anions."
 20202021: Paul Froese, Honors thesis, "Multiband BCS".
 20202021: James Wu, "Excitonexciton interactions".
 summer 2020: Rodrigo Chavez Zavaleta, SBQMI Quantum Pathways, "Metalinsulator transition in a 2D nickelate layer".
 20182019: Teresa Kulka, Phys 447 project, "Groundstate and spinon dispersion for the 1D AFM Heisenberg chain"
 20142015: Cam Sture, Phys 349 project, "Spin waves in a 1D antiferromagnetically ordered system"
 summer 2013: Alfred Cheung, Dean of Science Summer Research Award, "Polaron dynamics in an electric field"
 summer 2012: Alfred Cheung, USRANSERC,
"Exciton dissociation near interfaces"
 20092010: Ashley Cook, Honors thesis,
"Impurity bands in doped 3D semiconductors"
 summer 2009: Ashley Cook,
"Impurity bands in doped 1D semiconductors"
 20032004: Mandy Man Chu Wong, Honors thesis, "Zeeman localization of charge carriers in DMSpermalloy hybrids"
 20032004: Kiri Moana Nichol, Honors thesis, "Local currents on novel lattice geometries in the Hubabrd model"
 20032004: Alex Kubanek, (with Jeff Young), Honors thesis, "Semiconductor quantum dots and photonic microcavities"
 Postdoctoral fellows:
 20242026: Kyle Monkman (with Marcel Franz); SBQMI Fellow.
 20202022: Leon Ruocco SBQMI QCGC Fellow
 20182021: Krzysztof Bieniasz MPIUBCUT Fellow
 20172020: Mi Jiang SBQMI Fellow
 20172018: Mirko Moeller SBQMI Fellow
 20122013: Steven Johnston (with George Sawatzky) MPIUBC Fellow
 20112013: Anamitra Mukherjee (with George Sawatzky)
 20112011: Dominic Marchand
 20092010: Glen Goodvin
 20062009: Lucian Covaci
 20042006: Nicolas Laflorencie (with Ian Affleck)
 Other affiliations:
Research Interests (in chronological order). Note that the more recent topics may not be uptodate. To get a better idea of my recent interests, check my publication list which is kept uptodate.
The meronvortex model for high Tc superconductivity:
The occurrence of high Tc
superconductivity in layered perovskite materials has sparked broad
interest in the quantum properties of magnetically correlated electron
systems. At low temperature all parent compounds (such as LaCuO or
YBaCuO, see figures 1 and 2) are insulators with longrange
antiferromagnetic order in the CuO planes. As charge carriers are
introduced in the CuO planes by doping, this long range AFM order
disappears, leading to a metallic phase with striking nonFermiliquid
properties. Superconductivity emerges from this unconventional metal
as the system is cooled. The development of a microscopic model of
this unconventional metal is one of the outstanding issues in quantum
manybody theory.
Fig. 1: LaCuO parent compound 
Fig. 2: YBaCuO parent
compound 
I have a longstanding interest in this problem, which started with my
Ph.D. work with Prof. Sajeev John from
University of Toronto. He proposed a new microscopic model (the
spinflux Hamiltonian) for a strongly repulsive electron gas on a 2D
square lattice. The rough idea is that nearest neighbor Coulomb
repulsion stabilizes a state in which electrons undergo a "somersault"
in their internal spinspace (spinflux). We have shown that when this
spin1/2 antiferromagnetic (AFM) insulator is doped, the charge
carriers nucleate mobile, charged, bosonic vortex solitons accompanied
by unoccupied states deep inside the MottHubbard chargetransfer gap.
Our model provides a unified microscopic basis for (i)
nonFermiliquid transport properties, (ii) midinfrared optical
absorption, (iii) destruction of AFM long range order with doping,
(iv) angled resolved spectroscopy (ARPES), and (v) dwave preformed
charged carrier pairs. The approximation used to study the 2D
spinflux Hubbard model is the Configuration Interaction Method. In
1D, this approximation leads to excellent agreement with the exact
Bethe Ansatz results, as well as a clear demonstration of the
spincharge separation: the charge is carried by charged bosonic
domainwalls, while the spin is carried by neutral fermionic
domainwalls.
Details are presented in this online
presentation as well as in this
review article. You can also look
at this
more recent poster or
this
older poster. Future work will
concentrate on investigations of various magnetic, transport and
optical properties of this model, so that a detailed comparison with
available experiments can be carried out.
Publications
 Midgap states of a twodimensional antiferromagnetic Mott insulator:
Electronic structure of meronvortices Sajeev John, Mona Berciu and Andrey Golubentsev, Europhys. Lett.
41, pp. 3136 (1998).
 Charged bosons in a doped Mott insulator: Electronic properties of domain
wall solitons and meron vortices Mona Berciu and Sajeev John, Phys. Rev. B 57, pp. 95219543 (1998).
 Numerical study of multisoliton configurations in a doped antiferromagnetic
Mott insulator Mona Berciu and Sajeev John, Phys. Rev. B 59, pp. 1514315159 (1999).
 Quantum dynamics of charged and neutral magnetic solitons: Spincharge separation in
the onedimensional Hubbard model Mona Berciu and Sajeev John, Phys. Rev. B 61, pp. 1001510028 (2000).
 Microscopic model for dwave chargecarrier pairing and nonFermiliquid behavior
in a purely repulsive twodimensional electron system Mona Berciu and Sajeev John, Phys. Rev. B 61, pp. 1645416469 (2000).
 Microscopic model for dwave pairing in cuprates: what happens when electrons somersault?
Mona Berciu and Sajeev John, Physica B 296, pp. 143155 (2001).
 Magnetic structure factor in cuprate superconductors: the meronvortex model, Mona Berciu and Sajeev John, Phys. Rev. B 69, 224515 (2004).
go to top
Diluted magnetic semiconductors:
Diluted magnetic semiconductors (DMS) are semiconductors of the
general type (A,Mn)B, where AB is either a IIVI or a IIIV
semiconductor and M a magnetic element, most commonly Mn. Substitution
of a small fraction x of the element A by Mn impurities (and in the
case of IIVI semiconductors an additional charge dopant, such as valenceV P on
the B site) leads to the emergence of a semiconductor with
ferromagnetic properties. This is due to the
interactions of the S=5/2 Mn spins (coming from the
halffilled 3d shell of the Mn) with the spins of the charge carriers
introduced by the Mn dopants, or, in the case of IIVI semiconductors,
by the additional dopant.
Thus, DMS cane be thought of as an inert host
semiconductor doped with both localized spins and
carriers (electrons or holes) that are either itinerant, or localized
on a much longer length scale. In that sense, they belong to the
general family of correlated electron systems, which include a number
of fascinating materials such as cuprates, manganites, heavy fermions
and other Kondo lattice systems.
Electronic materials containing local moments have been studied for
a long time. What makes the DMS so fascinating is that they belong to a
regime that has previously been neglected. While the name diluted
magnetic semiconductors implies (correctly) that the system has only a
small percentage of localized spins, they are at the opposite extreme
of the dilute magnetic alloys such as Fe or Mn in Cu, the canonical
systems involving itinerant fermion and localized spin degrees of
freedom, which have been studied extensively. In the
dilute magnetic metallic alloys, the low density of spins are a
perturbation on the Fermi liquid representing the nonmagnetic host
metal. As a result, depending on the concentration of the local moments, they
may be studied in terms of dilute Kondo systems, or amorphous magnetic
systems with a spinspin coupling mediated by the Fermi sea of
conduction electrons (RKKY coupling), which lead often to spin glass
behavior.
By contrast, in the regime of interest, the carrier density in DMS is
significantly lower than the (low) localized moment density, so
the spins become an integral part of the description of the system and
its magnetic phase, rather than a mere perturbation on a metallic
Fermi sea. In that sense, the situation is even more extreme than
e.g., in Kondo lattice and heavy Fermion materials, where the two
species have comparable densities. This large, inverted, ratio of
local moments to carriers is in fact similar to that in the high Tc
cuprates. However, unlike the cuprates, the density of local moments
is low and incommensurate with the lattice, and the carriers and the
spins are not in the same band. As a consequence of the low moment
density, the exchange between local moments is not standard direct or
superexchange, as in the cuprates, but is mediated by the carriers,
even though their density is so small. Thus, the DMS are in rather
different region of phase space of electronic materials with local
moments, than other correlated electron systems.
Despite this difference, most models of diluted magnetic
semiconductors start from the high carrier density limit, where the
carriers may be modeled as free carriers moving in the conduction or
valence band. This is understandable, since in
the high density limit the carrier kinetic energy is the largest
energy in the problem, and calculations may be done perturbatively
starting from the noninteracting Fermi gas. However, most of the
interesting behavior is seen at low carrier densities, where the
system is insulating, or not too far from the metalinsulator
transition. Consequently, in collaboration with Prof. R. N. Bhatt
from Princeton University, we have concentrated on the
low density regime, investigating a simple model describing
carriers in an impurity band formed from the bound impurity states.
We found that disorder in the position of Mn atoms has significant influence on the
properties of the system; for instance, increased disorder leads to a higher critical temperature.
Here is a recent poster on this work.
We have performed meanfield and Monte Carlo investigations of this Hamiltonian, as well as used randomphase
approximation to compute the corresponding spinwave spectra. Future work will concentrate on computation of
various response functions, as well as on the possible use of such materials for devices.
Publications
 Effects of disorder on ferromagnetism in diluted magnetic semiconductors
Mona Berciu and R.N. Bhatt, Phys. Rev. Lett. 87, 107203 (2001).
 Twocomponent approach for thermodynamic properties in diluted magnetic semiconductors
Malcolm P. Kennett, Mona Berciu and R.N. Bhatt, Phys. Rev. B 65, 115308 (2002).
 MeanField Approach to Disorder Effects on Ferromagnetism in GaMnAs
Mona Berciu and R. N. Bhatt, Physica B 312313 , pp. 815817 (2002).
 Numerical simulations of random spins (and fermionic) models with a wide distribution of energy scales
R. N. Bhatt, Xin Wan, Malcolm P. Kennett and Mona Berciu, Comp. Phys. Comm. 147, 684689 (2002).
 Monte Carlo simulations of an impurityband model for IIIV diluted magnetic semiconductors
Malcolm P. Kennett, Mona Berciu and R.N. Bhatt, Phys. Rev. B 66, 045207 (2002).
 Diluted Magnetic Semiconductors in the Low Carrier Density Regime"
R. N. Bhatt, Mona Berciu, Malcolm P. Kennett and Xin Wan, J. of Supercond.: INM 15 , pp. 7183 (2002).
 Spinwaves in disordered IIIV diluted magnetic semiconductors
Mona Berciu and R.N. Bhatt, Phys. Rev. B 66, 085207 (2002).
 Meanfield approach to disorder effects on ferromagnetism in (III,Mn)V at low carrier densities,
Mona Berciu and R. N. Bhatt, Phys. Rev. B 69, 045202 (2004).
 Magnetic susceptibility of diluted magnetic semiconductors at low carrier densities, Adel Kassaian and Mona Berciu, Phys. Rev. B 71, 125203 (2005).
go to top
Photonic bandgap materials:
Photonic crystals are the optical analog of electronic crystals: the dispersion relations for photons have
bands of allowed photonic states separated by gaps in which light cannot propagate. Various photonic crystal
structures are known
to have full, 3D gaps. However, some of the proposed structures are extremely hard to realize experimentally. Thus,
there is a significant effort to find optimized structures, which have large gaps, and which can also be easily
created experimentally. My interests in this field are also related
to investigating the role of the symmetries of the photonic lattice on the properties of its dispersion bands and
those of various defects that can be introduced in such crystals. I am also very interested in studying the role
of disorder on the density of photonic states.
 Photonic band gaps based on tetragonal lattices of slanted pores, Ovidiu Toader, Mona Berciu and Sajeev John, Phys. Rev. Lett. 90, 233901 (2003).
go to top
Integer Quantum Hall Effect:
My interest in this field has started with a collaboration with experimentalists from Princeton University, who were hoping to measure the Hofstadter butterfly in a 2DES modulated with a triangular periodic gate, with a lattice constant of around 39nm. The butterly cannot be observed directly, due to the fact that disorder is still significantly larger than the periodic modulation. However, the small periodic potential has a significant effect on the nature of the electron wavefunctions, particularly close to the critical regime (see pictures below for some local densities of states projected onto the lowest Landau level, without (a) and with (b) a periodic modulation; (d) shows the semiclassical prediction). This results in a reproducible fluctuationlike pattern in the longitudinal conductivity, which was indeed measured experimentally.
After some work, in collaboration primarily with Chenggang Zhou, to try to understand these effects, we then studied fluctuations of the longitudinal and Hall resistivities of mesoscopic samples, close to the transition between integer Quantum Hall plateaus. We performed firstprinciple simulations to compute these quantities and found excellent agreement with measurements done in D. Shahar's group, at Weismann Institute. We then explained the behaviour observed based on a simple generalization of the LandauerButtiker model, which takes into account both tunneling and chiral currents  the coexisting mechanism for transport.
 A Laterally Modulated 2D Electron System in the Extreme Quantum Limit, Sorin Melinte, Mona Berciu, Chenggang Zhou et. al., Phys. Rev. Lett. 92, 036802 (2004).
 The longitudinal conductance of mesoscopic Hall samples with arbitrary disorder and periodic modulations, Chenggang Zhou and Mona Berciu, Phys. Rev. B 70, 165318 (2004).
 Resistance fluctuations near integer quantum Hall transitions in mesoscopic samples, Chenggang Zhou and Mona Berciu, Europhys. Lett. 69, 602608 (2005).
 Effects of large disorder on the Hofstadter butterfly, Chenggang Zhou, Mona Berciu and R. N. Bhatt, Phys. Rev. B 71, 125310 (2005).
 Correlated mesoscopic fluctuations in integer quantum Hall transitions, Chenggang Zhou and Mona Berciu, Phys. Rev. B 72, 085306 (2005).
go to top
Spintronic devices:
One of the central features of magnetic semiconductors (both the new IIIMnV materials and the more established IIMnVI) is that a relatively small external magnetic field causes enormous Zeeman splitting. This effect can be used in conjunction
with external inhomogeneous magnetic fields to engineer spinpolarized
chargecarrier eigenstates with certain desirable features. Producing the required nonuniform magnetic fields with nanoscale spatial variation is a formidable problem beyond present nanolithographic capabilities. We overcome this challenge with hybrid structures that couple the magnetic semiconductor with a superconductor in its vortex phase and/or with nanomagnets of various shapes. The spinpolarized carrier states trapped in the regions of high magnetic field exhibit various interesting properties that can be controlled by controlling the applied magnetic field.
Publications
 Nanoscale Zeeman localization of charge carriers in diluted magnetic semiconductorpermalloy hybrids, Mona Berciu and Boldizsar Janko, Phys. Rev. Lett. 90 , 246804 (2004).
 Manipulating nanoscale spin and charge textures in diluted magnetic semiconductors using superconducting vortices, M. Berciu, T. Rappoport and B. Janko, Nature 435, 7175 (2005).
 The effect of the Abrikosov vortex phase on spin and charge states in magnetic semiconductorsuperconductor hybrids, Tatiana G. Rappoport, Mona Berciu, and Boldizsar Janko, Phys. Rev. B 74, 094502 (2006).
go to top
Polarons and bipolarons
We have developed a novel, highly efficient yet accurate analytical
approximation for the Green's function of a Holstein polaron. It is
obtained by summing all the selfenergy diagrams, but with each
selfenergy diagram averaged over the momenta of its free
propagators. The result becomes exact for both zero bandwidth and
for zero electronphonon coupling, and is accurate everywhere in the
parameter space. The resulting Green's function satisfies exactly
the first six spectral weight sum rules. All higher sum rules are
satisfied with great accuracy, becoming asymptotically exact for
coupling both much larger and much smaller than the free particle
bandwidth. Comparison with existing numerical data also confirms
this accuracy. We used this approximation to analyze in detail the
redistribution of the spectral weight as the coupling strength
varies. This method is currently being generalized for more complicated models.
Publications
 Green's function of a dressed particle, Mona Berciu, Phys. Rev. Lett. 97, 036402 (2006).
 The Green's function of the Holstein polaron, Glen L. Goodvin, Mona Berciu, and George A. Sawatzky, submitted to PRB;
go to top

