Physics Department, University of Tennessee

Additional information about the instructor can be found in:

This course web page contains:

(I) Summary of main topics to be covered in Dagotto's lectures

(II) Summary of main topics to be covered by invited speakers

(III) Homework description and projects

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** ****
(I) SUMMARY OF MAIN TOPICS TO BE COVERED IN DAGOTTO'S LECTURES:**

** (1) Aschroft+Mermin, chapter 17. Here we start with
the analysis of screening in metals.
We will define epsilon(q) and
chi(q), using the ``linear approximation'' or ``linear response''
assumption. Before this
subject, details about the homework projects will be discussed in the
first class.
**

(2) To evaluate chi(q), we will use the Thomas-Fermi approx. where the potential is

assumed to change slowly with position. A formula will be derived in q-space and transformed

to real space, showing the presence of an exponential suppression of the original 1/r potential

of a charged impurity. The screening length associated with the cloud of electrons around

the impurity will be estimated for standard metals and it will be found to be ~0.5 - 1 Angstrom.

The Lindhard theory of screening is discussed next. The perturbative calculation will be sketched.

The limit q-->0 will be shown to give Thomas Fermi. The real space total potential has oscillations

(Friedel oscillations) related with singular behavior at q=2kF in q-space. The formula for

epsilon(q,omega) will also be briefly discussed.

(3) Special cases of Lindhard function. Kohn anomalies in phonons. Explanation of singular

behavior at 2kF based on Fermi surface arguments. RKKY interaction between spins. Plasma

oscillations in electrons. Exponential or oscillatory decay of electric fields inside the sample.

Plasmon and its energy vs. k relation. Explanation in terms of long-range forces.

Plasmon frequency for positive ions without electrons. Inclusion of electrons and relation

between phononic and Fermi velocities.

by positive ions. Retardation. Overscreening, intuitive meaning. Attraction between carriers,

once retardation caused by heavy ions is considered. Switch from external charges to a pair of

electrons within the sample (q->k-k', omega -> Delta E/hbar). Representation in terms of Feynman

diagramms, conservation of momentum and energy at every vertex. Discussion of Cooper's paper

(Phys. Rev. 104, 1189 (1956)), following Kittel's book. We will end with the writing of the

Schroedinger equation in terms of the energy e and the matrix element <e|V|e'> of the potential.

** (5) Solution
of the 2 electron Cooper problem in the
presence of many other inert el****ectrons
near the Fermi level.**

Formula for Delta as a function of omega_Debye, density-of-states at EFermi, and attraction V. Size of pair.

Relevant Hamiltonian in the space of pairs. BCS wave function. Values of uk and vk, and gap Delta.

(6) We will start with high temperature
superconductors following Rev. Mod. Phys. 66, 763 (1994).

**
Introduction on page 765: typical chemical
compositions, Tc's, structures, ionic charge of
the many elements. Coppers are in a state
of spin 1/2. We will discuss single- and bi-layer
materials.
Chemical doping such as La replaced by Sr. Phase diagram of LSCO.
Optimal,
underdoped
and overdoped doping. Parent insulating antiferromagnetic compound
(x=0).
Doping is through holes, but it
can also be electrons (NdCeCuO). Linear resistivity, as opposed
to
quadratic as in standard metals.
**

(7) Then, we move to the brief notes ``High-Tc Superconductors'' (copy to be provided), which

describe the degrees of freedom of relevance in Cu++. The splitting of levels analysis leads to

a ``hole'' picture where at each Cu++ there is a S=1/2 spin. The antiferromagnetic J is

estimated using a 4-steps process (perturbation in t_{pd}). J is found to be ~0.1eV,

much smaller than other energy scales, yet still 10 times the value of kBTc for superconductivity.

Heisenberg model for x=0 will be discussed. We will finish the brief set of notes on high-Tc,

addressing the t-J model. It will be discussed the Zhang-Rice singlet and the reduction from a

problem with Cu and O degrees of freedom to just Cu. We will also discuss the Hubbard model

and the large U/t limit.

** (8) We will study the instructor's
notes (hardcopy to be provided) on the mean-field solution of the
Hubbard
model. Since the math is fairly clear in the notes, only the important
concepts
will be
addressed. We will discuss up to page
10 approx. The presence of a gap induced by the interaction
U is
important (Hubbard gap , aka Mott gap; Hubbard-Mott insulator).
Similarities with formulas for
BCS superconductors are clear. The
mean-field Hamiltonian appears to not conserve momentum
since electrons with momentum p can be
transformed into p+Q. Explanation will be
provided based
on the spin-waves of momentum Q which
can be easily created**. We
will continue with the study

of the one-band Hubbard model, analyzing the ``quasiparticles'' gamma

electrons with momentum p and spin sigma, and electrons with momentum p+Q and spin sigma.

We will discuss the similarities with the BCS formalism.

** (9) We will study
spin-ladders as examples of materials that have a ``spin gap''. Basic
ideas and
suggestion of superconductivity upon
doping. Experimental realizations. Reference:
**

** (10) We will learn
what ``manganites'' are, namely the materials with the colossal
magnetoresistance (CMR).
Hamiltonians for manganites will be
presented and they will involve many active orbitals, and strong
Hund couplings. The experimental phase
diagram will be discussed, as well as the magnetoresistance
effects. Monte Carlo results will
suggest the main competing phases and also the relevance of phase
separation ideas. Orbital order will be
explained. Experimental evidence of inhomogeneities will be
discussed. Numerical results suggesting that
the inhomogeneities are crucial to understand the CMR
will be given.
**

** We will follow E. Dagotto et al.,
Phys. Reports 1, 344 (2001). A copy can be found
here.
**

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(in random order, and this is a tentative list):

** (1) Molecular conductors, by Khaled Al-Hassanieh (1 week)
(2) Spin systems and MC techniques, by Roger Melko
(1 class)
(3) Numerical studies of Manganese oxides, by Gonzalo Alvarez (1 week)
(4) Density Matrix Renormalization Group and quantum
spin chains,
by Fabian Heidrich-Meisner
(1 week)
(5) Diluted Magnetic Semiconductors, by Yucel Yildirim (1 week)
**

** Other possible speakers are Cengiz Sen (manganites),
Yucel Yildirim (DMS), Florentin
Popescu (DMFT),...**

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** (III) HOMEWORK DESCRIPTION AND
PROJECTS:
**

**
Below you can find the titles
of several possible projects. The presentation of the project
will consist of: (1) a brief (~5-10 pages) document
with the main ideas, followed by (2)
an oral presentation. The main goals of this ``homework''
are: (i) to learn the
most basic aspects of
each item, collecting the most relevant
references, and (ii) to provide
an opportunity to the student to practice lecturing
in a friendly environment.
**

**
The enfasis of both the written document and the oral seminar must be
on the
intuitive main issues.
Each document/seminar must contain a set of simple ideas, cartoons,
concepts that are easy to remember.
Complicated math or
complicated experimental details
are not of
interest, but ideas and concepts.
**

** If you have preferences
on
what project to select, let me know and I will try to adapt
to your wishes. Otherwise I will assign at random.
**

** The first set of
projects must
be ready
by mid February, the second set by mid/late March.
The presentation of the report must be in PDF or PS or DOC
format. The students
will deliver a talk on the subject of their research,
which will must last approx. 20-30 minutes.
The
best would be
if this talk is given using powerpoint tools.
**

** POSSIBLE
TOPICS:
**

**
*
Bose-Einstein condensation in atoms -- recent
Nobel
* Quantum computing (introduction)
**

* Semiconducting and metallic polymers. -- recent Nobel

* Conductance quantization

* Neutron Scattering at ORNL

*

* ORNL nanocenter. Organization, areas of expertise

* Protein Folding, introduction

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** Email
addresses: (*=@)
**