We present a review of the basic ideas and techniques of the spectral density functional theory which are currently used in electronic structure calculations of strongly{correlated materials where the one{electron description breaks down. We illustrate the method with several examples where interactions play a dominant role: systems near metal{insulator transition, systems near volume collapse transition, and systems with local moments.

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Electronic structure calculations with dynamical mean-field theory

In this review article a comprehensive analysis of the developments in ultraviolet (UV) detector technology is described. At the beginning, the classification of UV detectors and general requirements imposed on these detectors are presented. Further considerations are restricted to modern semiconductor UV detectors, so the basic theory of photoconductive and photovoltaic detectors is presented in a uniform way convenient for various detector materials. Next, the current state of the art of different types of semiconductor UV detectors is presented. Hitherto, the semiconductor UV detectors have been mainly fabricated using Si. Industries such as the aerospace, automotive, petroleum, and others have continuously provided the impetus pushing the development of fringe technologies which are tolerant of increasingly high temperatures and hostile environments. As a result, the main efforts are currently directed to a new generation of UV detectors fabricated from wide band‐gap semiconductors the most promising ...

Semiconductor ultraviolet detectors

We review studies of the electromagnetic response of various classes of correlated electron materials including transition metal oxides, organic and molecular conductors, intermetallic compounds with $d$- and $f$-electrons as well as magnetic semiconductors. Optical inquiry into correlations in all these diverse systems is enabled by experimental access to the fundamental characteristics of an ensemble of electrons including their self-energy and kinetic energy. Steady-state spectroscopy carried out over a broad range of frequencies from microwaves to UV light and fast optics time-resolved techniques provide complimentary prospectives on correlations. Because the theoretical understanding of strong correlations is still evolving, the review is focused on the analysis of the universal trends that are emerging out of a large body of experimental data augmented where possible with insights from numerical studies.

/pdf/electrodynamics-of-correlated-electron-materials-2qg1x9aaaz.pdf

Electrodynamics of correlated electron materials

본 연구는 현존하는 여덟 개의 〈뿔피리를 부는 제빵사〉 작품이 산발적으로 우연히 등장한 것이 아닌 서로 유기적인 영향관계를 주고 받으며 생산된 하나의 도상으로 살펴보았다. “뿔피리를 부는 제빵사” 그림에 대한 현재까지의 연구는 대부분 그림에 나타나 있는 다양한 빵들을 성만찬을 의미하는 종교적 상징으로 해석하고 있다. 이에 반해 본 연구는 “뿔피리를 부는 제빵사” 도상이 유독 17세기 중후반 네덜란드에서 생산된 사실에 주목하고, 그 이유를 동시대 네덜란드인들의 일상에 직접적인 영향을 주었던 빵의 생산 및 공급 그리고 유통과정에서 찾고자 한다. 여기에는 빵이 당대 네덜란드인들의 식생활 뿐 아니라 그들의 경제생활 더나아가 국가경제 전체에 어떤 중요한 역할을 했는지에 대한 연구가 선행되어야 한다. 주로 풍속화의 전통으로 해석되었던 “뿔피리를 부는 제빵사” 그림은 본 연구에서 풍속화와 초상화 두 장르가 혼합한 도상으로 다루어졌으며, 이제까지의 연구에서 미흡했던 초상화로서의 가능성이 적극적으로 제기되었다. 제빵사의 초상화 또는 화가의 자화상으로 생산된 〈뿔피리를 부는 제빵사〉의 시대적 배경에는 제빵사의 경제적 여유와 사회적 지위상승 그리고 제빵사를 영혼의 양식을 공급하는 고귀한 직업으로 승화시킨 칼비니즘의 영향이 있다. 제빵사들이 부를 축적할 수 있었던 배경에는 국가가 나서서 빵의 생산과 공급을 철저하게 관리해준 덕분이었다. 17세기 네덜란드의 제빵사들은 길드에 속해야만 빵을 팔 수 있었고 각 제빵사들의 빵 만드는 법은 외부인들에게 절대로 공개되지 않았다. 동시에 당시 네덜란드인들은 세금을 내지 않고는 빵을 먹기가 힘들었다. 이런 이유로 개인의 집에 오븐을 설치하는 것은 금지되었고, 빵은 꼭 관헌들에게 세금을 내는 합법적인 제빵사에게서만 살 수 있었다. 이렇게 17세기 네덜란드는 직업으로서 빵 굽는 사람을 보호하는 동시에 단속하기 위해 매우 엄격한 규정을 제정하였다. 17세기 네덜란드인들에게 빵은 그들의 식생활 뿐 아니라 그들의 다양한 직업과도 관련이 있었으며 빵의 생산과 유통과정에 개입한 국가의 권위와 통제를 상징하기도 한다. 빵의 생산과 유통에 붙는 세금은 국가의 경제를 지탱하고 발전시키는데 중요한 재원이었으며, 동시에 국민들을 엄격하게 관리함으로써 국가의 질서를 세우기 위한 효과적인 방편이었다. 빵가격은 변동이 심한 곡물가격과 빵의 크기에 따라서 엄격하게 결정되었다. 네덜란드정부는 평상시에 빵의 공식적인 무게와 가격을 고정시켰는데, 이는 다른 음식에서는 찾아볼 수 없는 유일한 관리방식이었다. 따라서 동시대 네덜란드인들에게 빵은 국가의 적극적인 개입과 관리체계를 통해서만 그들의 입에 넣을 수 있는 음식이었다. 이러한 맥락에서 볼 때, 본 연구에서 다룬 〈뿔피리를 부는 제빵사〉 그림에서 풍성하게 차려진 다양한 빵은 경제적인 풍요와 정치적인 안정을 의미한다. 여러 가지 다양한 음식 중에서도 빵이 이러한 의미를 가장 효과적으로 전달한다는 점에서 〈뿔피리를 부는 제빵사〉 그림에서 빵은 당시 네덜란드의 물질적 풍요뿐만 아니라 국가의 번영과 안녕을 의미한다고 볼 수 있다.

17세기 네덜란드 미술에 나타난 빵의 사회사

Magnetic ions embedded in a nonmagnetic host exhibit a number of unusual low-temperature properties known collectively as the Kondo effect. Conventional small-parameter expansions for the Kondo effect are plagued by infrared divergences, and the theory of dilute magnetic alloys has for many years centered on nonperturbative techniques. This paper reviews a novel perturbative approach to the magnetic alloy problem based on expansion in $\frac{1}{N}$, with $N$ the ionic angular momentum degeneracy. This approach is analogous to perturbative expansions in statistical mechanics and field theory based on an integer-valued parameter (such as the number of spin components or the number of colors). The large-$N$ expansion reproduces the essential features of the Kondo effect at $O(1)$ and appears to yield convergent (or asymptotic) expressions for ground-state properties. In contrast with previous nonperturbative approaches, the expansion provides information on dynamic, as well as static, properties. The evidence for convergence of the expansion is reviewed, and large-$N$ calculations are compared with exact results for static properties. A number of independent, but essentially equivalent, approaches to the large-$N$ expansion have been developed during the last five years. These techniques are reviewed pedagogically, and their relative strengths and weaknesses emphasized. A guide to notation in the recent literature is provided.

Review of techniques in the large-N expansion for dilute magnetic alloys

Volume collapse in the Kondo lattice

The density of states of yttrium compounds YM2 (M = Fe, Co, Ni) is calculated within a tight-binding scheme. Using the Stoner model, conclusions are drawn about their magnetic and electronic properties. In YCo2, we show that one has to introduce paramagnon effects in order to explain the electronic properties. The additional d-f interaction introduced by rare-earth atoms (R) in RM2 is treated with a Hartree-Fock approximation. Little change is obtained in iron and nickel compounds. Conversely, in RCo 2 compounds, the exchange field due to the rare-earth atoms can produce collective metamagnetism as described by Wohlfarth and Rhodes [1] and cobalt becomes magnetic. This effect stems from the position of the Fermi level of these compounds in a steep decrease of the density of states. Curie temperature, order of the transition and effect of an external magnetic field are studied following this method. Other consequences are drawn from the model.

https://hal.archives-ouvertes.fr/jpa-00209160/document

Density of states and magnetic properties of the rare-earth compounds RFe2, RCO2 and RNi2

The gamma - alpha transition and the related volume collapse are explained within a compressible Kondo lattice where the variation of mod J mod /D with volume is taken into account. The authors show that, contrary to the promotion model, the Kondo contribution is sufficient to induce a first-order transition at low temperature from a magnetic to a Kondo phase. The alpha phase is then characterised by an extremely high Kondo temperature. The calculations are also made at zero and finite temperatures and the existence of a critical point is demonstrated. Finally, they propose a generalisation to the effect of a chemical pressure. Applications are made to Ce, CeAl2 and Ce1-xThx.

/pdf/the-g-a-transition-in-cerium-compounds-4zup5dwe3p.pdf

The γ-α transition in cerium compounds

We calculate the dielectric function of the lattice Anderson model via an auxiliary-boson large-N method suitably generalized to include the effects of the long-range part of the Coulomb interaction. We show that the model exhibits a low-lying plasma oscillation at a frequency \ensuremath{\omega} on the order of the Kondo temperature of the model, in addition to the usual high-frequency plasma oscillation. We also analyze the Anderson model without the long-range Coulomb interaction, computing the Landau parameter ${F}_{0s}$, and showing that the model has a zero-sound mode whose velocity we compute. We derive the version of the f sum rule applicable to our model and show that our results satisfy it.

/pdf/plasma-oscillations-in-heavy-fermion-materials-3bx7z9zdaf.pdf

Plasma oscillations in heavy-fermion materials

A systematic extension of the model of almost localized fermions to finite temperatures is presented. It consists in accounting for the quantum fluctuations beyond the Gutzwiller approximation (GA). We use the slave-boson formulation of the Hubbard model which has been proved to be equivalent at the saddle-point level to the GA. We determine the values of the Lagrange multipliers introduced to enforce the constraints, and show how they involve a Mott gap at nearly half filling for {ital U}{gt}{ital U}{sub {ital c}} (localization edge). The results of the GA for the correlation functions are reproduced at the saddle point when taking into account the implicit dependence of the boson fields with the external excitation. The Gaussian fluctuations are then considered in a renormalized basis of the boson fields which brings out two distinct channels (symmetric {ital s} and antisymmetric {ital a}). We show how the free energy and the correlation functions can be simply expressed as a function of the same Landau parameters {ital F}{sub 0}{sup {ital s}} and {ital F}{sub 0}{sup {ital a}} defined in the GA.

/pdf/functional-integral-approach-to-strongly-correlated-fermi-477uarex0s.pdf

Mireille Lavagna

Papers

Volume collapse in the Kondo lattice

Density of states and magnetic properties of the rare-earth compounds RFe2, RCO2 and RNi2

The γ-α transition in cerium compounds

Plasma oscillations in heavy-fermion materials

Functional-integral approach to strongly correlated Fermi systems: Quantum fluctuations beyond the Gutzwiller approximation