One of the most interesting and puzzling issues in the research of strongly correlated electron systems is anomalous transport phenomena. In conventional metals, the fundamental transport coefficients, such as the dc-resistivity, Hall coefficient, and magnetoresistance, are well-described by Landau Fermi liquid theory. The dc-resistivity ρ(xx) which stems from electron-electron scattering varies as ρ(xx) ∝ T2. The Hall coefficient RH signifies the topology of Fermi surface and carrier density. It is given as RH = 1/ne, where n is a carrier density, and is independent of temperature. The magnetoresistance due to an orbital motion of carriers obeys a simple scaling law, so-called "Kohler's rule", Δρ(xx)/ρ(xx)(0) = F(μ0H/ρ(xx)(0)), where Δρ(xx) ≡ ρ(xx)(H) - ρ(xx)(0) and F(x) is a function depending on the details of the electronic structure.

Within the last decade, however, it has been found that in strongly correlated electron systems, including heavy fermion compounds, organics, and high-Tc cuprates, these transport coefficients show a striking deviation from Fermi liquid behavior. Such a non-Fermi liquid is remarkably observed when the system approaches in the vicinity of a quantum critical point (QCP). Among these materials, the electron transport properties in high-Tc cuprates have been studied most extensively. In the optimally doped region, the resistivity exhibits a T-linear dependence ρ(xx) ∝ T in a wide temperature range. The Hall coefficient displays peculiar temperature and doping dependence and its value becomes much larger than 1/ne at low temperatures. It has been pointed out that the Hall problem can be simplified when analyzed in terms of cotΘH, where ΘH ≡ tan(-1)(ρ(xy)/ρ(xy)) is the Hall angle and ρ(xy) is the Hall resistivity. In high-Tc cuprates, the cotangent of Hall angle approximately varies as cotΘH ≡ ρ(xx)/ρ(xy) ∝ T2. The magnetoresistance violates Kohler's rule and a new scaling relation, Δρ(xx)/ρ(xx)(0) ∝ tan2 ΘH, has been proposed to explain the temperature and field dependence of the magnetoresistance. Moreover, a giant enhancement of the Nernst coefficient νH, which is three orders of magnitude larger than that expected for conventional metals, is observed. Despite of many experimental and theoretica studies, a common agreement on the origin of the observed unusual transport properties is still lacking. A most important question is whether these anomalous transport properties are specific to high-Tc cuprates or represent universal features in the strongly correlated electron systems.

The resent discovery of heavy fermion compound CeMIn5 (M = Co, Rh, and Ir) has attracted great interest in the strongly correlated systems. CeMIn5 crystalizes in a quasi two-dimensional structure which can be viewed as alternating layers of CeIn3 and MIn2 stacked sequentially along c-axis. Figures 1 (a) and (b) show the schematic pressure-temperature phase diagram for CeRhIn5 and CeCoIn5, respectively. CeRhIn5 is an antiferromagnetic (AF) metal with TN = 3.8 K at ambient pressure. Above Pc 〓 2 GPa, CeRhIn5 shows superconductivity without long-range magnetic order, indicating an AF QCP. CeCoIn5 and CeIrIn5 are superconductors with Tc = 2.3 K and 0.4 K, respectively, at ambient pressure. It is speculated that CeCoIn5 is located in the vicinity of the AF QCP, which is in a slightly negative pressure region. Below P*, at which Tc shows a broad maximum, non-Fermi liquid behavior observed in transport or thermodynamic properties. On the other hand, at P > P* the non-Fermi liquid behavior is strongly suppressed and recovery of the Fermi liquid behavior is observed. CeMIn5 bears some resemblance to high-Tc cuprates. The main Fermi surface is cylindrical and it is widely believed that the superconducting gap of CeMIn5 has d-wave symmetry with line nodes perpendicular to the plane. The superconductivity occurs in the neighborhood of AF ordered state.

CeMIn5 is very suitable for the detailed study of the tranpsport phenomena in strongly correlated systems. This is because the ground state can be tuned, ranging from the AF ordered state metal to the Fermi liquid state through the non-Fermi liquid state, by applying pressure without introducing additional scattering centers and with keeping the carrier number unchanged (Fig. 1 (a) and (b)). In addition, CeMIn5 is in a very clean regime. In fact, the quasi particle mean free path in the superconducting state of CeCoIn5 is as long as a few μm. This is important because the intrinsic transport properties relevant to the electron-electron correlation are often masked by the impurity scattering. Moreover, while anomalous Hall term due to skew scattering masks ordinary Hall effect in most heavy fermion systems, the skew scattering term in CeMIn5 is absent or very small. This enable us to make a detailed study of normal Hall effect and magnetoresistance.

In order to clarify the anomalous transport properties observed in strongly correlated systems, we have performed the systematic studies of the dc-resistivity, Hall effect, and magnetoresistance for CeMIn5 as a function of temperature, magnetic field, and pressure. We have revealed several unusual features in transport properties near the AF QCP in the present work. The amplitude of the Hall coefficient is dramatically enhanced with decreasing temperature and attains a value much larger than 1/|ne| at low temperatures. Furthermore, the magnetoresistance strongly violates the Kohler's rule. The resistivity shows T-linear dependence, the cotangent of Hall angle cotΘH varies as cotΘH ∝ T2, and the magnetoresistance is well-scaled by the tan2 ΘH. These anomalous transport properties are pronounced when the AF spin fluctuations are enhanced in the vicinity of the AF QCP, indicating that AF spin fluctuations strongly affect the transport phenomena. We have shown that the anomalous transport properties can be accounted for in terms of a resent spin fluctuation theory. All of the anomalous behavior of dc-resistivity, Hall effect, and magnetoresistance are well scaled by a single parameter, the AF correlation length ξ(AF) , for a wide range of the parameters, temperature, magnetic field, and pressure. These anomalous behavior observed in the transport phenomena of CeMIn5 bear a striking resemblance to the normal-state properties of high-Tc cuprates, indicating universal transport properties of strongly correlated quasi two-dimensional electron systems in the presence of strong AF fluctuations.

Figure 1: Schematic P -T phase diagrams for (a) CeRhIn5 and (b) CeCoIn5. At P 〜 P*, a non-Fermi liquid liquid (nFL) to Fermi liquid (FL) crossover occurs and Tc reaches a broad maximum. In CeRhIn5, Pc is a critical pressure which separates the AF metallic and superconducting state, indicating that the AF QCP is located at Pc. In CeCoIn5, the AF QCP appears to be at a slightly negative inaccessible pressure.

　銅酸化物高温超伝導物質，重い電子系，有機導体などのいわゆる強相関電子系を特徴づけるもののひとつはその異常な伝導特性にある．ランダウのフェルミ液体論によれば，(最も単純な場合)ホール係数はキャリア密度とRH=1/neの関係にあり，電子電子散乱による抵抗率の温度依存性はρ∝T2に従う．また磁気抵抗はいわゆるケーラー則(Koehler's rule)Δρ(H)/ρ(0)=f(μ0H/ρ(0))に従うことが期待される．しかしながら強相関電子系では単純なフェルミ液体から大きく逸脱したふるまいが見られる．そのような非フェルミ液体的ふるまいは量子臨界点の近傍において特に顕著となる．たとえば銅酸化物高温超伝導物質の最適ドープ領域では電気抵抗の温度依存性がρ∝Tに従うことや，ホール係数が顕著な温度依存性を示すことなどが特徴として認識されている．本研究は，最近発見された擬2次元的重い電子系物質CeMIn5(M:Co，Rh，Ir)を採りあげ，温度-圧力相図の各部分，特に量子相転移点近傍における輸送特性を詳細に調べたものである．

　本論文は7章からなる．第1章は序論，第2章は強相関電子系の輸送現象に関するレビュー，第3章はCeMIn5の結晶構造やこれまでに知られている相図および物性をまとめたものである．第4章では本研究の実験手法が述べられ，第5章では輸送現象の実験結果が提示されている．第6章ではそれらの実験結果と理論との比較が行われ，第7章で全体の結論が述べられている．

　CeRhIn5の低温相は常圧ではTN〓3.8Kの反強磁性であるが，2GPa程度以上の圧力下では長距離磁気秩序は失われTc〓2Kの超伝導が発現する．すなわちPc〓2GPaに量子臨界点がある．これに比してCeCoIn5およびCeIrIn5は常圧から超伝導を示しており(それぞれTc〓2.4Kおよび〓O.4K)，量子臨界点は負の圧力側に位置しているものと想定されている．

　一般にf電子系を含む重い電子系では異常ホール効果が大きいためにintrinsicな輸送特性を抽出するのが困難であるが，CeMIn5では例外的に異常ホール効果の寄与が小さく，しかも不純物散乱の少ないクリーンな電子系であるため輸送特性の解析に曖昧さが少ない．また，圧力によって系を量子臨界点の近傍に設定することができるという特徴をもつ．本研究ではこれらの特徴を活かして，量子臨界点近傍の強相関電子系に特徴的な異常輸送特性を明らかにした．異常輸送特性の特徴として;(1)ホール係数は十分高温ではRH=1/neに相当する値を示すが，低温ではその絶対値が大きく増大する．(2)磁気抵抗は通常のケーラー則には従わず，むしろtan2ΘHでよくスケールされる(ΘHはホール角)．(3)電気抵抗およびホール抵抗の温度依存性は，ρ∝T，cotΘH∝T2でよく表される．

　学位申請者はこれらの特徴が銅酸化物高温超伝導物質の常伝導相において観測される性質と極めて類似していることに着目し，提唱されているいくつかの理論モデルとの比較検討を行っている．銅酸化物高温超伝導物質の異常輸送特性については，スピン・電荷分離という描像(非フェルミ液体からのアプローチ)と反強磁性的スピン揺らぎによる異方的散乱(フェルミ液体からのアプローチ)が提唱されている．学位申請者は基底状態がモット絶縁体である銅酸化物系と反強磁性金属であるCeMIn5系が類似のふるまいを示すことを根拠に前者を退けている．後者の立場では，フェルミ面上で散乱緩和時間が著しく異なる領域(ホットスポットとコールドスポット)が存在するというモデルが提唱されているが，学位申請者は，単純にこのモデルを適用するとホール係数の実験結果が説明できないこと，バックフローの効果を採り入れた理論とは整合すること，を結論している．また低温でホール係数の絶対値が再び減少するという実験事実は，不純物散乱によって散乱時間の異方性が減少するためにバックフローの効果が抑制される結果との解釈を提示している．

　以上のように，本研究は擬2次元重い電子系CeMIn5(M:Co，Rh，Ir)の異常輸送現象を詳しく調べ，その特徴が銅酸化物高温超伝導物質と類似していることを指摘したもので，強相関電子系の輸送現象の理論的解明に有益な材料を提供したものと評価される．本論文の中核をなす研究内容は指導教員らとの共著論文として学術誌に印刷公表されているが，実験の遂行および結果の解析の大部分は論文提出者が主体となって行なったものと判断される．よって，博士(理学)の学位授与に値するものと認める．