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Strange metals and exotic superconductivity in strongly correlated materials

Strange metals and exotic superconductivity in strongly correlated materials

Simple on its own

Gather, a realm of wonder

More is different


One might think that the best way to deal with an extremely complex problem is to break it into small parts and study how each part work. However, many real-world challenges refuse this approach. All too often, macroscopic scale behavior takes the form of "emergent' properties" that are completely beyond our imagination based on the knowledge of an individual building block. Indeed, there is nothing in a single water molecule that can guide us to predict the snowflake’ dazzling variety of pattern; our understanding of brain cells is far from enough for comprehending human consciousness – we are surrounded by fascinating emergent properties, which seek universal organizing principles that link the enigmatic interaction network in the microscopic realm with descriptions at the macroscopic level.

In condensed matter physics, the emergent phenomena involve the beautiful and mind-boggling aspects of quantum mechanics. A celebrated example is the high-temperature superconductivity: the strong correlation among electrons massively entangles the spin, orbital, and charge degrees of freedoms, leading to dissipationless charge transport at surprisingly high temperatures and novel quasiparticles that are radically different from the underlying electrons. With its fundamental significance and prospects for technological innovations, the discovery of the high-temperature superconductivity sparks a tremendous expansion of quantum materials research that branches out into various fronts. Yet, the origin of the high-temperature superconductivity remains an open question despite decades of research effort. Quite often, exotic SC appears near the non-Fermi-liquid (NFL) or strange metal phase - a highly entangled electronic state featuring a complete breakdown of the quasiparticle picture and singular behavior of physical properties. Understanding the NFL may hold the key to unveiling the primary driver of unconventional superconductivity and a wide variety of quantum states in condensed matter systems.

By combining material synthesis, high-precision measurements under extreme conditions, and a global collaborative network involving world-leading experts, our group's research leads the way to establishing new paradigms in quantum material research. We discovered the first Yb-based heavy-fermion superconductor and demonstrated an intimate connection between the exotic superconductivity and the quantum criticality of valence fluctuations. Another highlight is the multipolar Kondo material, in which conduction electrons interact strongly with local moments carrying high-rank quadrupoles and octupoles but no magnetic dipole. Such a system opens a new horizon for exploring spin-orbital entangled quantum phases and exotic SC. Please feel free to check out the following selected publications for an in-depth exploration of topic.


[1] Y. Shimura et al., Phys. Rev. Lett. 122, 256601 (2019).

[2] M. Tsujimoto et al., Phys. Rev. Lett. 113, 267001 (2014).

[3] K. Matsubayashi et al., Phys. Rev. Lett., 109, 187004 (2012).

[4] A. Sakai and S. Nakatsuji, J. Phys. Soc. Jpn., 80, 063701 (2011).

[5] S. Nakatsuji et al., Nat. Phys., 4, 603 (2008).

[6] T. Matsumoto et al., Science, 331, 316 (2011).

[7] T. Tomita et al., Science, 349, 506 (2015).

[8] K. Kuga et al., Science Adv., 8, 3547 (2018).


Extremely large magnetoresistance and anisotropic transport in the multipolar Kondo system PrTi2Al20

T. Isomae, A. Sakai, M. Fu, T. Taniguchi, M. Takigawa, S. Nakatsuji, Phys. Rev. Research 6, 013009 (2024).

Multipolar Kondo systems offer unprecedented opportunities to design astonishing quantum phases and functionalities beyond spin-only descriptions. A model material platform of this kind is the cubic heavy fermion system PrT2Al20 (T=Ti, V), which hosts a nonmagnetic crystal-electric-field ground state and substantial Kondo entanglement of the local quadrupolar and octopolar moments with the conduction electron sea. Here, we explore the magnetoresistance (MR) and Hall effect of PrTi2Al20, which develops ferroquadrupolar (FQ) order below TQ∼2 K, and compare their behavior with that of the MR and Hall effect of the non-4f analog, LaTi2Al20. In the FQ ordered phase, PrTi2Al20 displays extremely large magnetoresistance (XMR) of ∼103%. The unsaturated, quasilinear field (B) dependence of the XMR violates Kohler's scaling and defies description based on carrier compensation alone. By comparing the MR and the Hall effect observed in PrTi2Al20 and LaTi2Al20, we conclude that the open-orbit topology on the electron-type Fermi surface sheet is key for the observed XMR. The low-temperature MR and the Hall resistivity in PrTi2Al20 display pronounced anisotropy in the [111] and [001] magnetic fields, which is absent in LaTi2Al20, suggesting that the transport anisotropy ties in with the anisotropic magnetic field response of the quadrupolar order parameter.


© Isomae et al. (2024) (CC BY 4.0)

High-temperature antiferromagnetism in Yb based heavy fermion systems proximate to a Kondo insulator

S. Suzuki, K. Takubo, K. Kuga, W. Higemoto, T. U. Ito, T. Tomita, Y. Shimura, Y. Matsumoto, C. Bareille, H. Wadati, S. Shin, S. Nakatsuji, 
Phys. Rev. Res.
3, 023140 (2021).

Given the parallelism between the physical properties of Ce- and Yb-based magnets and heavy fermions due to the electron-hole symmetry, it has been rather odd that the transition temperature of the Yb-based compounds is normally very small, as low as ∼1K or even lower, whereas Ce counterparts may often have the transition temperature well exceeding 10 K. Here, we report our experimental discovery of the transition temperature reaching 20 K in a Yb-based compound at ambient pressure. The Mn substitution at the Al site in an intermediate valence state of α−YbAlB4 not only induces antiferromagnetic transition at a record high temperature of 20 K but also transforms the heavy-fermion liquid state in α−YbAlB4 into a highly resistive metallic state proximate to a Kondo insulator.


©︎ Suzuki et al.(2021), published under the terms of the Creative Commons Attribution 4.0 International license (CC BY 4.0)

Unveiling Quadrupolar Kondo Effect in the Heavy Fermion Superconductor PrV2Al20

M. Fu, A. Sakai, N. Sogabe, M. Tsujimoto, Y. Matsumoto, S. Nakatsuji, J. Phys. Soc. Jpn. 89, 013704 (2020).

Heavy fermion metals hosting multipolar local moments set a new stage for exploring exotic spin–orbital entangled quantum phases. In such systems, the quadrupolar Kondo effect serves as the key ingredient in the orbital-driven non-Fermi liquid (NFL) behavior and quantum critical phenomena. The cubic heavy fermion superconductor PrTr2Al20 (Tr: Ti, V) is a prime candidate for realizing the quadrupolar Kondo lattice. Here, we present a systematic study of the NFL phenomena in PrV2Al20 based on magnetoresistance (MR), magnetic susceptibility and specific heat measurements. Upon entering the NFL regime, we observe a universal scaling behavior expected for the quadrupolar Kondo lattice in PrV2Al20, which indicates a prominent role of the quadrupolar Kondo effect in driving the NFL behavior. Deviations from this scaling relation occur below ∼8 K, accompanied by a sign change in the MR and a power-law divergence in the specific heat. This anomalous low-temperature state points to the presence of other mechanisms which are not included in the theory, such as heavy fermion coherence or multipolar quantum critical fluctuations.


©2020 The Physical Society of Japan

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