The study of strongly correlated electron systems has yielded a wealth of fascinating phenomena, including heavy fermion behavior and strange metallicity. Heavy fermion systems, characterized by enormously enhanced effective electron masses, typically involve localized f-electrons interacting with conduction electrons via the Kondo mechanism. Strange metals deviate significantly from Fermi liquid theory, exhibiting unconventional transport properties such as T-linear resistivity. While f-electron systems are a well-established source of these phenomena, recent observations of Kondo-like behavior in 3d transition metal oxides and 4d/5d van der Waals heterostructures suggest that the underlying physics might be more general. This paper investigates the emergence of heavy fermion and strange metal behavior in a kagome metal, Ni3In, providing a new platform to study these phenomena. The focus is on the role of geometric frustration and band topology in driving strong correlations. Understanding these mechanisms could pave the way for designing novel correlated materials with tailored properties. The kagome lattice, known for its unique geometric properties and potential for hosting topological states, provides an ideal setting to explore the interplay between geometric frustration, electronic structure, and emergent many-body behavior. Previous research has hinted at the potential for flat bands in kagome metals to lead to strong correlations. This study investigates whether such flat bands in Ni3In, originating from destructive interference effects, can indeed induce heavy fermion and strange metal behavior, potentially bridging the gap between f-electron Kondo lattices and other systems exhibiting similar properties.

Landau Fermi liquid theory successfully describes many metals, portraying charge carriers as quasiparticles adiabatically connected to weakly interacting electrons. However, deviations from this theory occur in systems like Kondo lattices where conduction electrons interact with localized magnetic moments. These interactions can lead to either a non-magnetic renormalized Fermi liquid (heavy fermion state) or a metallic antiferromagnet. These phases can be connected by a quantum critical point (QCP), resulting in a strange metal state exhibiting unusual transport properties, particularly T-linear resistivity. Strange metal behavior has been observed in high-Tc superconductors and more recently, in moiré heterostructures. The exploration of kagome metals as potential platforms for exhibiting such behavior is relatively new, and past research has focused on their topological and superconducting phases, alongside their connection to Dirac, van Hove, and flat band states. The realization of ideal flat bands in these materials remains challenging due to issues such as hopping beyond nearest neighbors and orbital effects. This study aims to build upon existing knowledge by demonstrating that correlated metallic phases driven by electronic degeneracy can be realized when narrow or partially flat bands are situated at the Fermi level, focusing on Ni3In as a model system.

The study employed a combination of experimental and computational techniques to investigate the properties of Ni3In and its isostructural counterpart Ni3Sn. Single crystals of Ni3In were synthesized using an I2-catalyzed reaction, while Ni3Sn crystals were grown via chemical vapor transport. Polycrystalline samples of Ni3In1-xSnx were prepared for heat capacity measurements. Electrical transport measurements were performed on single crystals using a standard four-probe method, including measurements under high magnetic fields and hydrostatic pressure. Heat capacity measurements were carried out on sintered polycrystals using a two-relaxation time method. Magnetic susceptibility was measured using a Vibrating Sample Magnetometer (VSM). High-resolution magnetization measurements were performed at the NHMFL pulsed field facility. Scanning Transmission Electron Microscopy (STEM) was employed for structural characterization, and Angle-Resolved Photoemission Spectroscopy (ARPES) was used to determine the electronic structure. Density Functional Theory (DFT) calculations, using VASP and FPLO codes, were performed to determine the electronic band structure and construct effective tight-binding models. These models aided in the analysis of the experimental results and provided insight into the underlying microscopic mechanisms. Specifically, the effective 6-band model projected from the locally rotated dxz orbitals was crucial in understanding the formation of local magnetic moments. The local magnetic susceptibility χloc, defined by equation (1) in the paper, was calculated to examine Curie-Weiss-like temperature dependence. The authors contrasted this result with a simple single-band 3D cubic lattice model to highlight the unique characteristics of Ni3In.

The electrical resistivity ρab(T) of Ni3In in the kagome planes exhibits a broad shoulder above 100 K, transitioning to approximately T-linear behavior below this temperature. This deviation from conventional Fermi liquid behavior (ρ(T) ~ Tα with α=2) persists down to 1.5 K, where a T2 dependence is finally observed. The coefficient A of the T2 term is significantly larger than that of Ni3Sn, indicating enhanced electron-electron scattering in Ni3In. Heat capacity measurements reveal a low-temperature upturn in Cp/T for Ni3In, deviating from the expected Fermi liquid behavior (γ+βT2). The Sommerfeld coefficient γ for Ni3In is approximately five times larger than that of Ni3Sn, suggesting significant renormalization and correlation effects due to the presence of flat electronic states at EF. The Kadowaki-Woods ratio A/γ2 for Ni3In is three orders of magnitude larger than for elemental transition metals, placing it within the range of heavy fermion metals and correlated oxides, confirming strongly enhanced correlations. Applying magnetic fields suppresses the T-linear resistivity and drives the system towards a Fermi liquid state, as indicated by a decrease in A and an increase in TFL. Hydrostatic pressure has a similar effect. The observed crossover between non-Fermi liquid (NFL) and Fermi liquid (FL) states with temperature, magnetic field, and pressure suggests proximity to a quantum critical point (QCP). DFT calculations reveal partially flat bands near EF, which are mainly composed of dxz orbitals. Calculations of local and momentum-resolved magnetic susceptibility suggest the formation of local moments due to the interaction of these partially flat bands with the conduction electron sea. This can explain the unusual transport properties, analogous to heavy fermion behavior in Kondo lattice systems. The pressure- and field-driven crossover is consistent with the Doniach phase diagram, where pressure enhances the Kondo interaction J, and magnetic field suppresses fluctuations.

The findings strongly support the notion that the heavy fermion and strange metal behavior in Ni3In originates from the interplay of partially flat bands near the Fermi level and the resulting strong electron correlations. The observed T-linear resistivity, enhanced Kadowaki-Woods ratio, and response to magnetic field and pressure all point toward a quantum critical system akin to f-electron Kondo lattices. However, the source of the localized moments here is fundamentally different, arising from the geometrically frustrated hopping of d-electrons within the flat band states of Ni3In's kagome lattice. This represents a new paradigm for realizing heavy fermion and strange metal behavior, suggesting a route to design such systems based on band engineering and geometric frustration rather than relying solely on f-electrons. The similarities in phenomenology across drastically different materials emphasize that 'flat band' metallic states could be the unifying feature driving heavy fermion and strange metal behavior. This opens a new avenue of investigation into strongly correlated gapless phases.

This research demonstrates a novel route to realizing heavy fermion and strange metal behavior in kagome metal Ni3In, driven by hopping frustration in a flat band originating from 3d electrons, distinctly different from the conventional f-electron Kondo lattice paradigm. The observed T-linear resistivity, enhanced Kadowaki-Woods ratio, and responses to external tuning parameters highlight a system at the brink of a quantum phase transition. Future research directions could include exploration of the phase space spanned by other tuning parameters to pinpoint potential nearby electronic instabilities and further investigate the interaction between flat band electrons and conduction electrons. Comparisons with structurally related systems, such as CoSn, can offer valuable insights into the role of orbital character and band dimensionality. These findings expand the scope of materials exhibiting these correlated behaviors and inspire a new design paradigm for strongly correlated topological states.

While the study provides strong evidence for the flat-band-driven emergence of heavy fermion and strange metal behavior in Ni3In, some limitations exist. The multiband nature of the system makes precise quantitative analysis challenging, especially regarding the estimation of effective mass and the detailed nature of the scattering processes. Further experiments exploring different external tuning parameters are necessary to definitively map out the complete phase diagram and elucidate the nature of any nearby ordered phases. The proposed model for the formation of local moments needs further refinement to capture fully the complex interplay between lattice geometry, orbital degrees of freedom, and electron-electron interactions. Also, the specific nature of the quantum critical point remains to be fully established.