In the heavy-fermion metal CePdAl long-range antiferromagnetic order coexists with geometric frustration of one third of the Ce moments. At low temperatures the Kondo effect tends to screen the frustrated moments. We use magnetic fields B to suppress the Kondo screening and study the magnetic phase diagram and the evolution of the entropy with B employing thermodynamic probes. We estimate the frustration by introducing a definition of the frustration parameter based on the enhanced entropy, a fundamental feature of frustrated systems. In the field range where the Kondo screening is suppressed the liberated moments tend to maximize the magnetic entropy and strongly enhance the frustration. Based on our experiments, this field range may be a promising candidate to search for a quantum spin liquid.If competing exchange interactions prevent magnetic systems from developing long-range order, the frustrated magnetic moments can form fluid-like states of matter, so-called spin liquids (SLs) [1]. If the moments act as effective spin-1/2 particles, quantum fluctuations dominate and impede the moments from freezing or ordering at low temperatures T [2]. The ground states of these quantum SLs are characterized by massive many-body entanglement rendering them particularly attractive for investigations of new types of quantum matter. Ever since the first notion of SLs was advertised, there has been continual effort to search for materials that might host SLs, mainly in geometrically frustrated magnets [3][4][5][6][7]. Up to now only very few candidates for metallic SLs have been discovered [2, 8].CePdAl belongs to a class of heavy-fermion (HF) metals with ZrNiAl-type crystal structure (space group P62m) that display geometric frustration owing to the fact that the Ce ions form a distorted kagomé network in the hexagonal ab plane [9][10][11]. In HF compounds the magnetic moments are formed by nearly localized 4f or 5f states. Magnetic correlations are enabled by the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction which competes with the Kondo effect tending to screen the moments at low T . The presence of a Kondo effect in CePdAl is manifest through a logarithmic increase of the resistivity with decreasing T [12, 13] and an extremum of the thermopower at low T [14-16].CePdAl stands out due to the coexistence of geometric frustration with antiferromagnetic (AF) order below T N = 2.7 K [9, 14]. Neutron diffraction experiments [9] and 27 Al NMR measurements [17] reveal that one third of the Ce moments do not participate in the longrange order down to 30 mK. Theoretical models considering a quasi-two-dimensional magnetic structure based on the neutron experiments performed on polycrystals [9] suggest that the Ce moments of the hexagonal basal plane order in ferromagnetic chains which are antiferromagnetically coupled and separated from each other by the frustrated, interjacent moments [inset of Fig. 1(b)] [18, 19]. In the c direction this structure is repeated with an incommensurate AF modulation. Due to the crystal-electric...
CePdAl with Ce 4f moments forming a distorted kagomé network is one of the scarce materials exhibiting Kondo physics and magnetic frustration simultaneously. As a result, antiferromagnetic (AF) order setting in at TN = 2.7 K encompasses only two thirds of the Ce moments. We report measurements of the specific heat, C, and the magnetic Grüneisen parameter, Γmag, on single crystals of CePd1−xNixAl with x ≤ 0.16 at temperatures down to 0.05 K and magnetic fields B up to 8 T. Field-induced quantum criticality for various concentrations is observed with the critical field decreasing to zero at xc ≈ 0.15. Remarkably, two-dimensional (2D) AF quantum criticality of Hertz-Millis-Moriya type arises for x = 0.05 and x = 0.1 at the suppression of 3D magnetic order. Furthermore, Γmag(B) shows an additional contribution near 2.5 T for all concentrations which is ascribed to correlations of the frustrated one third of Ce moments.New quantum states of matter, arising in materials with competing interactions and multiple energetically degenerate configurations are of strong interest in condensed matter physics. For example, unconventional superconductivity is found near quantum critical points (QCPs) [1][2][3][4][5] and spin-liquid phases, driven by strong frustration have been realized in magnetic insulators [6,7]. However, there are only few studies on metallic frustrated magnets [8], and the effect of frustration on quantum criticality in metals has rarely been investigated experimentally. Rare-earth heavy-fermion (HF) metals, consisting of 4f moments coupled to conduction electrons by an exchange interaction J, are ideally suited for this purpose. Since J governs the competition between the indirect Ruderman-Kittel-KasuyaYosida (RKKY) exchange and the Kondo interaction [9], QCPs can be realized by variation of J with pressure, chemical composition or magnetic field [10][11][12]. Unconventional quantum criticality with a Kondo breakdown and a spin-liquid phase of localized 4f moments being decoupled from conduction electrons has been predicted for high degree of frustration [13]. The 'global phase diagram', which classifies the electronic and magnetic ground states for HF systems, treats J and the strength of quantum fluctuations arising from frustration as two independent parameters [14-16].The effect of geometrical frustration in Kondo lattices has been recognized in hexagonal systems crystallizing in the ZrNiAl structure. Here, the 4f electrons form a structure of equilateral corner-sharing triangles in the ab plane, which can be described as a distorted kagomé network. For YbAgGe, the geometrical frustration leads to a series of almost degenerate magnetic states tuned by magnetic fields and novel quantum bicritical behavior [17][18][19]. CeRhSn does not display long-range order and is located close to a QCP driven by geometrical frustration [20]. In CePdAl the magnetic frustration gives rise to unusual magnetic ordering [21][22][23][24]. It has been shown previously using polycrystals that the material can be tuned throug...
Magnetic frustration, which is well-defined in insulating systems with localized magnetic moments, yields exotic ground states like spin ices, spin glasses, or spin liquids. In metals magnetic frustration is less well defined because of the incipient delocalization of magnetic moments by the interaction with conduction electrons, viz., the Kondo effect. Hence, the Kondo effect and magnetic frustration are antithetic phenomena. Here we present experimental data of electrical resistivity, magnetization, specific heat and neutron diffraction on CePdAl, which is one of the rare examples of a geometrically frustrated Kondo lattice, demonstrating that the combination of Kondo effect and magnetic frustration leads to an unusual ground state. arXiv:1609.01551v1 [cond-mat.str-el]
We report on a single-crystal neutron diffraction study of the evolution of the antiferromagnetic order in the heavy-fermion compound CePd1−xNixAl which exhibits partial geometric frustration due to its distorted Kagomé structure. The magnetic structure is found to be unchanged with a propagation vector QAF ≈ (0.5 0 0.35) for all Ni concentrations x up to xc ≈ 0.14. Upon approaching the quantum critical concentration xc, the ordered moment vanishes linearly with Néel temperature TN, in good agreement with CePdAl under hydrostatic pressure. For all Ni concentrations, substantial short-range magnetic correlations are observed above TN as a result of frustration.Competing interactions in condensed matter systems attract both experimentalists and theorists due to the large variety of resulting phases and new physics involved. Depending on the relative strength of the interactions, different ground states can occur. In particular, metallic systems close to a magnetic instability at zero temperature, i.e., a quantum critical point in the case of a continuous transition, are interesting: Here the quantumcritical fluctuations of the order parameter determine the thermodynamic and transport properties also at finite temperatures resulting, e.g., in non-Fermi-liquid behavior [1-3]. Heavy-fermion compounds are particularly well-suited model systems to study quantum criticality because the strength of the competing interactions can be tuned by, e.g., alloying, hydrostatic pressure, or magnetic field. Recently, magnetic frustration was suggested as a further parameter to tune quantum criticality [4][5][6].The heavy-fermion compound CePdAl combines both the vicinity to a quantum critical point in a metallic magnet and the appearance of geometric magnetic frustration. Although at first sight magnetic frustration and heavy-fermion magnetism seem to be contradictory, the magnetic 4f moments are neither fully localised nor itinerant and therefore allow for frustration in a metallic system. CePdAl crystallises in the hexagonal ZrNiAl structure with the cerium atoms located on a distorted Kagomé lattice in the hexagonal basal plane [7]. It shows the typical properties of a heavy-fermion compound with an enhanced Sommerfeld coefficient of the specific heat being γ ≈ 270 mJ/molK 2 [8], and orders antiferromagnetically below the Néel temperature T N = 2.7 K in an incommensurate magnetic structure with a propagation vector Q AF ≈ (0.5 0 0.35) as revealed by neutron diffraction [9][10][11]. The cerium magnetic moments of ≈ 1.7 µ B
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