Quantum critical fluctuations in layered YFe ₂ Al ₁₀

Abstract

The absence of thermal fluctuations at T = 0 makes it possible to observe the inherently quantum mechanical nature of systems where the competition among correlations leads to different types of collective ground states. Our high precision measurements of the magnetic susceptibility, specific heat, and electrical resistivity in the layered compound YFe 2 Al 10 demonstrate robust field-temperature scaling, evidence that this system is naturally poised without tuning on the verge of ferromagnetic order that occurs exactly at T = 0, where magnetic fields drive the system away from this quantum critical point and restore normal metallic behavior.quantum criticality | ferromagnet | dynamical scaling T he interplay of competing interactions is responsible for the array of ground states that are possible in correlated electron systems. It is of particular importance to understand how one such ground state gives way to another when the system is tuned at temperature T = 0, without the complications of thermal fluctuations. Consequently, much interest has focused on quantum critical points (QCPs), where an ordered phase can be created by an infinitesimal modifications of pressure, composition, or field. The onset of ferromagnetic order is perhaps the simplest T = 0 phase transition, and indeed much experimental and theoretical effort has been directed toward understanding its essential features (1-6). It is generally believed that ferromagnetic order occurs at T = 0 via a discontinuous or first order transition, as is observed in the clean ferromagnets ZrZn 2 (7) and MnSi (8) under pressure. Disorder is known to render the ferromagnetic transition continuous, leading to the mean field behavior that is found when doping drives T C = 0 in Ni 1−x Pd x (9), Zr 1−x Nb x Zn 2 (10), and Nb 1−y Fe 2+y (11). More controversial is the possibility that strong quantum fluctuations, such as those that destabilize order in low-dimensional systems, may be significant near the T C = 0 ferromagnetic transition and perhaps may even destroy its first-order character (5). A complete experimental investigation of the critical phenomena and their scaling behaviors in a carefully selected system where disorder is minimal is needed to establish that quantum critical fluctuations are both present and relevant to the destabilization of ferromagnetic order.Progress toward obtaining this information has been painstaking, although a number of systems have been identified where the Curie temperature T C has been driven to zero. High pressure experiments evade the disorder that necessarily accompanies doping, but thus far only resistivity and susceptibility measurements have been reported. Complete experimental access is possible when doping is used to suppress T C → 0, but even small amounts of compositional inhomogeneity can obscure any intrinsic critical fluctuations of T C = 0 ferromagnetic transitions, resulting in interesting complications such as the Griffiths phase (12,13,14), as well as short ranged order, including spin glasses (15). Alterna...

Local quantum phase transition in YFe 2 Al 10

“…1D) [21][22][23]. Consistent with the T /B 0.6 scaling observed in the magnetization and specific heat [24], the scattering is strongly suppressed by magnetic fields B (Fig. 1A).…”

“…shown that a single variable T /B 0.6 controls the QC fluctuations for a wide range of fields and temperatures in YFe 2 Al 10 [24]. This variable is also observed in neutron scattering measurements.…”

“…In previously investigated systems the E/T scaling is always associated with the collapse of magnetic order, and it is only observed over a limited range of wave vectors that are associated with incipient magnetic order. In contrast, the E/T scaling of χ in YFe 2 Al 10 extends over the entire range of wave vectors accessed in this experiment, amounting to more than an entire Brillouin zone.The energy and temperature dependencies of χ provide the needed connection between the neutron scattering measurements and the previously reported temperature dependence 6 of the static susceptibility χ 0 (T ) ∼ T −1.4 [24], since the Kramers-Kronig relation giveswhere χ E 1.4 ∝ tanh(x), and x = E/k B T . Agreement between these two independent determinations of χ 0 (T ) requires that ∆ = 1.4, a value that is wholly within the experimental bounds of the neutron scattering experiment (Fig.…”

“…The energy and temperature dependencies of χ provide the needed connection between the neutron scattering measurements and the previously reported temperature dependence of the static susceptibility χ 0 (T ) ∼ T −1.4 [24], since the Kramers-Kronig relation gives…”

Metallic quantum ferromagnets

“…Initial experiments identified correlated FM behavior (Strydom and Peratheepan, 2010). Further detailed studies on single crystals found anomalies in the magnetic susceptibility and the specific heat which obey a peculiar NFL fieldtemperature scaling (Park et al, 2011;Wu et al, 2014). In addition, FM correlations have been found in NMR experiments .…”