#
APS March Meeting 2014

## Volume 59, Number 1

##
Monday–Friday, March 3–7, 2014;
Denver, Colorado

### Session L40: Invited Session: New Horizons for Magnetism and Competing Phases in Heavy Fermions

8:00 AM–11:00 AM,
Wednesday, March 5, 2014

Room: Mile High Ballroom 2B-3B

Sponsoring
Unit:
DCMP

Chair: Paul Canfield, Ames Laboratory

Abstract ID: BAPS.2014.MAR.L40.1

### Abstract: L40.00001 : Tuning magnetism by Kondo effect and frustration

8:00 AM–8:36 AM

Preview Abstract
Abstract

####
Author:

Hilbert v.L\"{o}hneysen

(Karlsruhe Institute of Technology (KIT), 76021 Karlsruhe, Germany)

Heavy-fermion systems are an ideal playground for studying the quantum phase
transition (QPT) between paramagnetic and magnetically ordered ground states
arising from the competition between Kondo and RKKY interactions [1]. Two
different routes have been identified by various experiments, i. e., the
more traditional spin-density-wave (SDW) [2] and the Kondo-breakdown [3]
approaches. However, up to now an \textit{a-priori} assignment of a given system to these
different routes has not been possible. Yet another route to quantum
criticality not included in the above approaches might be geometric
frustration of magnetic moments, a route well known for insulating magnets
with competing interactions [4]. First experiments on metallic systems have
recently been conducted.
In the canonical partially frustrated antiferromagnetic system
CePd$_{\mathrm{1-x}}$Ni$_{\mathrm{x}}$Al, the N\'{e}el temperature
$T_{\mathrm{N}}(x)$ decreases, with $T_{\mathrm{N}}\to $ 0 at the critical
concentration $x_{c}\approx $ 0.144. The low-temperature specific heat
$C(T)$ evolves toward $C$/$T\alpha $ ln($T_{\mathrm{0}}$/$T)$ for $x\to x_{c}$ [5]. The
unusual $T$ dependence of $C$/$T$ is compatible with the Hertz-Millis-Moriya (HMM)
scenario of quantum criticality [2] if the quantum-critical fluctuations are
two-dimensional in nature. Here two-dimensionality might arise from
antiferromagnetic planes that are effectively decoupled by the frustrated Ce
atoms in between. An exciting possibility is that the planes of frustrated
Ce moments form a two-dimensional spin liquid.
In the prototypical heavy-fermion system
CeCu$_{\mathrm{6-x}}$Au$_{\mathrm{x}}$ the experiments by Schr\"{o}der et
al.[6] provided the initial evidence of local quantum criticality. While
concentration and pressure tuning of the quantum phase transition (QPT) are
described by this scenario, magnetic-field tuning the QPT is in line with
the SDW scenario [7]. Elastic neutron scattering experiments on
CeCu$_{\mathrm{5.5}}$Au$_{\mathrm{0.5}}$ under hydrostatic pressure $p$ [8]
show that at $p=$ 8 kbar, $T_{\mathrm{N}}$ and the magnetic propagation vector
attain almost the values of CeCu$_{\mathrm{5.7}}$Au$_{\mathrm{0.3}}$. This
$x-p$ analogy away from the QPT is highly remarkable since the ambient-pressure
magnetic structures for $x=$ 0.3 and 0.5 are quite different. These results
give clues to a general ($x$,$p$,$B)$ phase diagram at $T=$ 0 and might explain the
existence of different universality classes.
\\[4pt]
[1] H. v. L\"{o}hneysen et al., Rev. Mod. Phys. \textbf{79}, 1015 (2007).\\[0pt]
[2] J. A. Hertz, Phys. Rev. B \textbf{14}, 1165 (1976); A. J. Millis, Phys.
Rev. B \textbf{48}, 7113 (1993); T. Moriya and T. Takamoto, J. Phys. Soc.
Jpn. \textbf{64}, 960 (1995).\\[0pt]
[3] Q. Si et al., Nature \textbf{413}, 804 (2001).\\[0pt]
[4] B. Keimer and S. Sachdev, Physics Today \textbf{64} (2), 29 (2011).\\[0pt]
[5] V. Fritsch et al., arXive 1301.6062, submitted for publication (2013).\\[0pt]
[6] A. Schr\"{o}der et al., Nature \textbf{407}, 351-355 (2000).\\[0pt]
[7] O. Stockert et al., Phys. Rev. Lett. \textbf{99}, 237203 (2007).\\[0pt]
[8] A. Hamann et al., Phys. Rev. Lett. \textbf{110}, 096404 (2013).

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2014.MAR.L40.1