Session F19: Kondo Physics, Heavy Fermions, and Quantum Criticality

Robust antiferromagnetism in the R$_{1-x}$La$_x$Cu$_2$Ge$_2$ series: comparison of Ce$_{1-x}$La$_x$Cu$_2$Ge$_2$ and Nd$_{1-x}$La$_x$Cu$_2$Ge$_2$

Recently, remarkably robust and correlated coherence and antiferromagnetism were found in the Ce$_{1-x}$La$_x$Cu$_2$Ge$_2$ series [H. Hodovanets et al., {\it PRL} {\bf 114}, 236601 (2015)]. Whereas Ce is known to hybridize and its compounds often show a strongly correlated behavior, Nd magnetism is associated with a local moment nature. In this talk, we report new measurements on the Ce$_{1-x}$La$_x$Cu$_2$Ge$_2$ series that extend the antiferromagnetic and coherence lines even further and then compare the data for Ce$_{1-x}$La$_x$Cu$_2$Ge$_2$ and the data for a local moment based Nd$_{1-x}$La$_x$Cu$_2$Ge$_2$ series to separate effects of Ce - hybridization from the behavior that might be common in the R$_{1-x}$La$_x$Cu$_2$Ge$_2$ (R=magnetic rare earth) family.   

Possible Kondo-Lattice-Enhanced Magnetic Ordering at Anomalously High Temperature in Nd Metal under Extreme Compression

Most elemental lanthanides order magnetically at temperatures $T_{o}$ well below ambient, the highest being 292 K for Gd. Sufficiently high pressure is expected to destabilize the well localized magnetic 4$f$ state of the heavy lanthanides, leading to increasing influence of Kondo physics on the RKKY interaction. For pressures above 80 GPa, $T_{o}$ for Dy and Tb begins to increase dramatically, extrapolating for Dy to a record-high value near 400 K at 160 GPa.\footnote{J. Lim G. Fabbris, D. Haskel, J. S. Schilling, Phys. Rev. B 91, 045116 & 174428 (2015).} This anomalous increase may be an heretofore unrecognized feature of the Kondo lattice state; if so, one would expect $T_{o}$ to pass through a maximum and fall rapidly at even higher pressures. A parallel is suggested to the ferromagnet CeRh$_{3}$B$_{2}$ where $T_{o}=$ 115 K at ambient pressure, a temperature more than 100-times higher than anticipated from simple de Gennes scaling.\footnote{S. A. Shaheen, J. S. Schilling, R. N. Shelton, Phys. Rev. B 31, 656(R) (1985).} Here we discuss recent experiments on Nd where anomalous behavior in $T_{o}(P)$ is found to occur at lower pressures, perhaps reflecting the fact that Nd's 4$f$ wave function is less localized.   

Competition between Kondo and indirect exchange at the edges and bulk of graphene, and 2D materials.

We study the problem of two magnetic impurities at the surface of graphene, BN, MoS$_2$, phosphorene, silicene and germanene using exact numerical methods. We map the band structure of these materials onto one dimensional tight-binding chains in the same spirit as Wilson's numerical renormalization group. We use the density matrix renormalization group to solve the problem exactly, keeping all the information about the underlying lattice. Competition between Kondo and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions is non-trivial, due to strong non-perturbative effects. Depending on the presence of a pseudogap, or gap, we identify an important directionality and position dependence of the correlations. We present scenarios and regimes where impurities prefer to form their own Kondo clouds instead of an RKKY singlet state, or remain as uncoupled local moments. In the particular case of graphene, ferromagnetism is only stable at half-filling. In addition, we study the effects of spin-orbit coupling, and the presence of edge states.   

Revisiting the Toulouse limit of a Kondo junction

Following the development of a scheme to bosonize and debosonize consistently [1,2], we present in detail the Toulouse-point analytic solution of the two-lead (nonequilibrium) Kondo junction model. The existence and location of the solvable point is not modified, but the calculational methodology and the final expressions for observable quantities change markedly as compared to the previously accepted results. \\ \vskip 1mm \noindent [1] See arXiv:1508.03078 and arXiv:1508.03079 \\ \noindent [2] See also N.~Shah, invited talk   

Magnetic-field tuned ground states of CeAuBi$_2$ single crystals

We present detailed temperature- and field-dependent data obtained from magnetization, resistivity and heat capacity measurement performed on nearly stoichiometric CeAuBi$_2$ single crystals. The compound orders antiferromagnetically at $\sim$ 13 K and shows large magnetic anisotropy at low temperatures with the c-direction being an easy axis. The field-dependent magnetization data at low temperatures reveal the existence of a spin-flop transition for \textbf{H}$\|$\textbf{c} ($H_c \sim$75kOe and $T$=1.8K). The zero-field resistivity and heat capacity data show features characteristic of a Ce-based intermetallic with crystal electric field splitting and possible correlated, Kondo lattice effects. The constructed $T-H$ phase diagram, for the magnetic field applied along the easy, [001], direction shows that the magnetic field required to suppress $T_N$ is $\sim$ 75 kOe. The possibility of realization of the field-tuned quantum critical point (QCP) in CeAuBi$_2$ will be discussed.   

Neutron scattering, magnetic, and transport properties of non-centrosymmetric UIrSi$_{\mathrm{3}}$.

Heavy-fermion superconductivity in the non-centrosymmetric crystal structure has drawn much attention [1]. It is argued that the order parameter contains not only a spin-singlet part, but also an admixture of a spin-triplet state. The compound UIrSi$_{\mathrm{3\thinspace }}$crystallizes in the non-centrosymmetric BaNiSn$_{\mathrm{3}}$ structure which is closely related to the well-known ThCr$_{\mathrm{2}}$Si$_{\mathrm{2}}$-type [2]. Preliminary study on polycrystalline UIrSi$_{\mathrm{3}}$ shows antiferromagnetic order below Neel temperature $T_{\mathrm{N}}=$42 K [2]. Its lanthanide analog CeIrSi$_{\mathrm{3}}$ shows heavy-fermion superconductivity under pressure [1]. Therefore, further investigation on UIrSi$_{\mathrm{3}}$ would be meaningful. We would like to present the results of our investigation on UIrSi$_{\mathrm{3\thinspace }}$by neutron scattering, magnetic, and transport measurement on poly and single crystals grown by Czochralski method in a tetra-arc-furnace. [1] Onuki \textit{et al}., J. Phys. Soc. Jpn. \textbf{77}, suppl. A 37 (2008). [2] Buffat \textit{et al.}, J. Mag. Mag. Mat. 62, 53 (1986).   

Single to multiparticle excitations in the itinerant helical magnet CeRhIn$_{5}$

CeRhIn$_{5\, }$is an itinerant magnet where the Ce$^{3+}$ spins order in a simple helical phase. We investigate the spin excitations in this material using triple-axis neutron spectroscopy and observe sharp spin waves at low energies consistent with previous reports and a nearest neighbour exchange of \textasciitilde 1 meV [1]. At higher energies, the fluctuations are heavily damped where the single-quasiparticle excitations are replaced by a momentum and energy-broadened continuum constrained by kinematics of energy and momentum conservation [2]. The delicate energy balance between localized and itinerant characters results in the breakdown of the single-quasiparticle picture in CeRhIn$_{5}$. [1] P. Das \textit{et al.} Phys. Rev. Lett \textbf{113}, 246403 (2014). [2] C Stock \textit{et al.} Phys. Rev. Lett. \textbf{114}, 247005 (2015). [3] T. Park \textit{et al.} PNAS \textbf{105}, 6825 (2008).   

Physical properties of the van der Waals bonded ferromagnet Fe$_{3-x}$GeTe$_2$

Fe$_3$GeTe$_2$ is an itinerant ferromagnetic with a layered structure held together by van der Waals bonds. The material has been synthesized using a flux-growth technique that results in large single crystals suitable for neutron scattering, and its magnetic structure and phase diagram have been investigated. The flux-grown crystals possess a Curie temperature $T_C \approx$ 150$\,$K, which is less than that reported for polycrystalline Fe$_3$GeTe$_2$ with $T_C \approx$ 230$\,$K. The difference is explained by intrinsic Fe-deficiency in these single crystals. This talk will summarize the physical properties of the flux grown single crystals and a series of polycrystalline samples with varying concentrations of Fe, which reveal how Fe content is correlated to structural parameters and $T_C$. In combination with the magnetic properties, Hall effect and thermoelectric data reveal that Fe$_{3-x}$GeTe$_2$ compounds are multi-carrier type, itinerant ferromagnets. Research supported by the US DOE, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.   

Complex magnetic phases in non-centrosymmetric heavy fermion CeCoGe3

The non-centrosymmetric nature of the tetragonal heavy fermion system CeCoGe3 has attracted much interest in the high pressure superconducting state of the material. We have explored the related ambient pressure magnetism using neutron scattering. There are three successive phase transitions at $T_{N1}\sim21$K , $T_{N2}\sim12 $K and $T_{N3}\sim8$K. The upper transition greatly enhances the susceptibility and there are meta-magnetic transitions in the lower T phases. We confirmed the previously determined AFM spin structure for $T_{N2} [Preview Abstract]

Quantum criticality on ferromagnetic systems: it is not where you think it is!

When a ferromagnetic-paramagnetic transition is tuned to $0$~K by application of pressure in clean systems, the transition becomes of the first order at a tricritical point before disappearing. Instead of having a quantum critical point, i.e. a second order transition at $0$~K, there is a quantum phase transition of the first order. The quantum phase transition can be from a ferromagnetic to a paramagnetic phase, or to a spatially modulated phase. We illustrate this case on a new material: LaCrGe$_3$. We will present the temperature-pressure-magnetic field phase diagram of LaCrGe$_3$ and show that quantum criticality is avoided by the appearance of a modulated phase. We will also explain how quantum criticality can be re-introduced. Work at Ames Laboratory was supported by US DOE under the Contract No. DE-AC02-07CH11358. Magnetization measurements under pressure were supported by Ames Laboratory's laboratory-directed research and development (LDRD) funding.   

Theory of electronic and magnetic properties of weak antiferromagnetic TiAu

To date, only a few itinerant magnetic compounds have been found, viz. ZrZn$_2$, TiBe$_2$ and Sr$_3$In, all comprised of nonmagnetic elements. TiAu, a newly synthesized itinerant weak antiferromagnet, orders antiferromagnetically below 36 K. Neutron diffraction reveals an ordered local moment of only 0.15 $\mu_B$/Ti at a wave vector Q=(0,$\pi$/b,0). Hole doping, viz. Ti$_{1-x}$Sc$_x$Au, causes the magnetic moment to disappear at a quantum critical point x$_{Sc}$=0.13. We present results of an extensive study of the electronic and magnetic properties of TiAu. DFT calculations reveal van Hove singularities at (0,0.45$\pi$/b,0.49$\pi$/c), 4 meV above the Fermi level. Several types of analysis will be discussed: fixed spin moment studies and Stoner enchancement; magnetic energies; magnetism versus doping; Fermi surface nesting; corrections for spin fluctuations.   

Quantum Criticality in YFe$_2$Al$_{10}$

Quantum criticality has been studied in many systems, but there are few systems where observed scaling can be unified with a critical free energy $F$, or where the critical exponents form the basis for QC universality classes. We have identified a new layered material YFe$_2$Al$_{10}$ that shows remarkably strong QC behavior, where the scaling properties of the magnetic susceptibility and specific heat are consistent with the same $F$. Recent neutron scattering results paint a remarkable picture of the QC fluctuations in YFe$_2$Al$_{10}$. In contrast to classical transitions, where fluctuations are relatively long ranged and inelastic scattering is observed at a magnetic zone center, in YFe$_2$Al$_{10}$ the scattering is independent of wave vector in the critical plane, indicating that the fluctuations are spatially localized, while out of plane scattering indicates that the interplaner interactions are restricted to nearest neighbors. The dynamical susceptibility $\chi^{''}\simeq E^{-2}$, and is wholly temperature independent, indicating that E/T scaling is present, the signature of QC fluctuations. These results hint that the the criticality in YFe$_2$Al$_{10}$ is local, which until now has only been found in a few f-electron based compounds.   

Quantum criticality in single crystalline YFe2Al10 determined from zero-field and longitudinal-field muon spin relaxation

Muon spin relaxation ($\mu$SR) measurements were performed on single crystalline YFe$_2$Al$_{10}$ down to 19 mK and in magnetic fields up to $\sim$100 Oe. Zero-field-$\mu$SR measurements showed no evidence of magnetic order down to 19 mK, consistent with previous measurements. However, we also find that the depolarization rate $\Lambda$ is temperature independent above 1 K but increases in an exponential behavior for $T <$ 1 K. Longitudinal-field $\mu$SR measurements also reveals a time-field scaling where $G(t, H)$ = $G(t/H^{\gamma}$), with $\gamma$ = 0.67. This is further confirmed from the magnetic field dependence of $\Lambda$, which finds $\Lambda(H) \propto H^{0.67}$ at 19 mK. This is further evidence that single crystalline YFe$_2$Al$_{10}$ is in close proximity to a ferromagnetic quantum critical point.   

Doping-Induced Quantum Critical Point in an Itinerant Antiferromagnet TiAu

The recently discovered itinerant magnet TiAu is the first antiferromagnet composed of non-magnetic constituents. The spin density wave ground state develops below T$^{N}$ \textasciitilde 36 K, about an order of magnitude smaller than in Cr. Achieving a quantum critical point in this material would provide a better understanding of weak itinerant antiferromagnets, while giving long sought-after insights into the effects of spin fluctuations in itinerant electron systems. While the application of pressure increases the ordering temperature T$^{N}$, partial substitution of Ti provides an alternative avenue towards achieving a quantum critical point. The non-Fermi liquid behavior accompanies the quantum phase transition, as evidenced by the divergent specific heat coefficient and linear temperature dependence of the resistivity. The transition is accompanied by enhanced electron-electron correlations as well as strong spin-fluctuations, providing an experimental avenue for the verification of the self-consistent theory of spin fluctuations.   

Itinerant magnetism in quantum critical YFe${}_2$Al${}_{10}$

The absence of magnetic order and the scaling laws in thermodynamcal and transport properties in layered compound YFe${}_2$Al${}_{10}$ suggest competition among different types of collective quantum states. Measurements on magnetic susceptibility have demonstrated a Curie-Weiss (CW) behavior with a reduced fluctuating Fe moment of 0.45µB and $T_{CW} \simeq -28K$. Using first principle methods, we show that the correlation in YFe2Al10 is moderate and the Fe magnetism is itinerant. Competing ground states include a paramagnetic state, an in-plane antiferromagnetic ordering (G-type) state and an in-plane collinear ordering (C-type) state. Although a bulk ferromagnetic order is not favored in total energy, both the G-type and C-type ground state prefer ferromagnetic inter-layer coupling.