
Quantum criticality in FM disordered systems (chair: Theodore R. Kirkpatrick)

09:00  09:30

Ryan Baumbach
(Florida State University)
Disordered ferromagnetic quantum phase transition in \(CePd_2P_2\)with Pd \(\rightarrow\)Ni substitution
The BelitzKirkpatrickVojta (BKV) theory has emerged as a leading framework within which to describe ferromagnetic quantum phase transitions, and there already are many systems that have been described in this context. This includes both $d$ and $f$electron magnets, which often feature intertwining energy scales such as the magnetic exchange (both sign and strength), the spin orbit interaction, the Kondo interaction, magnetic ion valence stability, and other factors. Furthermore, chemical/structural disorder plays an important role. Thus, it remains important to probe the universality of this theory by studying even more examples that span different parts of the electronic phase space. In this talk I will report results for the isoelectronic chemical substitution series $Ce(Pd_{1x}Ni_x)_2Si_2$, where a possible ferromagnetic quantum phase transition is uncovered in the temperatureconcentration ($T−x$) phase diagram. This behavior results partly from the contraction of the unit cell volume, which (i) tunes the relative strengths of the Kondo and Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions and (ii) modifies the $f$electron valence stability. The behavior is also influenced by the introduction of disorder through alloying. Near the critical region at $x_{\rm{cr}}$ $\approx$ 0.7, the rate of contraction of the unit cell volume strengthens, indicating that the cerium $f$ valence crosses over from trivalent to a noninteger value. Consistent with this picture, xray absorption spectroscopy measurements reveal that while $CePd_2P_2$ has a purely trivalent cerium $f$ state, $CeNi_2P_2$ has a small (< 10 \%) tetravalent contribution. In a broad region around $x_{\rm{cr}}$, there is a breakdown of Fermiliquid temperature dependence seen in the heat capacity, signaling the influence of quantum critical fluctuations and disorder effects. Electrical transport measurements of clean $CePd_2P_2$ furthermore show that applied pressure has an initial effect similar to alloying on the ferromagnetic order. From these results, $CePd_2P_2$ emerges as a keystone system to test the BKV model in $f$electron metals, where distinct behaviors are expected in the dirty and clean limits.

09:30  10:00

Gregory R. Stewart
(University of Florida)
Evolution of physical properties with decreasing size in \(U_2Pt_2In\), an undoped nFl compound
In 2009, Kim, Stewart, and Samwer (PRB 79, 165119) investigated how the magnetic susceptibility, the magnetization, and the specific heat evolved with decreasing size in quantum critical CeRu0.8Rh1.2Si2 down to a particle size of 0.6 microns. They found evidence for Griffiths phase behavior with decreasing particle size, which below 3 microns began to be masked by uncompensated local moment defects seen in small particles of Ce compounds before. In order to avoid this masking behavior in smallsized CeRu0.8Rh1.2Si2, we report here on a similar study on samples of small particles of the undoped nFl compound U2Pt2In, also down to 0.6 microns in size. Uncompensated local moment defect peaks in the low temperature specific heat of CeRu0.8Rh1.2Si2 are avoided in our data for U2Pt2In, allowing a clearer look at the properties of this bulk nFl compound as a function of decreasing size. Properties of particles of U2Pt2In in the following size windows are reported: 10  20 microns; 3  10 microns; 1.2  3 microns; and 0.6  1.2 microns.

10:00  10:30

Matthias Vojta
(Technische Universität Dresden)
Clusterglass phase in pyrochlore XY magnets with quenched disorder
We study the impact of quenched disorder (random exchange couplings or site dilution) on easyplane pyrochlore antiferromagnets. In the clean system, orderbydisorder selects a magnetically ordered state from a classically degenerate manifold. In the presence of randomness, however, different orders can be chosen locally depending on details of the disorder configuration. Using a combination of analytical considerations and classical MonteCarlo simulations, we argue that any longrangeordered magnetic state is destroyed beyond a critical level of randomness where the system breaks into magnetic domains due to random exchange
anisotropies, becoming, therefore, a glass of spin clusters, in accordance
with the available experimental data. These random anisotropies originate from offdiagonal exchange couplings in the microscopic Hamiltonian, establishing their relevance to other magnets with strong spinorbit coupling.

10:30  11:00

coffee break


Quantum criticality in FM disordered systems (chair: Michael Baenitz)

11:00  11:30

Almut Schroeder
(Kent State University)
Ferromagnetic order and disorder in NiV alloys
The Nialloy, Ni$_{1x}$V$_x$, presents the opportunity to study a ferromagnetic (FM) quantum phase transition (QPT) with strong ''disorder'' introduced by ''random'' chemical substitution. Using complementary methods, neutron scattering, muon spin relaxation ($\mu $ SR) and magnetization, we show evidence of disorder and probe signatures of quantum Griffiths singularities close to the quantum critical concentration $x_c \approx11.6\%$ where the FM magnetic order is suppressed [1].
The key findings in NiV are: A detailed pair density function (PDF) analysis confirms that NiV is one of the rare binary Nialloys that form a solid solution (with random atomic occupation on a fcclattice) in the relevant Nirich regime. Fielddependent magnetization data display power laws with nonuniversal exponents in an extended concentration region around $x_c$. This signals the first evidence of quantum Griffiths phase (QGP) in the FM phase. $\mu$SR data also recognize a remaining dynamic magnetic cluster distribution besides the FM ordered response in the FM phase close to $x_c$ [1]. New small angle neutron scattering (SANS) data reveal a shortrange magnetic cluster contribution and evidence for longrange FM domains for the same samples.
The experimental signatures of a QGP with its limits are discussed in this Nialloy with strong short range and weak longrange interaction.
in collaboration with:
A. Gebretsadik, S. Bhattarai, J.G. Lussier, A. Alyami, R. Wang, Kent State University;
T. Vojta, Missouri University of S & T; K. Page, L. DebeerSchmitt, ORNL; K. Krycka, NIST;
P. J. Baker, F. L. Pratt, ISIS, STFC RAL; S. J. Blundell, T. Lancaster, J. S. Möller, Oxford
[1] R. Wang, A. Gebretsadik, S. UbaidKassis et al., Phys. Rev. Lett. 118, 267202 (2017)

11:30  12:00

Hilbert von Löhneysen
(Karlsruher Institut für Technologie)
Anomalous quantumcritical dynamic scaling the weak itinerant ferromagnet \(Sr_{1x}Ca_{x}RuO_{3}\)
The dynamics of continuous phase transitions is governed by the dynamic scaling exponent relating the correlation length and correlation time. For transitions at finite temperature, thermodynamic critical properties are independent of the dynamic scaling exponent z. In contrast, at quantum phase transitions where the transition temperature becomes zero, static and dynamic properties are inherently entangled by virtue of the uncertainty principle. Consequently, thermodynamic scaling equations explicitly contain the dynamic exponent. Basic thermodynamic measurements of thr magnetization M and the specific heat C (as a function of temperature and magnetic field) for the itinerant ferromagnet $Sr{1−x}Ca_xRuO_3$ show that the transition temperature becomes zero for x = 0.7. For samples of this Ca concentraton, we find dynamic scaling of M and C with highly unusual quantum critical dynamics. We observe a small dynamic scaling exponent z of 1.76 strongly deviating from current models of ferromagnetic quantum criticality and likely being governed by strong disorder in conjunction with strong electron–electron coupling.
Work done in collaboration with C. L. Huang, D. Fuchs, M. Wissinger, R. Schneider, M. C. Ling, M. S. Scheurer, and J. Schmalian

12:00  12:30

Brian C Sales
(Oak Ridge National Laboratory)
Quantum criticality in the asymptotic limit of high disorder: a new twist on structural alloys

12:30  13:30

lunch

13:30  14:30

discussion


Ubased FM compounds (chair: Daniel Braithwaite)

14:30  15:00

Anne de Visser
(University of Amsterdam)
Superconducting and ferromagnetic phase diagram of UCoGe
The intermetallic compound UCoGe has become a laboratory tool to study the unusual coexistence of itinerant ferromagnetism ($T_C$ = 3 K) and superconductivity ($T_s$ = 0.5 K) on the microscopic scale at ambient pressure [1]. Superconductivity in UCoGe is unconventional, as is demonstrated by the strongly anisotropic upper critical field and the extremely large $B_{c2}(0)$ values for magnetic field directions perpendicular to the ordered moment [2]. Here we report thermal expansion measurements in zero and applied magnetic fields on a single crystal of UCoGe around the magnetic and superconducting phase transitions [3]. The thermal expansion cell was mounted on a piezoelectric rotator in order to allow for a precise alignment of the magnetic field with the main crystal axes. The superconducting and magnetic phase diagram has been determined. With our bulk technique we confirm the Sshape of the uppercritical field for $B \parallel b$ and reinforcement of superconductivity above 6 T. At the same time the Curie point shifts towards lower temperatures when the field is applied along the $b$axis. Our results lend further support to theoretical proposals of spinfluctuation mediated reinforcement of superconductivity for $B \parallel b$.
[1] N.T. Huy et al., Phys. Rev. Lett. 99, 067006 (2007).
[2] D. Aoki et al., J. Phys. Soc. Jpn 78, 113709 (2009).
[3] A.M. Nikitin et al., Phys. Rev. B 85, 115151 (2017).

15:00  15:30

Hisashi Kotegawa
(Kobe University)
NMR study on ferromagnetic critical point of \(UGe_2\) and the investigation of phase diagrams of some ferromagnets
We report new NMR results on UGe2 with focusing the electronic state near the critical point between FM1 and FM2 phases. At the phase boundary, NMR can evaluate the microscopic states in each phase separately and detect the magnetic fluctuations originated from the critical point. We will discuss the relationship between superconductivity and the observed fluctuations. In the latter part of the presentation, we will show the pressuretemperaturemagnetic field phase diagrams of some felectrons ferromagnets.

15:30  16:00

Malte Grosche
(University of Cambridge)
Superconductivity in metallic magnets
The threshold of metallic magnetism throws up fundamental questions that continue to challenge our understanding of correlated systems. Among these are the complex phase diagrams arising from the interplay of ferromagnetic and spindensity wave order, the nonFermi liquid states observed near second order but in some cases also near first order quantum phase transitions, the consequences of disorder and inhomogeneity, the opportunities for topological defects to form, and the possibility of unconventional superconductivity.
I will review progress with the layered ironbased superconductor YFe$_2$Ge$_2$ [1], which according to DFT [3] and neutron scattering studies [3] is closely balanced near the threshold of ferromagnetic and antiferromagnetic order. The high Sommerfeld ratio of order 100 mJ/molK$^2$ and a $T^{3/2}$ temperature dependence of the electrical resistivity at low temperature $T$ indicate that YFe$_2$Ge$_2$ is governed by strong electronic correlations. Our crystal growth study found that superconductivity in YFe$_2$Ge$_2$ is very sensitive to disorder scattering, pointing towards an unconventional pairing state. Only the most recent crystals, which reach resistance ratios of order 400, show a sharp superconducting heat capacity anomaly and enable a first examination of the superconducting state.
1. J. Chen et al. Phys. Rev. Lett. 116, 127001 (2016) and Phys. Rev. B 99, 020501 (2019).
2. D. J. Singh, Phys. Rev. B 89, 024505 (2014); A. Subedi Phys. Rev. B 89, 024504 (2014).
3. H. Wo, H. et al. condmat 180807262 (2018).

16:00  16:30

coffee break


Strong anisotropic FM systems and spin liquids (chair: Maria Teresa Mercaldo)

16:30  17:00

Natalia Perkins
(University of Minnesota)
Observing spin fractionalization in the Kitaev spin liquid
Motivated by the ongoing effort to search for highresolution signatures of quantum spin liquids, we investigate the temperature dependence of the indirect resonant inelastic xray scattering (RIXS) response for the Kitaev honeycomb model. We find that, as a result of spin fractionalization, the RIXS response changes qualitatively at two wellseparated temperature scales, $T_L$ and $T_H$, which correspond to the characteristic energies of the two kinds of fractionalized excitations, Z2 gauge fluxes and Majorana fermions, respectively. While thermally excited Z2 gauge fluxes at temperature TL lead to a general broadening and softening of the response, the thermal proliferation of Majorana fermions at temperature $T_H\sim 10$ $T_L$ results in a significant shift of the spectral weight, both in terms of energy and momentum. Due to its exclusively indirect nature, the RIXS process we consider gives rise to a universal magnetic response and, from an experimental perspective, it directly corresponds to the Kedge of Ru$^{3+}$ in the Kitaev candidate material $\alpha$RuCl$_3$.

17:00  17:30

Anton Jesche
(Universität Augsburg)
Anisotropy driven spontaneous magnetization
A seemingly ferromagnetic state can emerge in an Ising system with sufficiently large anisotropy even when effective exchange interactions are prohibited by strong dilution. A prime example is given by irondoped lithium nitride, $Li_2(Li_{1x}Fe_x)N$ with x << 1. The basic magnetic unit is not a cluster or a magnetic domain but the magnetic moment of single, isolated iron atoms, which are embedded in the nonmagnetic lithium nitride matrix [1]. This novel model system shows an extremely large magnetic anisotropy and allows to study quantum tunneling of the magnetization in a rather simple, inorganic material. At the origin of the outstanding properties is the special geometry that the iron finds itself in: the linear coordination between two nitrogen atoms is not subject to a JahnTeller distortion and gives rise to an unquenched orbital moment. Accordingly, this rareearthfree material shows a huge hysteresis with coercivity fields of more than 11 Tesla.
Recently we have found an extreme field dependence of the spin reversal process in $Li_2(Li_{1x}Fe_x)N$ [2]. The spinflip probability strongly increases in transverse magnetic fields that proves the resonant character of this magnetic tunneling process. Applied Longitudinal fields, on the other hand, lift the groundstate degeneracy and destroy the tunneling condition. An increase of the relaxation time by four orders of magnitude in applied fields of only a few milliTesla reveals exceptionally sharp tunneling resonances. Therefore, it is possible either to freeze the orientation and mimic ferromagnetic ordering or to promote the flip of a spinstate by tiny applied fields. The up and down states of the iron atom's spin have been made switchable and provide an 'atomic quantum bit' at easily accessible liquid helium temperatures.
[1] A. Jesche et al. Nat. Commun. 5:3333 (2014) doi: 10.1038/ncomms4333
[2] M. Fix, J. H. Atkinson, P. C. Canfield, E. del Barco & A. Jesche Phys. Rev. Lett. 120, 147202 (2018)

19:00  22:00

workshop dinner
