Y. Janssen

Stony Brook University, Stony Brook, New York, United States

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Publications (64)96.08 Total impact

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    ABSTRACT: We present here the results of electrical resistivity ρ, magnetization M, ac susceptibility χac′, and specific heat CM measurements that have been carried out on single crystals of Yb3Pt4 over a wide range of fields and temperatures. The 2.4-K Néel temperature that is found in zero field collapses under field to a first-order transition TN=0 at BCEP=1.85 T. In the absence of antiferromagnetic order, the specific heat CM(T,B), the magnetization M(T,B), and even the resistivity ρ(T,B) all display B/T scaling, indicating that they are dominated by strong paramagnetic fluctuations, where the only characteristic energy scale results from the Zeeman splitting of an energetically isolated, Yb doublet ground state. This paramagnetic scattering disappears with the onset of antiferromagnetic order, revealing Fermi liquid behavior Δρ=AT2 that persists up to the antiferromagnetic phase line TN(B), but not beyond. The first-order character of TN=0 and the ubiquity of the paramagnetic fluctuations imply that non-Fermi-liquid behaviors are absent in Yb3Pt4. In contrast to heavy fermions such as YbRh2Si2, Yb3Pt4 represents an extremely simple regime of f-electron behavior where the Yb moments and conduction electrons are almost decoupled, and where Kondo physics plays little role.
    Physical review. B, Condensed matter 08/2012; 86(5). · 3.77 Impact Factor
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    ABSTRACT: Yb3Pt4 is an antiferromagnet that orders at TN=2.4K. Magnetic fields B suppress TN, and the B-T phase line TN(B) terminates almost vertically at T=0, BC=2.0 T. Specific heat measurements find a mean-field transition at TN(B), and the magnetocaloric effect shows that the antiferromagnetic transition is continuous at all fields, with no associated latent heat. However, neutron diffraction measurements performed for B˜BC find that a distinct step in the magnetization deltaM occurs near the transition, with a magnitude that increases for T0. We argue that a nonzero magnetization step deltaM is required to give deltaS=0 for T=0, since the vertical phase line at T=0 implies dTN/dB=-deltaM/deltaS->-∞. We argue that TN (B) terminates at BC in a T=0 first order transition.
    03/2011;
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    ABSTRACT: Single crystals of HfMnGa2, space group Pnma, were grown using a Ga self flux technique. A sharp peak in the AC susceptibility chiAC shows a phase transition at TC 26 K, followed by Curie-Weiss behavior at higher temperatures. Arrott plot analysis confirms this transition is ferromagnetic with a spontaneous moment of mu00.3 muB/ Mn. HfMnGa2 has a coercive field of ˜ 0.1 T as well as a large magnetic anisotropy that restricts the moments to point in the [010] direction. Both a large Rhodes-Wohlfarth parameter mufluct/mu03.6 and low TC suggest HfMnGa2 is comparable to other itinerant ferromagnets MnSi, ZrZn2 and Ni3Al. Resistivity rho(T) shows HfMnGa2 to be metallic with rho(T)-rho0 having a T^5/3 dependance in the ordered state. This non-Fermi liquid relationship was also observed in Ni3Al and ZrZn2 over a more limited range of temperatures.
    03/2011;
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    ABSTRACT: Muon spin rotation and relaxation measurements have been carried out on the unconventional antiferromagnet Yb(3)Pt(4). Oscillations are observed below T(N) = 2.22(1) K, consistent with the antiferromagnetic (AFM) Néel temperature observed in bulk experiments. In agreement with neutron diffraction experiments the oscillation frequency ω(μ)(T)/2π follows an S = 1/2 mean-field temperature dependence, yielding a quasistatic local field of 1.71(2) kOe at T = 0. A crude estimate gives an ordered moment of ∼ 0.66 μ(B) at T = 0, comparable to 0.81 μ(B) from neutron diffraction. As [Formula: see text] from above the dynamic relaxation rate λ(d) exhibits no critical slowing down, consistent with a mean-field transition. In the AFM phase a T-linear fit to λ(d)(T), appropriate to a Fermi liquid, yields highly enhanced values of λ(d)/T and the Korringa constant K(μ)(2)T/λ(d), with K(μ) the estimated muon Knight shift. A strong suppression of λ(d) by applied field is observed in the AFM phase. These properties are consistent with the observed large Sommerfeld-Wilson and Kadowaki-Woods ratios in Yb(3)Pt(4) (although the data do not discriminate between Fermi-liquid and non-Fermi-liquid states), and suggest strong enhancement of q≈0 spin correlations between large-Fermi-volume band quasiparticles in the AFM phase of Yb(3)Pt(4).
    Journal of Physics Condensed Matter 02/2011; 23(9):094220. · 2.22 Impact Factor
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    ABSTRACT: We present measurements of the magnetization M, ac susceptibility chi', electrical resistivity rho, and specific heat C in single crystals of metallic YFe2Al10. The magnetic susceptibility follows a Curie-Weiss temperature dependence for 75K
    Physical review. B, Condensed matter 01/2011; 84(9). · 3.77 Impact Factor
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    ABSTRACT: We present measurements of the specific heat, magnetization, magnetocaloric effect, and magnetic neutron diffraction carried out on single crystals of antiferromagnetic Yb3Pt4, where highly localized Yb moments order at TN=2.4 K in zero field. The antiferromagnetic order was suppressed to TN-->0 by applying a field of 1.85 T in the ab plane. Magnetocaloric effect measurements show that the antiferromagnetic phase transition is always continuous for TN>0, although a pronounced step in the magnetization is observed at the critical field in both neutron diffraction and magnetization measurements. These steps sharpen with decreasing temperature, but the related divergences in the magnetic susceptibility are cut off at the lowest temperatures, where the phase line itself becomes vertical in the field-temperature plane. As TN-->0, the antiferromagnetic transition is increasingly influenced by a quantum critical end point, where TN ultimately vanishes in a first-order phase transition.
    Physical review. B, Condensed matter 01/2011; 84. · 3.77 Impact Factor
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    ABSTRACT: We present a report on the physical properties of the transition-metal-based ferromagnets HfFeGa2 and HfMnGa2. The magnetic susceptibility in both displays Curie-Weiss behavior at high temperature that is replaced by the critical susceptibility just above the Curie temperatures, which are 47.9 K in HfFeGa2 and 25.6 K in HfMnGa2. The ferromagnetically ordered state has a coercive field of 1700 Oe in HfFeGa2 and 320 Oe in HfMnGa2, with strong anisotropy that largely confines the moments to the b axis. Critical exponents that are derived from neutron diffraction measurements and Arrott plot analyses of the magnetization confirm the mean-field character of the ferromagnetic transitions. Phonons dominate the specific heat at all temperatures, but clear ordering anomalies accompany the onset of ferromagnetic order, as well as an electronic component that is larger in the ordered than paramagnetic states. Both HfFeGa2 and HfMnGa2 are metallic, and we observe an anomalous exponent in the temperature-dependent resistivity ρ(T), where ρ(T)-ρ0=BT5/3, signaling that the ordered state is a marginal Fermi liquid. Overall, the robustness of ferromagnetic order, the Curie temperatures, and the impact of fluctuations in both HfFeGa2 and HfMnGa2 are very similar to those of previously studied ferromagnets, such as MnSi, ZrZn2, Ni3Al, and Sc3In.
    Physical review. B, Condensed matter 01/2011; 83(18). · 3.77 Impact Factor
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    ABSTRACT: Aiming to study quantum critical behavior in itinerant-electron transition-metal ferromagnets, we have recently been first to characterize the intermetallic compound HfFeGa2. We have found that HfFeGa2 is a ferromagnet with a low ordering temperature and a small ordered magnetic moment. We present extensive magnetization and neutron diffraction studies of single crystals of pristine HfFeGa2. ac-magnetization data indicates the Curie temperature TC ˜ 49 K, and above TC we find Curie-Weiss behavior with an effective moment of ˜ 2.2 muB/Fe. HfFeGa2 is ferromagnetic with the ordered magnetic moments parallel to the easy c-axis of the orthorhombic crystal structure, and with a zero-temperature spontaneous magnetization of ˜ 0.6 muB/Fe. By a scaling analysis of magnetization and neutron diffraction data we have determined TC = 48.3 K as well as the critical exponents beta= 0.494 and gamma= 1.21. Our results indicate HfFeGa2 is comparable to the well-known itinerant ferromagnet ZrZn2.
    03/2010;
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    ABSTRACT: A unique system among f-electron based quantum critical systems, Yb3Pt4 orders antiferromagnetically at 2.4 K. Heat capacity, magnetocaloric effect and neutron diffraction experiments show the magnetic order can be suppressed to lower temperatures by magnetic fields applied in the easy ab plane of the rhombohedral structure. A mean-field-like anomaly in temperature-dependent heat capacity is reduced with increasing field (H //a), and disappears at (1.5K,1.6T). However, the anomaly seen in the field-dependent heat capacity at temperatures as low as 0.1K, indicates that the phase boundary line continues, showing a possible quantum critical point at about 1.8 T. Isentropes determined by direct measurements of the magnetocaloric effect (H//a) show a slope change, consistent with a continuous phase transition at all temperatures below 2.4K. Field-dependent (H//b) diffracted magnetic peak intensity is consistent with both thermodynamic measurements down to 1.5K. At lower temperatures, observations indicate two phase transitions. The upper field transition, a step in magnetization, appears to be first order. Details of the experiments and the H-T phase diagram will be discussed.
    03/2010;
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    ABSTRACT: We present a study of the magnetic and thermodynamic properties of HfFe1-xRuxGa2 single crystals grown using flux techniques. Having found a low temperature ferromagnetic intermetallic compound HfFeGa2 we try to suppress the Curie Temperature (Tc) by doping with Ru as a means to investigate the evolution of critical phenomena and perhaps realize a ferromagnetic quantum critical point (QCP). Magnetization measurements have shown changes in Tc of HfFe1-xRuxGa2 from approximately 48K to below 1.8K as a function of Ru concentration (x). We will show recent data as well as discuss the development of the spontaneous moment (m0), susceptibility chi along with heat capacity upon doping and present the resulting magnetic phase diagram.
    03/2010;
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    ABSTRACT: We review here the results of magnetization, specific heat, and inelastic neutron scattering measurements conducted on Yb3Pt4, Yb2Pt2Pb, Yb5Pt9, and YbRh2Pb, which indicate that the Yb moments in these heavy electron compounds are appreciably localized, at least in their paramagnetic states. The magnetic ground states in each are isolated magnetic doublets, and we show that magnetic fields suppress long ranged magnetic order and lead to a characteristic magnetic field-temperature phase diagram where order vanishes suddenly above a critical value for the field. We argue that the stability of magnetic order in these compounds arises from the competition between the Zeeman splitting gμ B H of the ground state doublet, which favors a spin polarized state with minimal entropy and without long range order, and the exchange splitting Δ of the doublet, which enables long ranged magnetic order. KeywordsQuantum critical points-Ytterbium compounds
    Journal of Low Temperature Physics 01/2010; 161(1):98-116. · 1.18 Impact Factor
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    ABSTRACT: We have used neutron-diffraction measurements to study the zero-field magnetic structure of the intermetallic compound Yb3Pt4, which was earlier found to order antiferromagnetically at the Néel temperature TN=2.4 K, and displays a field-driven quantum-critical point at 1.6 T. In Yb3Pt4, the Yb moments sit on a single low-symmetry site in the rhombohedral lattice with space group R3̅ . The Yb ions form octahedra with edges that are twisted with respect to the hexagonal unit cell, a twisting that results in every Yb ion having exactly one Yb nearest neighbor. Below TN, we found new diffracted intensity due to a k=0 magnetic structure. This magnetic structure was compared to all symmetry-allowed magnetic structures and was subsequently refined. The best-fitting magnetic-structure model is antiferromagnetic and involves pairs of Yb nearest neighbors on which the moments point almost exactly toward each other. This structure has moment components within the ab plane as well as parallel to the c axis although the easy magnetization direction lies in the ab plane. Our magnetization results suggest that besides the crystal-electric-field anisotropy, anisotropic exchange favoring alignment along the c axis is responsible for the overall direction of the ordered moments. The magnitude of the ordered Yb moments in Yb3Pt4 is 0.81μB/Yb at 1.4 K. The analysis of the bulk properties, the size of the ordered moment, and the observation of well-defined crystal-field levels argue that the Yb moments are spatially localized in zero field.
    Physical Review B 01/2010; 81(6). · 3.66 Impact Factor
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    ABSTRACT: Heavy electron systems provide ideal venues to study a range of issues associated with quantum criticality, including unconventional electronic phases, moment formation, and complex phase diagrams with exotic critical phenomena. In the heavy electron antiferromagnets studied so far, magnetic order occurs via a second order phase transition which can be tuned via pressure or field to a quantum critical point. Fermi liquid behavior is found beyond the quantum critical point, and the quasiparticle mass diverges at the quantum critical point, nucleating the moments required to enable magnetic order itself. We review here our experimental results on a new heavy electron system, Yb3Pt4, where antiferromagnetic order is weakly first order in zero field, but becomes second order at a critical endpoint with the application of magnetic field. No divergence of the quasiparticle mass is observed near the quantum critical field, and instead magnetic order is driven by the exchange enhancement of the Fermi liquid itself. These data support the thesis that there are multiple routes to quantum criticality in the heavy electron compounds.
    06/2009: pages 243-260;
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    ABSTRACT: DC and AC magnetic susceptibilities, specific heat and resistivity are measured for YbFe2Al10-structure compounds, ReT2Al10 where Re =Y, and La, and T = Fe, Ru, and Os. The YT2Al10 show systematically enhancing paramagnetic behavior in magnetic properties from Os to Ru and to Fe, and Fermi-liquid behaviour below around 100 K. With the linear term of the specific heat (gamma0) and the temperature independent susceptibility (chi0) at low temperature, the Stoner enhancement parameter, Z, is utilized to find how close the compounds are to the ferromagnetic ordering, where Z = 1-(3muB^2/pi^2kB^2)(gamma0/ chi0) . Specifically, YFe2Al10 shows a larger Z (0.98) than that (0.83) of Pd, a well known example of nearly ferromagnetic materials. The implied proximity to the quantum criticality is tested by a power law analysis , where 1/(chi-chi0)=AT^lambda can describe well a wide range (2K to 100K) of AC magnetic susceptibility for YFe2Al10 with lambda = 1.19, which is between the mean-field value (lambda = 1) and that of the three-dimensional Ferromagnetic Heisenberg model (lambda = 1.33).
    03/2009;
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    ABSTRACT: We have used flux techniques to explore the Yb-Pt-Pb ternary phase diagram, and have grown a number of intermetallic compounds including YbPt, Yb3Pt5, and the new Yb5Pt9, YbPt2, Yb3Pt4 and Yb2Pt2Pb, as well as Yb3Pt5Si and YbPtSi. The crystal structure of these different compounds will be compared. A particular focus has been the synthesis of single crystals of quantum critical antiferromagnet (AF) Yb3Pt4, and we show that it is possible to synthesize crystals which are large enough for neutron diffraction measurements. Laue patterns and neutron rocking curves along with other methods show that these cyrstals are of very high quality. Initial results of neutron diffraction and inelastic scattering experiments on single Yb3Pt4 crystals and arrays of multiple Yb3Pt4 crystals will be presented.
    03/2009;
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    ABSTRACT: The antiferromagnetic (AF) intermetallic compound Yb3Pt4 shows a magnetic phase diagram which includes a quantum critical point, but is different from other Yb-containing quantum critical compounds. We elucidated the zero-field behavior by neutron scattering on both polycrystal and single-crystal samples. The magnetic structure due to the single-site-low-symmetry Yb moments was determined by diffraction. The AF unit cell coincides with the crystallographic unit cell, and shows pairs of Yb nearest-neighbor moments pointing directly towards each other. The order parameter is consistent with a continuous transition at the N'eel temperature (2.4 K) and can be described by a simple mean-field model. The ordered moment amounts to ˜ 1.2 muB/Yb at 0 K. Inelastic neutron scattering reveals that the crystal electric field lifts the degeneracy of the Yb 4f ground state into 4 doublets, consistent with specific heat results.
    03/2009;
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    ABSTRACT: We have studied the evolution of the weakly first order antiferromagnetic transition in heavy fermion Yb$_3$Pt$_4$ using a combination of specific heat, magnetic susceptibility, and electrical resistivity experiments. We show that magnetic fields suppress the Neel temperature, as well as the specific heat jump, the latent heat, and the entropy of the transition, driving a critical endpoint at 1.2 K and 1.5 T. At higher fields, the antiferromagnetic transition becomes second order, and this line of transitions in turn terminates at a quantum critical point at 1.62 T. Both the ordered and high field paramagnetic states are Fermi liquids at low temperature, although the former has a much larger magnetic susceptibility and stronger quasiparticle scattering. Unlike previously studied quantum critical systems, the quasiparticle mass in Yb$_3$Pt$_4$ does not diverge at the quantum critical point, implying that here the quasiparticle interactions drive the zero temperature transition. The Fermi liquid parameters are nearly field-independent in the ordered state, indicating that the fluctuations never become fully critical, as their divergences are interrupted by the first order antiferromagnetic transition.
    01/2009;
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    ABSTRACT: We have studied single crystals of the orthorhombic intermetallic compound YbPtSi. Here we present results of anisotropic field-and-temperature dependent magnetization and specific heat. Magnetization isotherms measured between 1.8 K and the ordering temperature of 4.65 K show a sudden anomalous increase in magnetization. This phase transition becomes sharper and takes place at progressively higher fields as temperature decreases. Measurements of field-and-temperature dependent specific heat above 0.4 K confirm this magnetic phase transition above 1.8 K, and allow us to draw anisotropic magnetic phase diagrams down to 0.4 K. These phase diagrams indicate that the magnetic phase transition in YbPtSi can be brought close to zero temperature in relatively modest applied fields, ˜ 3.5 T for the hard magnetization direction. The critical line is well described by TN(H)/TN(0) = (1-(HC(T)/HC(0))^2)^0.3, very different from quasi-one-dimensional or two-dimensional quantum critical antiferromagnets.
    03/2008;
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    ABSTRACT: We report the first measurements on YbPtSi, which we synthesized as single crystals by means of flux growth from Indium. Single crystal x-ray diffraction measurements find that the YbPtSi crystal structure is the orthorhombic Co2Si-type, with one crystallographic site for Yb. Magnetic susceptibility measurements find Curie-Weiss behavior above 100 K with a moment of 4.35 muB, close to the 4.54 muB expected for trivalent Yb. Measurements of the heat capacity find a mean field-like magnetic ordering transition at T=4.65K. DC-susceptibility measurements show substantial single ion anisotropy, but also exhibit a peak at 4.65 K, indicating possible antiferromagnetic ordering. The electrical resistivity is metallic, and the magnetic ordering is accompanied by a change in slope. The magnetic entropy amounts to only about 65 % of Rln2 expected from an ordering Yb ground state doublet at the ordering temperature, suggesting a possible role for the Kondo effect. Our measurements indicate that YbPtSi is an unusual example of an Yb-based Kondo lattice system, ordering at an unusually high temperature.
    03/2008;
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    ABSTRACT: The ternary intermetallic compound Pr6Ni2Si3, is a member of a structure series of compounds based on a triangular structure where the number of Pr atoms in the prism cross section can be systematically varied. Pr6Ni2Si3 contains two distinct Pr lattice sites which result in complex interactions between the magnetic ions. Extensive measurements of specific heat and magnetization on single crystal samples indicate that Pr6Ni2Si3 orders with both a ferromagnet and an antiferromagnet component, with ordering temperatures of 39.6 K and ~ 32 K, respectively. The ferromagnetic component // c-axis is accompanied by a large hysteresis, and the antiferromagnetic component,_|_ c-axis is accompanied by a spin-flop-type transition. More detailed measurements, of the vector magnetization, indicate that the ferromagnetic and the antiferromagnetic order appear independent of each other. These results not only clarify the behavior of Pr6Ni2Si3 itself, but also of the other members of the structure series, Pr5Ni2Si3 and Pr15Ni7Si10. Comment: 9 pages, 13 figures, submitted to PRB
    Physical review. B, Condensed matter 01/2008; · 3.77 Impact Factor

Publication Stats

132 Citations
96.08 Total Impact Points

Institutions

  • 2011
    • Stony Brook University
      • Department of Physics and Astronomy
      Stony Brook, New York, United States
    • University of California, Riverside
      • Department of Physics and Astronomy
      Riverside, CA, United States
  • 2009–2011
    • Brookhaven National Laboratory
      • Condensed Matter Physics & Materials Science Department
      New York City, NY, United States
  • 2007–2008
    • Iowa State University
      • • Department of Physics and Astronomy
      • • Ames Laboratory
      Ames, IA, United States
  • 1997–2004
    • University of Amsterdam
      • Van der Waals-Zeeman Institute
      Amsterdamo, North Holland, Netherlands
  • 2000
    • Northeast Institute of Geography and Agroecology
      • Institute of Physics
      Beijing, Beijing Shi, China