arXiv subjects

K. Grube

Publications and source records attributed to K. Grube.

At least 19 recordsLinked to original sources

arXiv2020

Quantum critical point in the itinerant ferromagnet Ni$_{1-x}$Rh$_x$

We report a chemical substitution-induced ferromagnetic quantum critical point in polycrystalline Ni$_{1-x}$Rh$_x$ alloys. Through magnetization and muon spin relaxation measurements, we show that the ferromagnetic ordering temperature is suppressed continuously to zero at $x_{crit} = 0.375$ while the magnetic volume fraction remains 100% up to $x_{crit}$, pointing to a second order transition. Non-Fermi liquid behavior is observed close to $x_{crit}$, where the electronic specific heat $C_{el}/T$ diverges logarithmically, while immediately above $x_{crit}$ the volume thermal expansion coefficient $α_{V}/T$ and the Grüneisen ratio $Γ= α_{V}/C_{el}$ both diverge logarithmically in the low temperature limit, further indication of a ferromagnetic quantum critical point in Ni$_{1-x}$Rh$_x$.

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arXiv2019

Emergent XY electronic nematicity in iron-based superconductors

Electronic nematicity, a correlated state that spontaneously breaks rotational symmetry, is observed in several layered quantum materials. In contrast to their liquid-crystal counterparts, the nematic director cannot usually point in an arbitrary direction (XY nematics), but is locked by the crystal to discrete directions (Ising nematics),resulting in strongly anisotropic fluctuations above the transition. Here, we report on the observation of nearly isotropic XY-nematic fluctuations, via elastoresistance measurements, in hole-doped Ba$_{1-x}$Rb$_{x}$Fe$_{2}$As$_{2}$ iron-based superconductors. While for $x=0$ the nematic director points along the in-plane diagonals of the tetragonal lattice, for $x=1$ it points along the horizontal and vertical axes. Remarkably, for intermediate doping, the susceptibilities of these two symmetry-irreducible nematic channels display comparable Curie-Weiss behavior, thus revealing a nearly XY-nematic state. This opens a new route to assess this elusive electronic quantum liquid-crystalline state, which is a candidate to host unique phenomena not present in the Ising-nematic case.

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arXiv2018

Multidimensional entropy landscape of quantum criticality

The Third Law of Thermodynamics states that the entropy of any system in equilibrium has to vanish at absolute zero temperature. At nonzero temperatures, on the other hand, matter is expected to accumulate entropy near a quantum critical point (QCP), where it undergoes a continuous transition from one ground state to another. Here, we determine, based on general thermodynamic principles, the spatial-dimensional profile of the entropy S near a QCP and its steepest descent in the corresponding multidimensional stress space. We demonstrate this approach for the canonical quantum critical compound CeCu6-xAux near its onset of antiferromagnetic order. We are able to link the directional stress dependence of S to the previously determined geometry of quantum critical fluctuations. Our demonstration of the multidimensional entropy landscape provides the foundation to understand how quantum criticality nucleates novel phases such as high-temperature superconductivity.

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arXiv2018

Magnetic and structural quantum phase transitions in CeCu6-xAux are independent

The heavy-fermion compound CeCu$_{6-x}$Au$_x$ has become a model system for unconventional magnetic quantum criticality. For small Au concentrations $0 \leq x < 0.16$, the compound undergoes a structural transition from orthorhombic to monoclinic crystal symmetry at a temperature $T_{s}$ with $T_{s} \rightarrow 0$ for $x \approx 0.15$. Antiferromagnetic order sets in close to $x \approx 0.1$. To shed light on the interplay between quantum critical magnetic and structural fluctuations we performed neutron-scattering and thermodynamic measurements on samples with $0 \leq x\leq 0.3$. The resulting phase diagram shows that the antiferromagnetic and monoclinic phase coexist in a tiny Au concentration range between $x\approx 0.1$ and $0.15$. The application of hydrostatic and chemical pressure allows to clearly separate the transitions from each other and to explore a possible effect of the structural transition on the magnetic quantum critical behavior. Our measurements demonstrate that at low temperatures the unconventional quantum criticality exclusively arises from magnetic fluctuations and is not affected by the monoclinic distortion.

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arXiv2016

Antiferromagnetic Phases of the Kondo Lattice

We discuss the paramagnetic and Neel-ordered phases of the Kondo lattice Hamiltonian on the 2D square lattice by means of bond Fermions. In the doped case we find two antiferromagnetic solutions, the first one with small ordered moment, heavy bands, and an antiferromagetically folded large Fermi surface - i.e. including the localized spins - the second one with large ordered moment, light bands and an antiferromagnetically folded conduction electron-only Fermi surface. The zero temperature phase diagram as a function of Kondo coupling and conduction electron density shows first and second order transition lines between the three different phases and agrees qualitatively with previous numerical studies. We compare to experiments on CeRh(1-x)Co(x)In(5) and find qualitative agreement.

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arXiv2015

Evolution of quasiparticle excitations with critical mass enhancement in superconducting AFe2As2 (A = K, Rb, and Cs)

In the heavily hole-doped iron-based superconductors $A$Fe$_2$As$_2$ ($A=$ K, Rb, and Cs), the electron effective mass increases rapidly with alkali-ion radius. To study how the mass enhancement affects the superconducting state, we measure the London penetration depth $λ(T)$ in clean crystals of $A$Fe$_2$As$_2$ down to low temperature $T\sim0.1$ K. In all systems, the superfluid stiffness $ρ_s(T)=λ^2(0)/λ^2(T)$ can be approximated by a power-law $T$ dependence at low temperatures, indicating the robustness of strong momentum anisotropy in the superconducting gap $Δ(k)$. The power $α$ increases from $\sim1$ with mass enhancement and approaches an unconventional exponent $α\sim 1.5$ in the heaviest CsFe$_2$As$_2$. This appears to be a hallmark of superconductors near antiferromagnetic quantum critical points, where the quasiparticles excited across the anisotropic $Δ(k)$ are significantly influenced by the momentum dependence of quantum critical fluctuations.

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arXiv2015

Pressure dependence of the charge-density-wave and superconducting states in GdTe$_3$, TbTe$_3$ and DyTe$_3$

We present electrical resistivity and ac-susceptibility measurements of GdTe$_3$, TbTe$_3$ and DyTe$_3$ performed under pressure. An upper charge-density-wave (CDW) is suppressed at a rate of $\mathrm{d}T_{\mathrm{CDW,1}}/\mathrm{d}P$ = $-$85 K/GPa. For TbTe$_3$ and DyTe$_3$, a second CDW below $T_{\mathrm{CDW,2}}$ increases with pressure until it reaches the $T_{\mathrm{CDW,1}}$($P$) line. For GdTe$_3$, the lower CDW emerges as pressure is increased above $\sim$ 1 GPa. As these two CDW states are suppressed with pressure, superconductivity (SC) appears in the three compounds at lower temperatures. Ac-susceptibility experiments performed on TbTe$_3$ provide compelling evidence for bulk SC in the low-pressure region of the phase diagram. We provide measurements of superconducting critical fields and discuss the origin of a high-pressure superconducting phase occurring above 5 GPa.

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arXiv2014

Approaching quantum criticality in a partially geometrically frustrated heavy-fermion metal

Quantum phase transitions have captured the interest of a large community in condensed-matter and atom physics research. The common feature of these very different material classes lies in the fact that the competition between low-energy scales can be tuned by a nonthermal parameter, such as pressure, magnetic or electric field, and chemical composition for the condensed-matter systems. In heavy-fermion materials, the strong exchange J between f-electrons and conduction electrons can lead to quenching of the f-electron-derived (nearly) localized magnetic moments via the Kondo effect or, if J becomes weaker, to long-range magnetic order via the Ruderman-Kittel-Kasuya-Yosida interaction mediated by the conduction electrons. In addition it has been suggested that magnetic order can be suppressed by quantum fluctuations which may be enhanced by geometric frustration. Here we report on the observation of a quantum phase transition in a partially frustrated antiferromagnetic metallic system. In antiferromagnetic CePdAl the magnetic Ce ions form a network of equilateral triangles in the (001) plane, similar to the kagomé lattice, with one third of the Ce moments not participating in long-range order. The Néel temperature T_N = 2.7 K can be driven to zero upon replacing 14.4% of Pd by Ni. Here the specific heat C exhibits a C/T ~ - log T dependence. Within the Hertz-Millis-Moriya model of quantum criticality, this behavior can be attributed to two-dimensional critical antiferromagnetic fluctuations arising from the decoupling of three-dimensional magnetic order by frustration. The intermediate planes of frustrated moments are a possible candidate for a two-dimensional spin-liquid. The simultaneous presence of magnetic order, geometric frustration, and Kondo effect in this system might thus entail a new route to quantum criticality.

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arXiv2014

Fermi Surface of KFe$_2$As$_2$ from Quantum Oscillations in Magnetostriction

We present a study of the Fermi surface of KFe$_2$As$_2$ single crystals. Quantum oscillations were observed in magnetostriction measured down to 50 mK and in magnetic fields $H$ up to 14 T. For $H \parallel c$, the calculated effective masses are in agreement with recent de Haas-van Alphen and ARPES experiments, showing enhanced values with respect to the ones obtained from previous band calculations. For $H \parallel a$, we observed a small orbit at a cyclotron frequency of 64 T, characterized by an effective mass of $\sim 0.8 m_e$, supporting the presence of a three-dimensional pocket at the Z-point.

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arXiv2013

Pauli-Limited Multiband Superconductivity in KFe2As2

The upper critical field Hc2(T) of the multiband superconductor KFe2As2 has been studied via low-temperature thermal expansion and magnetostriction measurements. We present compelling evidence for Pauli-limiting effects dominating Hc2(T) for H || a, as revealed by a crossover from second- to first-order phase transitions to the superconducting state in the magnetostriction measurements down to 50 mK. Corresponding features were absent for H || c. To our knowledge, this crossover constitutes the first confirmation of Pauli limiting of the Hc2(T) of a multiband superconductor. The results are supported by modeling Pauli limits for single-band and multiband cases.

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arXiv2012

Magnetic properties of single-crystalline CeCuGa3

The magnetic behavior of single-crystalline CeCuGa3 has been investigated. The compound forms in a tetragonal BaAl4-type structure consisting of rare-earth planes separated by Cu-Ga layers. If the Cu-Ga site disorder is reduced, CeCuGa3 adopts the related, likewise tetragonal BaNiSn3-type structure, in which the Ce ion are surrounded by different Cu and Ga layers and the inversion symmetry is lost. In the literature conflicting reports about the magnetic order of CeCuGa3 have been published. Single crystals with the centrosymmetric structure variant exhibit ferromagnetic order below approx. 4 K with a strong planar anisotropy. The magnetic behavior above the transition temperature can be well understood by the crystal-field splitting of the 4f Hund's rule ground-state multiplet of the Ce ions.

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arXiv2011

Complex magneto-elastic properties in the frustrated kagome-staircase compounds (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$

High resolution heat capacity and thermal expansion experiments performed on single crystalline kagome-staircase compounds (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$ are presented. The parent compounds Co$_3$V$_2$O$_8$ and Ni$_3$V$_2$O$_8$ undergo a complex sequence of first- and second-order magnetic phase transitions. The low-temperature ($T<40$ K) magnetic entropy evolves monotonously with the doping content $x$, from the full S=1 Ni$^{2+}$ magnetic entropy in Ni$_3$V$_2$O$_8$ to half of the S=3/2 Co$^{2+}$ magnetic entropy in Co$_3$V$_2$O$_8$. Thermal expansion coefficients $α_i$ ($i = a$, $b$ and $c$) show a strong anisotropy for all (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$ compounds. The low-temperature magnetic distortion indicates that Co-doping (Ni-doping) has similar effects to applying a uniaxial pressures along $a$ or $b$ ($c$). Linear Grüneisen parameters $Γ_i$ are extracted for the three main axes $i$ and exhibit a complex behavior with both temperature and doping. For each axis, $Γ_i$ and $α_i$ exhibit a sign change (at low temperature) at the critical concentration $x_c\simeq0.25$, at which the incommensurate magnetic propagation vector changes. Beyond our study, an understanding of the multiple and complex parameters (magnetic frustration, magnetic anisotropy, mixture of S=1 and S=3/2 ions, etc.) is now necessarily to bring light to the rich magneto-elastic properties of (Co$_{1-x}$Ni$_x$)$_3$V$_2$O$_8$.

cond-mat.str-el↗

arXiv2010

Towards the identification of a quantum critical line in the (p, B) phase diagram of CeCoIn5

The low-temperature thermal expansion of CeCoIn5 single crystals measured parallel and perpendicular to magnetic fields B oriented along the c axis yields the volume thermal-expansion coefficient $β$. Considerable deviations of $β(T)$ from Fermi-liquid behavior occur already within the superconducting region of the (B, T) phase diagram and become maximal at the upper critical field $B^0_{c2}$. However, $β(T)$ and the Grüneisen parameter $Γ$ are incompatible with a quantum critical point (QCP) at $B^0_{c2}$, but allow for a QCP shielded by superconductivity and extending to negative pressures for $B < B^0_{c2}$. Together with literature data we construct a tentative (p, B, T) phase diagram of CeCoIn5 suggesting a quantum critical line in the (p, B) plane.

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arXiv2009

Resistivity of Mn$_{1-x}$Fe$_x$Si single crystals: Evidence for quantum critical behavior

Resistivity measurements have been made on Mn$_{1-x}$Fe$_x$Si single crystals between 2 and 300K for $x$ = 0, 0.05, 0.08, 0.12 and 0.15. Fe doping is found to depress the magnetic ordering temperature from 30K for $x$ = 0 to below 2K for $x$ = 0.15. Although Fe doping results in a large increase of the low-temperature residual resistivity, the temperature dependence of the resistivity above the magnetic transition remains practically unaffected by increasing Fe content. An analysis of the temperature derivative of the resistivity provides strong evidence for the existence of a non-Fermi-liquid ground state near $x$ = 0.15 and thus for a quantum critical point tuned by Fe content.

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arXiv2007

Importance of In-Plane Anisotropy in the Quasi Two-Dimensional Antiferromagnet BaNi$_{2}$V$_{2}$O$_{8}$

The phase diagram of the quasi two-dimensional antiferromagnet BaNi$_{2}$V$_{2}$O$_{8}$ is studied by specific heat, thermal expansion, magnetostriction, and magnetization for magnetic fields applied perpendicular to $\mathbf{c}$. At $μ_0H^{*}\simeq1.5$ T, a crossover to a high-field state, where $T_N(H)$ increases linearly, arises from a competition of intrinsic and field-induced in-plane anisotropies. The pressure dependences of $T_N$ and $H^{*}$ are interpreted using the picture of a pressure-induced in-plane anisotropy. Even at zero field and ambient pressure, in-plane anisotropy cannot be neglected, which implies deviations from pure Berezinskii-Kosterlitz-Thouless behavior.

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arXiv2006

Thermal expansion of the quasi two-dimensional magnetic layered compound BaNi$_{2}$V$_{2}$O$_{8}$ under magnetic fields along c-axis

The quasi two-dimensional magnetic BaNi$_{2}$V$_{2}$O$_{8}$ is studied by using high-resolution thermal expansion in magnetic fields up to 10 T applied along the c-axis. A slight increase of about 1 % of the three-dimensional antiferromagnetic ordering temperature $T_N$ is observed at 10 T. Positive and negative pressure dependencies of $T_N$, respectively, are inferred from the thermal expansion $α(T)$ for pressures applied along the $a$- and $c$-axes.

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arXiv2001

High Pressure Studies of Tc and Lattice Parameters of MgB2

We performed ac-susceptibility measurements of magnesium diboride powder samples under pure hydrostatic pressures up to 0.4 GPa and under quasi-hydrostatic pressure conditions up to 8 GPa in a helium gas pressure cell and a diamond-anvil cell (DAC), respectively. Furthermore, the DAC has been used to investigate the lattice compression by X-ray diffraction. Under helium gas pressure the superconducting transition temperature shows a linear decrease dTc/dp = -1.13 K/GPa. The pressure effect is fully reversible with a return to the initial Tc of 37.5 K after pressure release. In the diamond-anvil cell the application of pressure leads to a stronger Tc-decrease than in the helium gas pressure cell, and after the release of pressure a degradation effect with lower Tc and a broader transition compared to the first measurement at ambient pressure occurs. Such degradation may be explained by shear stresses and uniaxial pressure components, which cannot be avoided in a DAC at low temperatures. Considering the anisotropic AlB2-type structure with alternating honeycomb boron and magnesium layers, MgB2 seems to be very sensitive to such non-hydrostatic pressure components, which would explain the spread of dTc/dp values reported in the literature.

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arXiv2001

Pressure Effect and Specific Heat of RBa2Cu3Ox at Distinct Charge Carrier Concentrations: Possible Influence of Stripes

In YBa2Cu3Ox, distinct features are found in the pressure dependence of the transition temperature, dTc/dp, and in DeltaCp*Tc, where DeltaCp is the jump in the specific heat at Tc: dTc/dp becomes zero when DeltaCp*Tc is maximal, whereas dTc/dp has a peak at lower oxygen contents where DeltaCp*Tc vanishes. Substituting Nd for Y and doping with Ca leads to a shift of these specific oxygen contents, since oxygen order and hole doping by Ca influences the hole content nh in the CuO2 planes. Calculating nh from the parabolic Tc(nh) behavior, the features coalesce for all samples at nh=0.11 and nh=0.175, irrespective of substitution and doping. Hence, this behavior seems to reflect an intrinsic property of the CuO2 planes. Analyzing our results we obtain different mechanisms in three doping regions: Tc changes in the optimally doped and overdoped region are mainly caused by charge transfer. In the slightly underdoped region an increasing contribution to dTc/dp is obtained when well ordered CuO chain fragments serve as pinning centers for stripes. This behavior is supported by our results on Zn doped NdBa2Cu3Ox and is responsible for the well known dTc/dp peak observed in YBa2Cu3Ox at x=6.7. Going to a hole content below nh=0.11 our results point to a crossover from an underdoped superconductor to a doped antiferromagnet, changing completely the physics of these materials.

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