arXiv subjects

R. E. Baumbach

Publications and source records attributed to R. E. Baumbach.

At least 19 recordsLinked to original sources

arXiv2025

Metallic layered materials with magnetic frustration: An ARPES view of the SmAuAl$_4$Ge$_2$ and TbAuAl$_4$Ge$_2$

Compounds of the new materials class LnTAl$_4$X$_2$ (Ln = lanthanide, X = tetrel, T = transition metal) host exotic magnetic phenomena due to geometric frustration induced by their triangular lattice. Complex spin arrangements, magnetic fluctuations and double magnetic transitions have been well observed by means of magneto-transport. Nevertheless, the experimental electronic structure of this family of materials has been poorly studied. We have investigated the experimental electronic structure of two members of this class of materials: SmAuAl$_4$Ge$_2$ and TbAuAl$_4$Ge$_2$. By means of Angle-Resolved PhotoEmission Spectroscopy (ARPES) accompanied by Density Functional Theory calculations (DFT), we reveal common trends and features, the important effect of localized spin moments on the electronic structure, the presence of surface-localized electronic states and the nature of the surface termination layer. Low-dimensionality, exchange interaction, and spin-orbit coupling are all important ingredients of the electronic structure.

cond-mat.str-el↗

arXiv2022

Pressure-induced superconductivity in the weak topological insulator Bi2TeI and the topological metal Bi3TeI

We report a series of high-pressure electrical transport, magnetic susceptibility, and x-ray diffraction measurements on single crystals of the weak topological insulator Bi2TeI and the topological metal Bi3TeI. Room temperature x-ray diffraction measurements show that both materials go through a series of pressure-induced structural transitions and eventually adopt a disordered bcc alloy structure at high pressure. A re-analysis of the published data on BiTeI indicates that this material also adopts a disordered bcc structure at high pressure, in contrast to the previously suggested P4/nmm structure. We find that Bi2TeI and Bi3TeI become superconducting at 13 GPa and 11.5 GPa, respectively. The superconducting critical temperature Tc of the bcc phase reaches maximum values of 7 K and 7.5 K for Bi2TeI and Bi3TeI, respectively and dTc/dP < 0 in both cases. The results indicate that disordered alloy bcc superconducting phases appear to be a universal feature of both the Bi-Te and Bi-Te-I systems at high pressure.

cond-mat.supr-con↗

arXiv2021

Resonance from antiferromagnetic spin fluctuations for superconductivity in UTe$_2$

Superconductivity originates from the formation of bound (Cooper) pairs of electrons that can move through the lattice without resistance below the superconducting transition temperature $T_c$. Electron Cooper pairs in most superconductors form anti-parallel spin singlets with total spin $S=0$, although they can also form parallel spin-triplet Cooper pairs with $S=1$ and an odd parity wavefunction. Spin-triplet pairing is important because it can host topological states and Majorana fermions relevant for quantum computation. Because spin-triplet pairing is usually mediated by ferromagnetic (FM) spin fluctuations, uranium based materials near an FM instability are considered to be ideal candidates for realizing spin-triplet superconductivity. Indeed, UTe$_2$, which has a $T_c\approx 1.6$ K, has been identified as a candidate for a chiral spin-triplet topological superconductor near an FM instability, although it also has antiferromagnetic (AF) spin fluctuations. Here we use inelastic neutron scattering (INS) to show that superconductivity in UTe$_2$ is coupled to a sharp magnetic excitation, termed resonance, at the Brillouin zone boundary near AF order. Because the resonance has only been found in spin-singlet unconventional superconductors near an AF instability, its observation in UTe$_2$ suggests that AF spin fluctuations may also induce spin-triplet pairing or that electron pairing in UTe$_2$ has a spin-singlet component.

cond-mat.supr-con↗

arXiv2021

Heat capacity of URu$_{2-x}$Os$_x$Si$_2$ at low temperatures

We perform measurements of the heat capacity as a function of temperature in URu$_{2-x}$Os$_x$Si$_2$ alloys. Our experimental results show that the critical temperature of the second-order phase transition increases while the value of the Sommerfeld coefficient in the ordered state decreases with an increase in osmium concentration. We also observe the increase in the values of the heat capacity at the critical temperature as well as a broadening of the critical fluctuations region with an increase in $x$. We analyze the experimental data using the Haule-Kotliar model which, in particular, identifies the 'hidden order' transition in the parent material URu$_2$Si$_2$ as a transition to a state with nonzero hexadecapole moment. We demonstrate that our experimental results are consistent with the predictions of that model.

cond-mat.str-el↗

arXiv2021

Spectroscopic Evidence for the Direct Involvement of Local Moments in the Pairing Process of the Heavy-Fermion Superconductor CeCoIn$_5$

The microscopic mechanism for electron pairing in heavy-fermion superconductors remains a major challenge in quantum materials. Some form of magnetic mediation is widely accepted with spin fluctuations as a prime candidate. A novel mechanism, 'composite pairing' based on the cooperative two-channel Kondo effect directly involving the f-electron moments has also been proposed for some heavy fermion compounds including CeCoIn$_5$. The origin of the spin resonance peak observed in neutron scattering measurements on CeCoIn$_5$ is still controversial and the corresponding hump-dip structure in the tunneling conductance is missing. This is in contrast to the cuprate and Fe-based high-temperature superconductors, where both characteristic signatures are observed, indicating spin fluctuations are likely involved in the pairing process. Here, we report results from planar tunneling spectroscopy along three major crystallographic orientations of CeCoIn5 over wide ranges of temperature and magnetic field. The pairing gap opens at T$_p$ ~ 5 K, well above the bulk T$_c$ = 2.3 K, and its directional dependence is consistent with d$_{x^2-y^2}$ symmetry. With increasing magnetic field, this pairing gap is suppressed as expected but, intriguingly, a gaplike structure emerges smoothly, increasing linearly up to the highest field applied. This field-induced gaplike feature is only observed below T$_p$. The concomitant appearance of the pairing gap and the field-induced gaplike feature, along with its linear increase with field, indicates that the f-electron local moments are directly involved in the pairing process in CeCoIn$_5$.

cond-mat.supr-con↗

arXiv2021

Experimental observation and spin texture of Dirac node arcs in tetradymite topological metals

We report the observation of a non-trivial spin texture in Dirac node arcs, novel topological objects formed when Dirac cones of massless particles extend along an open one-dimensional line in momentum space. We find that such states are present in all the compounds of the tetradymite M$_2$Te$_2$X family (M$=$Ti, Zr or Hf and X$=$P or As), regardless of the weak or strong character of the topological invariant. The Dirac node arcs in tetradymites are thus the simplest possible, textbook example, of a type-I Dirac system with a single spin-polarized node arc.

cond-mat.mes-hall↗

arXiv2020

Isoelectronic perturbations to $f$-$d$-electron hybridization and the enhancement of hidden order in URu$_2$Si$_2$

Electrical resistivity measurements were performed on single crystals of URu$_2-x$Os$_x$Si$_2$ up to $x$ = 0.28 under hydrostatic pressure up to $P$ = 2 GPa. As the Os concentration, $x$ , is increased, (1) the lattice expands, creating an effective negative chemical pressure $P_{ch}$($x$), (2) the hidden order (HO) phase is enhanced and the system is driven toward a large-moment antiferromagnetic (LMAFM) phase, and (3) less external pressure $P_{c}$ is required to induce the HO to LMAFM phase transition. We compare the $T(x)$, $T(P)$ phase behavior reported here for the URu$_2-x$Os$_x$Si$_2$ system with previous reports of enhanced HO in URu$_2$Si$_2$ upon tuning with $P$, or similarly in URu$_2-x$Fe$_x$Si$_2$ upon tuning with positive $P_{ch}$($x$). It is noted that pressure, Fe substitution, and Os substitution are the only known perturbations that enhance the HO phase and induce the first order transition to the LMAFM phase in URu$_2$Si$_2$. We present a scenario in which the application of pressure or the isoelectronic substitution of Fe and Os ions for Ru results in an increase in the hybridization of the U-5$f$- and transition metal $d$-electron states which leads to electronic instability in the paramagnetic phase and a concurrent stability of HO (and LMAFM) in URu$_2$Si$_2$. Calculations in the tight binding approximation are included to determine the strength of hybridization between the U-5$f$ electrons and each of the isoelectronic transition metal $d$-electron states of Fe, Ru, and Os.

cond-mat.str-el↗

arXiv2020

Origin of gap-like behaviors in URu$_2$Si$_2$: Combined study via quasiparticle scattering spectroscopy and resistivity measurements

We address two long-standing questions regarding the hidden order in URu2Si2: Is it associated with the hybridization process, and what are the distinct roles played by the localized and itinerant electrons? Our quasiparticle scattering spectroscopy reveals a hybridization gap ubiquitous in the entire phase space spanned by P and Fe substitutions in URu2Si2, including the no-order and antiferromagnetic regions, with minimal change upon crossing the phase boundary. This indicates its opening isn't associated with the ordering, and thus localized electrons must be the major player. Towards a consistent understanding of all the other gap-like behaviors observed only below transition temperatures, we analyze the electrical resistivity using a model in which gapped bosonic excitations are the dominant scattering source. With their stiffness set to follow an unusual temperature dependence (decreasing with decreasing temperature), this model fits all of our resistivity data well including the jump at the transition. Remarkably, the extracted gap increases slowly with increasing Fe content, similarly to the gap detected by inelastic neutron scattering at Q1 = (1.4, 0, 0), suggesting a common origin. Such a model can also naturally explain the Hall effect temperature dependence without invoking Fermi surface gapping.

cond-mat.str-el↗

arXiv2020

Pressure-induced suppression of ferromagnetism in CePd$_2$P$_2$

The correlated electron material CePd$_2$P$_2$ crystallizes in the ThCr$_2$Si$_2$ structure and orders ferromagnetically at 29 K. Lai et al. [Phys. Rev. B 97, 224406 (2018)] found evidence for a ferromagnetic quantum critical point induced by chemical compression via substitution of Ni for Pd. However, disorder effects due to the chemical substitution interfere with a simple analysis of the possible critical behavior. In the present work, we examine the temperature - pressure - magnetic field phase diagram of single crystalline CePd$_2$P$_2$ to 25 GPa using a combination of resistivity, magnetic susceptibility, and x-ray diffraction measurements. We find that the ferromagnetism appears to be destroyed near 12 GPa, without any change in the crystal structure.

cond-mat.str-el↗

arXiv2020

Tuning the structural and antiferromagnetic phase transitions in UCr$_{2}$Si$_2$: hydrostatic pressure and chemical substitution

Structural phase transitions in $f$-electron materials have attracted sustained attention both for practical and basic science reasons, including that they offer an environment to directly investigate relationships between structure and the $f$-state. Here we present results for UCr$_2$Si$_2$, where structural (tetragonal $\rightarrow$ monoclinic) and antiferromagnetic phase transitions are seen at $T_{\rm{S}}$ $=$ 205 K and $T_{\rm{N}}$ $=$ 25 K, respectively. We also provide evidence for an additional second order phase transition at $T_{\rm{X}}$ = 280 K. We show that $T_{\rm{X}}$, $T_{\rm{S}}$, and $T_{\rm{N}}$ respond in distinct ways to the application of hydrostatic pressure and Cr $\rightarrow$ Ru chemical substitution. In particular, hydrostatic compression increases the structural ordering temperature, eventually causes it to merge with $T_{\rm{X}}$ and destroys the antiferromagnetism. In contrast, chemical substitution in the series UCr$_{2-x}$Ru$_x$Si$_2$ suppresses both $T_{\rm{S}}$ and $T_{\rm{N}}$, causing them to approach zero temperature near $x$ $\approx$ 0.16 and 0.08, respectively. The distinct $T-P$ and $T-x$ phase diagrams are related to the evolution of the rigid Cr-Si and Si-Si substructures, where applied pressure semi-uniformly compresses the unit cell and Cr $\rightarrow$ Ru substitution results in uniaxial lattice compression along the tetragonal $c$-axis and an expansion in the $ab$-plane. These results provide insights into an interesting class of strongly correlated quantum materials where degrees of freedom associated with $f$-electron magnetism, strong electronic correlations, and structural instabilities are readily controlled.

cond-mat.str-el↗

arXiv2019

Origin of the butterfly magnetoresistance in a Dirac nodal-line system

We report a study on the magnetotransport properties and on the Fermi surfaces (FS) of the ZrSi(Se,Te) semimetals. Density Functional Theory (DFT) calculations, in absence of spin orbit coupling (SOC), reveal that both the Se and the Te compounds display Dirac nodal lines (DNL) close to the Fermi level $\varepsilon_F$ at symmorphic and non-symmorphic positions, respectively. We find that the geometry of their FSs agrees well with DFT predictions. ZrSiSe displays low residual resistivities, pronounced magnetoresistivity, high carrier mobilities, and a butterfly-like angle-dependent magnetoresistivity (AMR), although its DNL is not protected against gap opening. As in Cd$_3$As$_2$, its transport lifetime is found to be 10$^2$ to 10$^3$ times larger than its quantum one. ZrSiTe, which possesses a protected DNL, displays conventional transport properties. Our evaluation indicates that both compounds most likely are topologically trivial. Nearly angle-independent effective masses with strong angle dependent quantum lifetimes lead to the butterfly AMR in ZrSiSe.

cond-mat.str-el↗

arXiv2018

Pressure dependent intermediate valence behavior in YbNiGa$_{4}$ and YbNiIn$_{4}$

We report a comprehensive structural and valence study of the intermediate valent materials YbNiGa$_{4}$ and YbNiIn$_{4}$ under pressures up to 60 GPa. YbNiGa$_{4}$ undergoes a smooth volume contraction and shows steady increase in Yb-valence with pressure, though the Yb-valence reaches saturation around 25 GPa. In YbNiIn$_{4}$, a change in pressure dependence of the volume and a peak in Yb-valence suggest a pressure induced electronic topological transition occurs around 10-14 GPa. In the pressure region where YbNiIn$_{4}$ and YbNiGa$_{4}$ possess similar Yb-Yb spacings the Yb-valence reveals a precipitous drop. This drop is not captured by density-functional-theory calculations and implies that both the lattice degrees of freedom and the chemical environment play an important role in establishing the valence of Yb.

cond-mat.str-el↗

arXiv2018

The role of band filling in tuning the high field phases of URu2Si2

We present a detailed study of the low temperature and high magnetic field phases in the chemical substitution series URu$_2$Si$_{2-x}$P$_x$ using electrical transport and magnetization in pulsed magnetic fields up to 65T. Within the hidden order region (0 $\ x$$\ $ 0.035) the high field ordering is robust even as the hidden order temperature is suppressed. Earlier work shows that for 0.035 $\ x$ $\ $ 0.26 there is a Kondo lattice with a no-ordered state that is replaced by antiferromagnetism for 0.26 $\ x$ 0.5. We observe a simplified continuation of the high field ordering in the no-order $x$-region and an enhancement of the high field state upon the destruction of the antiferromagnetism with magnetic field. These results closely resemble what is seen for URu$_{2-x}$Rh$_x$Si$_2$\footnote{The concentration in this paper is defined as URu$_{2-x}$Rh$_x$Si$_2$ while the chemical formula in the literature is given as U(Ru$_{1-x}$Rh$_x$)$_2$Si$_2$ [24-26]}, from which we infer that charge tuning uniformly controls the ground state of URu$_2$Si$_2$, regardless of whether s/p or d-electrons are replaced. This provides guidance for determining the specific factors that lead to hidden order versus magnetism in this family of materials.

cond-mat.str-el↗

arXiv2018

Ferromagnetic Quantum Critical Point in CePd$_2$P$_2$ with Pd $\rightarrow$ Ni Substitution

An investigation of the structural, thermodynamic, and electronic transport properties of the isoelectronic chemical substitution series Ce(Pd$_{1-x}$Ni$_x$)$_2$P$_2$ is reported, where a possible ferromagnetic quantum critical point is uncovered in the temperature - concentration ($T-x$) phase diagram. This behavior results from the simultaneous contraction of the unit cell volume, which tunes the relative strengths of the Kondo and RKKY interactions, and 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 non-integer value. Consistent with this picture, x-ray absorption spectroscopy measurements reveal that while CePd$_2$P$_2$ has a purely trivalent cerium $f$-state, CeNi$_2$P$_2$ has a small ($<$ 10 \%) tetravalent contribution. In a broad region around $x_{\rm{cr}}$, there is a breakdown of Fermi liquid temperature dependences, signaling the influence of quantum critical fluctuations and disorder effects. Measurements of clean CePd$_2$P$_2$ furthermore show that applied pressure has a similar initial effect to alloying on the ferromagnetic order. From these results, CePd$_2$P$_2$ emerges as a keystone system to test theories such as the Belitz-Kirkpatrick-Vojta model for ferromagnetic quantum criticality, where distinct behaviors are expected in the dirty and clean limits.

cond-mat.str-el↗

arXiv2018

Bulk Fermi surfaces of the Dirac Type-II Semimetallic Candidates VAl3, NbAl3 and TaAl3

We report a de Haas-van Alphen (dHvA) effect study on the Dirac type-II semimetallic candidates \emph{M}Al$_3$ (where, \emph{M} = V, Nb and Ta). The angular-dependence of their Fermi surface (FS) cross-sectional areas reveals a remarkably good agreement with first-principle calculations. Therefore, dHvA supports the existence of tilted Dirac cones with Dirac type-II nodes located at 100, 230 and 250 meV above the Fermi level $\varepsilon_F$ for VAl$_3$, NbAl$_3$ and TaAl$_3$ respectively, in agreement with the prediction of broken Lorentz invariance in these compounds. However, for all three compounds we find that the cyclotron orbits on their FSs, including an orbit nearly enclosing the Dirac type-II node, yield trivial Berry phases. We explain this $via$ an analysis of the Berry phase where the position of this orbit, relative to the Dirac node, is adjusted within the error implied by the small disagreement between our calculations and the experiments. We suggest that a very small amount of doping could displace $\varepsilon_F$ to produce topologically non-trivial orbits encircling their Dirac node(s).

cond-mat.mtrl-sci↗

arXiv2017

Converting topological insulators into topological metals within the tetradymite family

We report the electronic band structures and concomitant Fermi surfaces for a family of exfoliable tetradymite compounds with the formula $T_2$$Ch_2$$Pn$, obtained as a modification to the well-known topological insulator binaries Bi$_2$(Se,Te)$_3$ by replacing one chalcogen ($Ch$) with a pnictogen ($Pn$) and Bi with the tetravalent transition metals $T$ $=$ Ti, Zr, or Hf. This imbalances the electron count and results in layered metals characterized by relatively high carrier mobilities and bulk two-dimensional Fermi surfaces whose topography is well-described by first principles calculations. Intriguingly, slab electronic structure calculations predict Dirac-like surface states. In contrast to Bi$_2$Se$_3$, where the surface Dirac bands are at the $Γ-$point, for (Zr,Hf)$_2$Te$_2$(P,As) there are Dirac cones of strong topological character around both the $\bar Γ$- and $\bar {M}$-points which are above and below the Fermi energy, respectively. For Ti$_2$Te$_2$P the surface state is predicted to exist only around the $\bar {M}$-point. In agreement with these predictions, the surface states that are located below the Fermi energy are observed by angle resolved photoemission spectroscopy measurements, revealing that they coexist with the bulk metallic state. Thus, this family of materials provides a foundation upon which to develop novel phenomena that exploit both the bulk and surface states (e.g., topological superconductivity).

cond-mat.str-el↗

arXiv2017

On the possibility to detect multipolar order in URu$_2$Si$_2$ by the electric quadrupolar transition of resonant elastic X-ray scattering

Resonant elastic X-ray scattering is a powerful technique for measuring multipolar order parameters. In this paper, we theoretically and experimentally study the possibility of using this technique to detect the proposed multipolar order parameters in URu$_2$Si$_2$ at the U-$L_{3}$ edge with the electric quadrupolar transition. Based on an atomic model, we calculate the azimuthal dependence of the quadrupolar transition at the U-$L_{3}$ edge. The results illustrate the potential of this technique for distinguishing different multipolar order parameters. We then perform experiments on ultra-clean single crystals of URu$_2$Si$_2$ at the U-$L_{3}$ edge to search for the predicted signal, but do not detect any indications of multipolar moments within the experimental uncertainty. We theoretically estimate the orders of magnitude of the cross-section and the expected count rate of the quadrupolar transition and compare them to the dipolar transitions at the U-$M_4$ and U-$L_3$ edges, clarifying the difficulty in detecting higher order multipolar order parameters in URu$_2$Si$_2$ in the current experimental setup.

cond-mat.str-el↗

arXiv2017

Possible devil's staircase in the Kondo lattice CeSbSe

The temperature ($T$) - magnetic field ($H$) phase diagram for the tetragonal layered compound CeSbSe, is determined from magnetization, specific heat, and electrical resistivity measurements. This system exhibits complex magnetic ordering at $T_{\rm{M}}$ $=$ 3 K and the application of a magnetic field results in a cascade of magnetically ordered states for $H$ $\lesssim$ 1.8 T which are characterized by fractional integer size steps: i.e., a possible Devil's staircase is observed. Electrical transport measurements show a weak temperature dependence and large residual resistivity which suggest a small charge carrier density and strong scattering from the $f$-moments. These features reveal Kondo lattice behavior where the $f$-moments are incompletely screened, resulting in a fine balanced magnetic interaction between different Ce neighbors that is mediated by the RKKY interaction. This produces the nearly degenerate magnetically ordered states that are accessed under an applied magnetic field.

cond-mat.str-el↗