# YbV3Sb4 and EuV3Sb4, vanadium-based kagome metals with Yb2+ and Eu2+ zig-zag
chains

Brenden R. Ortiz<EMAIL_ADDRESS>Materials Department, University of
California Santa Barbara, Santa Barbara, CA 93106, USA  Ganesh Pokharel
Materials Department, University of California Santa Barbara, Santa Barbara,
CA 93106, USA  Malia Gundayao  Materials Department, University of California
Santa Barbara, Santa Barbara, CA 93106, USA  Hong Li  Department of Physics,
Boston College, Chestnut Hill, MA 02467, USA  Farnaz Kaboudvand  Materials
Department, University of California Santa Barbara, Santa Barbara, CA 93106,
USA  Linus Kautzsch  Materials Department, University of California Santa
Barbara, Santa Barbara, CA 93106, USA  Suchismita Sarker  CHESS, Cornell
University, Ithaca, NY, 14853, United States  Jacob P. C. Ruff  CHESS, Cornell
University, Ithaca, NY, 14853, United States  Tom Hogan  Quantum Design, Inc.,
San Diego, CA 92121, USA  Steven J. Gomez Alvarado  Materials Department,
University of California Santa Barbara, Santa Barbara, CA 93106, USA  Paul M.
Sarte  Materials Department, University of California Santa Barbara, Santa
Barbara, CA 93106, USA  Guang Wu  Department of Chemistry and Biochemistry,
University of California Santa Barbara, Santa Barbara, CA 93106, USA  Tara
Braden  Physics Department, Colorado School of Mines, Golden, CO 80401, USA
Ram Seshadri  Materials Department, University of California Santa Barbara,
Santa Barbara, CA 93106, USA  Eric S. Toberer  Physics Department, Colorado
School of Mines, Golden, CO 80401, USA  Ilija Zeljkovic  Department of
Physics, Boston College, Chestnut Hill, MA 02467, USA  Stephen D. Wilson
<EMAIL_ADDRESS>Materials Department, University of California Santa
Barbara, Santa Barbara, CA 93106, USA

###### Abstract

Here we present YbV3Sb4 and EuV3Sb4, two new compounds exhibiting slightly
distorted vanadium-based kagome nets interleaved with zig-zag chains of
divalent Yb2+ and Eu2+ ions. Single crystal growth methods are reported
alongside magnetic, electronic, and heat capacity measurements. YbV3Sb4 is a
nonmagnetic metal with no collective phase transitions observed between 60 mK
and 300 K. Conversely, EuV3Sb4 is a magnetic kagome metal exhibiting easy-
plane ferromagnetic-like order below $T_{\text{C}}=32$ K with hints of
modulated spin texture under low field. Our discovery of YbV3Sb4 and EuV3Sb4
demonstrate another direction for the discovery and development of vanadium-
based kagome metals while incorporating the chemical and magnetic degrees of
freedom offered by a rare-earth sublattice.

††preprint: APS/123-QED

##  I Introduction

Research into layered kagome metals has accelerated dramatically in the past
few years, fueled in part by the discovery of the AV3Sb5 kagome
superconductors [1, 2, 3, 4] and the continued exploration of the AM6X6 phase
space [5, 6, 7, 8, 6, 9, 10, 11, 12, 13, 14, 15, 16]. Metals based on kagome
networks derive much of their fundamental interest via their potential to
realize an electronic structure replete with Dirac points, flat bands, and Van
Hove singularities [17, 18, 19, 20]. Depending on the alignment of the Fermi
level with the aforementioned features, a wide array of electronic
instabilities ranging from bond density wave order [18, 21], charge
fractionalization [22, 23], charge-density waves [24, 25, 19], and
superconductivity [18, 26, 19] are possible. Developing new compounds built
from kagome networks with variable band fillings and the ability to engineer
additional interactions, such as magnetic order, remains an ongoing challenge.

The nonmagnetic kagome network of vanadium ions filled near the Van Hove
points in the AV3Sb5 (A: K, Rb, Cs) class of kagome superconductors display a
unique intertwining of charge density wave (CDW) order and a superconducting
ground state [2, 3, 4, 27, 28, 29, 30, 31]. This renders them excellent
platforms for exploring the electronic interactions on a kagome lattice.
Interfacing the nonmagnetic vanadium kagome network with magnetic interstitial
ions in a manner similar to LnV6Sn6 compounds remains a challenge. Addressing
this challenge is motivated by the promise of engineering magnetic order
within itinerant kagome metals to stabilize an interesting range of magnetic
and electronic instabilities. These include, for instance, tunable Chern gaps
[16, 12, 15, 8], anomalous Hall effects [6, 32, 33], and spin-charge coupled
density waves [34].

One promising material class that deserves mention are metals of the form
AM3X4. These compounds exhibit slightly distorted M-based kagome sublattices
with zig-zag chains of A-site ions. The potential for magnetism through choice
of the A-site provides a degree of chemical flexibility analogous to the AM6X6
family. The AM3X4 structures known to date are limited almost exclusively as
ATi3Bi4 with (A3+: La3+, Ce3+, Sm3+) [35, 36]. The singular known exception is
CaV3Sb4, where the Ti–Bi sublattice is swapped for V–Sb. This substitution is
intriguing, and mirrors the reverse case of the AV3Sb5 family, wherein the
Ti–Bi variants RbTi3Bi5 [37] and CsTi3Bi5 [37, 38] arose following the initial
discovery of the V–Sb series [1]. The overall structure appears somewhat
tolerant of several different A-site valences, with both the trivalent rare-
earths and divalent alkali-earth compounds (e.g. CaTi3Bi4, CaV3Sb4) known.
However, to date, CaV3Sb4 remains the only known V–Sb AM3X4 [36]. Note that
other non-stoichiometric antimonides like NdTi3(Sb0.9Sn0.1)4 are known [39],
and while these compounds do not exist at the purely antimonide limit, they
suggest a degree of chemical tunability in AM3X4 metals. Regardless, little is
known about the physical properties of these materials.

In this work, we present the single crystal growth and characterization of two
new AM3X4 kagome metals: YbV3Sb4 and EuV3Sb4. Similar to the case of CaV3Sb4,
both YbV3Sb4 and EuV3Sb4 are instances of divalent Yb2+ and Eu2+ $A$-site
cations. Analogous to the AV3Sb5 and LnV6Sn6 compounds, the vanadium
sublattice appears nonmagnetic, leaving the magnetism to be dominated by the
rare-earth element. As expected of Yb2+, crystals of YbV3Sb4 are Pauli
paramagnetic metals with no clear thermodynamic phase transitions from 60 mK
to 300 K. In contrast, EuV3Sb4 exhibits a low-field ferromagnetic transition
with a $T_{\text{C}}$ of approximately 32 K. The magnetism presents with an
easy-plane anisotropy, and the susceptibility when $H\parallel c$ suggests a
more complex magnetic ground state in the zero field limit (e.g. moment
canting, helical states). Curie-Weiss and magnetic heat capacity analyses are
consistent with magnetism originating from the full moment of $S=7/2$ Eu2+
ions. Together our results continue to expand upon the known kagome metals,
introducing new routes forward in the realization of complex electronic and
magnetic ground states on kagome platforms.

##  II Experimental Methods

###  II.1 Single Crystal Synthesis

YbV3Sb4 single crystals are grown through a self-flux method. Note that all
handling of raw reagents, powders, and precursors was done within an argon-
filled glove box with oxygen and water levels $<$1 ppm. A stoichiometric
mixture is formed by combining Yb (rod, Alfa 99.9%), V (powder, Sigma 99.9%)
and Sb (shot, Alfa 99.999%) at a 1:3:4 ratio into tungsten carbide ball-mill
vials. The vials are sealed under argon and milled for 3 hr in a SPEX 8000D
dual mixer/mill. Note that the milling proceeds in three 1 hr segments with
intermediate (hand) grinding steps to dislodge agglomerates of Yb metal. As-
received Yb rod was mechanically cleaned of any residual oxides before it was
cleaved into $<$1 mm chunks. The as-received V powder was also cleaned through
sonication in a mixture of EtOH and HCl.

The resulting precursor powder can be annealed at 650°C to produce phase-pure
polycrystalline powders of YbV3Sb4 (see ESI)[40]. While we present some powder
data in the ESI, note that all characterization within the main body of this
manuscript was performed on single crystals. For single crystal methods, the
as-milled precursor is loaded in 2 mL high-density alumina (Coorstek)
crucibles and sealed in carbon-coated fused silica ampules under $\sim$0.7 atm
of argon. The samples are heated to 1050°C at a rate of 200°C/hr before
cooling to 800°C at a rate of 1-2°C/hr. Samples are then allowed to cool to
room temperature before extracting single crystals mechanically from the
solidified flux. Crystals are thin (100–500 $\upmu$m) hexagonal flakes with
side lengths approximately 1–2 mm. The samples are a lustrous silver and can
be exfoliated with some slight difficulty.

It is worth noting that the compound does not melt congruently. The heating of
the precursor phase above 1050°C causes the YbV3Sb4 powder to undergo
peritectic decomposition. The resulting liquid phase subsequently acts as the
flux upon cooling. Attempts to grow YbV3Sb4 through other fluxes (e.g. Sb, Bi,
Sn, Pb) have not yet been successful. Further optimization of the flux
conditions is still underway.

Single crystals of EuV3Sb5 are grown through a bismuth flux. A stoichiometric
mixture of Eu (rod, Alfa 99.9%), V (powder, Sigma 99.9%), and Sb (shot, Alfa
99.999%) is mixed with Bi (rod, Alfa 99.9999%) at a 1:3:4:40 ratio. The
elemental reagents are loaded in 2 mL Canfield crucibles fitted with a catch
crucible and a porous frit. The crucibles are sealed in carbon-coated fused
silica ampules under $\sim$0.7 atm of argon. The samples are heated to 1000°C
at a rate of 200°C/hr before cooling to 400°C at a rate of 2°C/hr. The samples
are centrifuged at 400°C to remove excess bismuth. Crystals are thin
(10-50$\upmu$m) hexagonal flakes with side lengths approximately 100-250
$\upmu$m. The samples have a brilliant silver luster.

###  II.2 X-ray Diffraction

The structure of YbV3Sb4 was first solved using powder diffraction data
collected at the Advanced Photon Source (11-BM) with 0.412619 Å radiation (see
ESI [40]). Powders were sealed in Kapton capillaries after being diluted at a
molar ratio of 1:4 with amorphous SiO2 to reduce X-ray absorption. Structural
solution was performed using charge flipping methods and the Topas V6 software
package [41, 42, 43, 44]. The composition of single crystals measured in this
work were characterized using Energy Dispersive Spectroscopy (EDS) using a
Hitachi TM4000 electron microscope equipped with an integrated Oxford
Instruments EDS probe. Samples are stoichiometric within the error of
unstandardized EDS.

Additional single-crystal measurements for both YbV3Sb4 and EuV3Sb4 were
performed on a Bruker KAPPA APEX II diffractometer equipped with an APEX II
CCD detector using a TRIUMPH monochromator with a Mo K$\alpha$ X-ray source
($\lambda$ = 0.71073 Å). Furthermore, synchrotron X-ray diffraction
experiments were carried out at the QM2 beam line at the Cornell High Energy
Synchrotron Source (CHESS). The incident X-ray wavelength of $\lambda$ =
0.41328 Å was selected using a double-bounce diamond monochromator. A
cryostream of flowing helium was used for temperature control. The diffraction
experiment was conducted in transmission geometry using a 6-megapixel photon-
counting pixel-array detector with a silicon sensor layer. Data was collected
in full 360° rotations with a step size of 0.1°. Scattering planes in
reciprocal space were visualized using the NeXpy software package. The
diffraction data was indexed and integrated using the APEX3 software package
including absorption and extinction corrections. Crystallographic structural
solutions were determined using the SHELX software package [45].

###  II.3 Computational Modeling

As no prior electronic structure calculations are reported, we ventured to
provide basic information about the electronic structure of the nonmagnetic
AM3X4 lattice. First-principles calculations based on density functional
theory (DFT) within the Vienna ab initio Simulation Package (VASP) were
performed [46, 47]. The projector augmented wave (PAW) method [48, 49] was
employed and relaxations of the ionic positions were conducted using an energy
cutoff of 520 eV. Reciprocal space k-point meshes were automatically generated
at a density of 40 Å-1 along each reciprocal lattice vector. The band
structure was calculated across a subset of the high symmetry points as
defined by Setyawan and Curtarolo [50].

###  II.4 Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) measurements were performed using a
customized Unisoku USM1300 microscope. Single-crystals of YbV3Sb4 were cleaved
at 20 K and immediately inserted into the 4.5 K STM head. Spectroscopic
measurements were made using a standard lock-in technique with 910 Hz
frequency and bias excitation as detailed in figure captions. STM tips used
were home-made chemically-etched tungsten tips, annealed in a UHV chamber to a
bright orange color prior to the experiment. We apply the Lawler-Fujita drift-
correction algorithm to all our data to align the atomic Bragg peaks onto
single pixels.

###  II.5 Bulk Characterization

Magnetization measurements of both YbV3Sb4 and EuV3Sb4 single crystals were
performed on a 7 T Quantum Design Magnetic Property Measurement System (MPMS3)
SQUID magnetometer in vibrating-sample magnetometry (VSM) mode. Samples were
mounted to quartz paddles using a small quantity of GE varnish. Two
measurements were performed for both samples, orienting the c-axis of the
single-crystals perpendicular and parallel to the applied magnetic field. For
consistency, the same crystal was used for both field orientations.

Electronic resistivity measurements on YbV3Sb4 were performed on a Quantum
Design 9 T Dynacool Physical Property Measurement System (PPMS). Single
crystals were mounted to the sample stage using a small quantity of cigarette
paper and GE varnish to ensure electrical isolation and thermal contact.
Samples were then exfoliated and contacts established using silver paint
(DuPont cp4929N-100) and gold wire (Alfa, 0.05 mm Premion 99.995%). We used a
dc current of 1 mA to measure the resistivity under zero-field conditions.

Heat capacity measurements on both YbV3Sb4 and EuV3Sb4 single crystals between
300 K and 1.8 K were performed on a Quantum Design 9 T Dynacool Physical
Property Measurement System (PPMS) equipped with the heat capacity option.
Measurements were repeated three times at each temperature and averaged
together. Further measurements on YbV3Sb4 were performed on a Quantum Design
14 T Dynacool Physical Property Measurement System (PPMS) equipped with the
dilution refrigerator (DR) option. Normalization of the DR heat capacity data
was done through a scaling to higher temperature YbV3Sb4 data using the
crossover from 2 K to 4 K.

##  III Results & Discussion

###  III.1 Crystalline and Electronic Structure

Figure 1: YbV3Sb4 and EuV3Sb4 (a) are orthorhombic (Fmmm) compounds that
exhibit a zig-zag sublattice of Ln ions (b) interwoven with staggered layers
of V-based kagome networks (c). Consistent with the orthorhombic structure,
the kagome networks are slightly distorted (d). The distortion is relatively
minor, and all V-atoms are within 0.1 Å of the idealized kagome lattice. Some
interatomic distances of interest are highlighted on the graphic. Figure 2:
(a) The electronic structure of Fmmm YbV3Sb4 calculated over an abbreviated
portion of the face-centered (type-1) orthorhombic high-symmetry points shows
Dirac-like and flatband-like features consistent with the vanadium kagome
network. Most cleavage surfaces exhibit Yb–Sb termination, resulting in the
scanning tunneling microscopy (STM) topograph and quasiparticle interference
(QPI) patterns shown in (b,c). Several energy- and momentum-dependent line
cuts through the QPI patterns (red, green blue) are shown (d) with bands
highlighted via colored arrows. Though less common, crystals occasionally
cleave along the V–Sb layers, resulting in the STM topograph shown in (e) and
the corresponding QPI (f).

YbV3Sb4 and EuV3Sb4 are new members of a relatively small class of compounds
AM3X4. The majority of the members of this family consist of mildly distorted
Ti–Bi networks with rare-earth cations [36, 51, 35], though divalent calcium
compounds (e.g. CaV3Sb4, CaTi3Bi4) were recently reported as well [36]. Unlike
the known Ti–Bi rare-earth compounds, which can contain trivalent ions, both
YbV3Sb4 and EuV3Sb4 form with rare-earth divalent A-site sublattices of Yb2+
and Eu2+. Figure 1(a) illustrates the overall crystal structure of YbV3Sb4 and
EuV3Sb4. We have chosen to omit the V–Sb and Ln–Sb bonds to highlight the Ln
and V sublattices. Figure 1(b) shows the Ln–Ln distances, which can be
visualized as zig-zag chains running in the a direction. The Ln–Ln distance
along the chain ($\sim$3.9 Å) is substantially closer than the nearest
neighbor Ln-Ln interchain distance ($\sim$5.6 Å).

Unlike the smaller prototype structures AM3X5 (P6/mmm) and AM6X6 (P6/mmm)
compounds, the AM3X4 (Fmmm) compounds have four kagome layers in each unit
cell. Figure 1(c) highlights the different layers, which are offset from one
another. For visual clarity we have left the nearest-neighbor Sb atoms
displayed in Figure 1(c), though they are not formally within the kagome
plane. The case is analogous to the AM6X6 (HfFe6Ge6) prototype, where the X
atom is displaced slightly above/below the kagome sheet. In fact, the analogy
with the AM6X6 compounds goes a bit further, as the AM3X4 structure actually
contains elements of the AM6X6 motif. If we consider stacking along the
c-axis, the AM3X4 structure consists of
$X_{4}$–$M_{3}$–$AX_{2}$–[$AX_{2}$–$M_{3}$–$X_{4}$–$M_{3}$–$AX_{2}$]–$AX_{2}$–$AX_{2}$–$M_{3}$–$X_{4}$
layers. The bracketed segment of the stacking represents the same stacking as
the HfFe6Ge6 prototype structure. It has been noted that the other layers of
AM3X4 compounds contain stacking elements drawing from many prototypical
kagome and quasi-2D compounds (e.g. CeCo3B2, Cs2Pt3S4, Zr4Al3, and CrSi2)
[36].

Figure 3: Bulk electronic properties characterization on single crystals of
YbV3Sb4. Temperature-dependent magnetization data are plotted in (a) with both
orientations (c parallel and perpendicular to H) revealing largely
temperature-independent Pauli paramagnetism with a weak Curie tail from
impurity spins. Isothermal magnetization (a,inset) shows no clear saturation
and a magnitude (10-2 $\upmu_{\text{B}}$ per Yb) consistent with a small
fraction of impurity spins. Zero-field resistivity measurements plotted in (b)
confirm the metallic nature of YbV3Sb4 crystals. A simple quadratic fit to the
low-temperature data is shown. Heat capacity results are plotted in (c). A
nuclear Schottky anomaly is noted below 0.1 K. A Sommerfeld fit (red) is shown
over a limited temperature range (blue) that avoids the Schottky anomaly.

Like other known AM3X4 compounds, the kagome layers in YbV3Sb4 and EuV3Sb4 are
slightly distorted (see Figure 1(d)). The planes are slightly buckled and the
V–V distances are not identical. The distortions are small, however, and the
vanadium atoms do not deviate more than 0.1 Å from their idealized kagome
positions. Corresponding CIF files have been included in the ESI [40].

Figure 2(a) shows an abbreviated trace of the electronic structure near the
Fermi level. An inset of the orthorhombic Brillouin zone showing a subset of
the high-symmetry points is shown in the bottom right. Owing to the more
complex structure, the band diagram near the Fermi level is substantially more
involved than the AM3X5 and AM6X6 phases; however, a somewhat flat band
feature from Y–$\Gamma$ seems to be close to the Fermi level. This is
potentially accessible to future ARPES or scanning tunneling spectroscopy
studies.

STM measurements were used to screen YbV3Sb4 crystals for any short-range
charge correlations not captured in the average structure. Crystals of YbV3Sb4
cleave in such a way that both the Yb–Sb and V–Sb terminations are available.
Figure 2(b) shows an STM topograph across a Yb–Sb cleavage plane. The lattice
parameters observed by STM (a=5.70 Å and b=9.87 Å) agree well with those
refined from diffraction (a=5.62 Å and b=9.82 Å). The associated Fourier
transform and quasiparticle interference (QPI) patterns are also shown in
Figure 2(b-c), where the atomic Bragg peaks have been highlighted with black
circles. The QPI is two-fold symmetric by inspection, consistent with the
orthorhombic nature of YbV3Sb4. Several energy/momentum line cuts are
highlighted on Figure 2(c) in red, green, and blue. These cuts correspond to
the energy- and momentum-dependent QPI linecuts shown in Figure 2(d). Bands
highlighted by colored arrows correspond directly to those highlighted in
Figure 2(c).

While substantially rarer, the V–Sb termination is also observed by STM. The
corresponding topograph is shown in Figure 2(e), and the QPI for the vanadium-
terminated surface is shown in Figure 2(f). The atomic Bragg peaks are
outlined with black circles and the two-fold axis is highlighted in orange. A
energy/momentum line cut through Figure 2(f) is shown in the ESI [40]. Due to
the relatively complex electronic structure, more detailed calculations will
be required to make direct comparisons between the bands extracted from the
STM QPI linecuts and our DFT calculations.

###  III.2 YbV3Sb4

Figure 4: Here we demonstrate a suite of magnetization data collected on
single crystals of EuV3Sb4. Temperature-dependent susceptibility collected on
a crystal oriented with H$\parallel$c (a) and H$\perp$c (b) both exhibit a
sharp upturn in magnetization, consistent with ferromagnetic-like ordering
near $T_{\text{C}}=32$ K. Note that an additional cusp and brief downturn in
the susceptibility is noted when H$\parallel$c. Field-cooled measurements over
a range of applied fields are plotted over the transition for H$\parallel$c
(c) and H$\perp$c (d). Isothermal ZFC magnetization data are plotted in (e)
highlighting rapid moment saturation and a weak coercivity (inset). Curie-
Weiss analysis of the low-field susceptibility for both field orientations are
plotted in (f).

We now turn to the characterization of the bulk electronic properties of
YbV3Sb4 single crystals. Figure 3(a) plots temperature-, field-, and
orientation-dependent magnetization data. Evidenced by the extremely weak
susceptibility (10-6 emu Oe-1 mol-1), YbV3Sb4 is a Pauli paramagnet with no
signatures of bulk, local moments. No significant qualitative difference is
noted when the crystal is mounted with the c-axis parallel or perpendicular to
the magnetic field. Similarly, isothermal magnetization on single crystals
(Figure 3(a,inset)) shows no saturation. Please note that the scale of the
inset plot is on the order of 10-2 $\upmu_{\text{B}}$ per Yb, consistent with
the polarization of impurity spins.

Temperature-dependent resistivity data showing metallic transport in YbV3Sb4
are plotted in Figure 3(b). The residual resistivity is approximately 11
$\upmu\Omega$ cm, though the residual resistivity ratio (RRR) is relatively
low ($\sim$4). As a result, no signatures of quantum oscillations in the
magnetoresistance were observed with I${}_{\text{ab}}$ and H$\parallel$c. The
zero-field, low temperature ($<50$ K) resistivity is well modeled via Fermi
liquid behavior and a simple quadratic fit $\rho=\rho_{0}+aT^{2}$ with
$\rho_{0}=11.1~{}\upmu\Omega$ cm and $a=1.09$ n$\Omega$ cm K-2.

No clear phase transitions are indicated in the magnetization or resistivity
results on YbV3Sb4 down to 2 K. Zero-field heat capacity were also collected
from 300 K down to 60 mK. Figure 3(c) plots the resulting $C_{\text{p}}/T$
data for a 1.1 mg single crystal. A feature consistent with a nuclear Schottky
anomaly emerges around 0.1 K, and another small feature is noted at 0.8 K (see
ESI) [40]. The magnitude of the 0.8 K feature suggests this is due to an
impurity effect, such as freezing of the paramagnetic impurities resolved in
magnetization measurements. To avoid contributions from the nuclear Schottky
anomaly, a Sommerfeld model was fit to a limited temperature range (4 K to 10
K). The resulting least-squares fit yields the parameters $\gamma=20.9$ mJ
mol-1 K-1 and $A=0.73$ mJ mol-1 K-3.

All experimental data characterizing YbV3Sb4 support the classification as a
nonmagnetic kagome metal. The structure is consistent with a Yb2+ rare-earth
sublattice, and no bulk magnetic, electronic, or structural instabilities are
noted from 60 mK to 300 K. These results resemble those seen in CaV3Sb4 and
CaTi3Bi4 [36]. As such, YbV3Sb4 provides an excellent comparison and
nonmagnetic standard for EuV3Sb4.

###  III.3 EuV3Sb4

Whereas Yb2+ results in a nonmagnetic rare-earth sublattice, divalent Eu2+ is
isoelectronic to Gd3+ ($S=7/2$) and should exhibit a magnetically ordered
ground state. Figure 4 shows the temperature-, field-, and orientation-
dependent magnetization data from a 10$~{}\upmu$g single crystal of EuV3Sb4.
Looking first at the temperature-dependent susceptibility, Figure 4(a,b) plots
the low-field susceptibility of EuV3Sb4 under an applied field of 100 Oe
oriented with H$\parallel$c and H$\perp$c, respectively.

Both orientations exhibit a dramatic increase in the magnetization near 36 K.
With H$\perp$c, a rapid polarization is observed that quickly saturates and
suggests a predominantly ferromagnetic transition. Upon changing the field
orientation such that H$\parallel$c, two main differences arise: (1) the
magnitude of the susceptibility is dramatically reduced, demonstrating an
easy-plane anisotropy, and (2) a low-field, sharp cusp appears near $T_{C}$
before continuing toward saturation. Similar low field cusps have been
observed in other magnetic kagome metals such as GdV6Sn6, suggesting a
modulated magnetic ground state [7].

Figure 5: Consistent with magnetization results, heat capacity measurements on
single crystals of EuV3Sb4 clearly show a lambda-like anomaly at
$T_{\text{C}}=32$ K. The transition is field-dependent (a,inset), broadening
and weakly shifting towards lower temperatures. The transition is more obvious
in $C_{\text{p}}/T$ (b), where direct subtraction of the scaled YbV3Sb4
nonmagnetic lattice reference yields the magnetic contribution to the total
heat capacity. Integration of the magnetic heat capacity results in the
magnetic entropy (c), yielding 97% of $R\ln 8$, consistent with divalent Eu2+.

Figure 4(c,d) reports the field-dependent and temperature-dependent
magnetization for both field orientations shown in Figure 4(a,b). The low-
field cusp in the magnetization for H$\parallel$c persists for fields up to
approximately 500 Oe. The isothermal magnetization (Figure 4(e)) demonstrates
that the magnetic response of H$\perp$c saturates much quicker ($\sim$500 Oe)
than H$\parallel$c ($\sim$2500 Oe), consistent with an easy-plane anisotropy.
Figure 4(e,inset) highlights the clear (albeit subtle) hysteresis in the 1.8 K
isothermal magnetization, consistent with the FC/ZFC irreversibility plotted
in Figure 4(a,b). Curiously, at high fields, both curves saturate near
5$\upmu_{\text{B}}$, which is well below the expected value of 7
$\upmu_{\text{B}}$ for Eu2+ assuming $S=7/2$ and $g=2$. Though Figure 4(f)
shows an abbreviated field range, no metamagnetic transitions were observed up
to 7 T.

A Curie-Weiss analysis (Figure 4(f)) of the low-field susceptibility in both
field orientations produces nearly identical results. Both orientations
exhibit $\uptheta_{\text{CW}}$ of approximately +36 K, consistent with both
the heat capacity peak (32 K, discussed later) and the first derivative of the
magnetization curve (33 K). The analysis supports predominantly ferromagnetic
correlations and minimal frustration (recall that the Ln-sublattice exhibits a
zig-zag motif independent of the vanadium kagome lattice). The effective
paramagnetic moment is 7.4 $\upmu_{\text{B}}$, which is close to that expected
for Eu2+ (7.9 $\upmu_{\text{B}}$).

It is worth noting that all results in Figure 4 were collected on the same
crystal. We suspect that the local moment obtained via the Curie-Weiss
analysis likely deviates from the full Eu2+ moment due to uncertainty in
determining the sample mass. An approximate 13% error in mass would account
for the deviation in the Curie-Weiss analysis, not outside expectations
considering the small mass of the single crystal (10 $\upmu$g). However, the
mass error still fails to account for the missing moment in the isothermal
magnetization saturated state. Even compensated for the mass error, we
approximate that $\approx$1 $\upmu_{\text{B}}$ remains unaccounted for, which
may hint at unforeseen moment polarization on the vanadium sites or
significant dynamic effects; however, future measurements on larger volume
crystals will be required to completely rule out unaccounted massing errors.

Figure 5 plots the heat capacity data for EuV3Sb4 utilizing YbV3Sb4 as a
nonmagnetic phonon reference. A cluster of crystals weighing 0.5 mg were used
for the heat capacity analysis. Figure 5(a) shows heat capacity data
overplotted for both compounds, including a small scaling factor that
normalized the high-temperature heat capacity signals $>$100 K together. A
clear anomaly can be seen in the heat capacity for EuV3Sb4 at
$T_{\text{C}}=32$ K, in good agreement with the magnetization results. We
briefly highlight the field-dependence of the heat capacity anomaly in EuV3Sb5
when H$\parallel$c, demonstrating that the transition broadens and is slightly
depressed in temperature with increasing fields.

Figure 5(b) shows the corresponding $C_{\text{p}}/T$ data for YbV3Sb4 and
EuV3Sb4. Neglecting data near the nuclear Schottky anomaly in YbV3Sb4, direct
subtraction of the nonmagnetic YbV3Sb4 lattice reference isolates the magnetic
entropy from the EuV3Sb4 heat capacity data. Integration of $C_{\text{p,mag}}$
(see ESI) from 1.8 K to 100 K is plotted in Figure 5(c), and the recovered
entropy plateaus near 16.8 J mol-1 K-1. This is 97% of the entropy expected
from Eu2+ ions in the fully ordered state ($R\ln 8$).

In aggregate, the data suggest that EuV3Sb4 adopts a noncollinear or modulated
(e.g. helical or cycloidal) magnetic ground state in the zero field limit.
This may account for the cusp in the magnetization immediately below
$T_{\text{C}}$ observed in the hard-axis susceptibility data. However, larger
samples and future scattering experiments (e.g., resonant x-ray or neutron
scattering) will be required to fully explore this possibility.

##  IV Conclusion

We presented two new vanadium-based kagome materials, YbV3Sb4 and EuV3Sb4.
These materials are members of a larger AM3X4 family, which have generally
consisted of Ti–Bi based kagome metals with rare-earth ions. Whereas the
vanadium kagome sublattice appears nonmagnetic, the rare-earth sublattice
creates zig-zag chains that form between kagome planes and introduce the
potential for magnetic degrees of freedom. YbV3Sb4 forms as a nonmagnetic
kagome metal with no bulk phase transitions down to 60 mK, while EuV3Sb4
realizes a ferromagnetic-like ground state below $T_{\text{C}}=32$ K. The
ordered state has an easy-plane anisotropy that shows hints of a canted or
modulated order in the zero-field limit, motivating future exploration of its
magnetic ground state. This work establishes the AV3Sb4 class of kagome metals
as new platforms for proximitizing A-site tuned magnetic order with the
topologically nontrivial features endemic to kagome band structures.

##  V Acknowledgments

This work was supported by the US Department of Energy (DOE), Office of Basic
Energy Sciences, Division of Materials Sciences and Engineering under Grant
No. DE-SC0020305 (B.R.O., G.P., S.D.W.). B.R.O. and P.M.S. acknowledge
financial support from the University of California, Santa Barbara, through
the Elings Fellowship. A portion of this research, including the undergraduate
internship program for M.G., F.K., and R.S. was supported by the National
Science Foundation (NSF) through Enabling Quantum Leap: Convergent Accelerated
Discovery Foundries for Quantum Materials Science, Engineering and Information
(Q-AMASE-i): Quantum Foundry at UC Santa Barbara (DMR-1906325). I. Z.
acknowledges support via NSF grant DMR 2216080. S.J.G.A. acknowledges
financial support from the National Science Foundation Graduate Research
Fellowship under Grant No. 1650114. A portion of this research, including
support for E.S.T. and T.B., was supported by the National Science Foundation
(NSF) under Grants No. DMR-1950924 and No. DMR-1555340, respectively. This
work is based upon research conducted at the Center for High Energy X-ray
Sciences (CHEXS), which is supported by the National Science Foundation under
Award DMR-1829070. Use of the Advanced Photon Source at Argonne National
Laboratory was supported by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The
research made use of the shared experimental facilities of the NSF Materials
Research Science and Engineering Center at UC Santa Barbara (Grant No.
DMR-1720256).

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