Semiconductor-insulator transition in a YbB6 nanowire with boron vacancy

Article abstract of DOI:10.1016/j.jssc.2018.03.029

In this paper, we report the study of transport and magnetic properties of ytterbium hexaboride (YbB₆) nanowires grown by a low trigger-temperature (200–240 °C) solid state method. The temperature dependence of resistivity shows that the YbB

Full text of DOI:10.1016/j.jssc.2018.03.029

Journal of Solid State Chemistry 262 (2018) 244–250
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Journal of Solid State Chemistry
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Semiconductor-insulator transition in a YbB6 nanowire with boron vacancy
⁎
Wei Hana, Zhen Wanga, Qidong Lia, Xin Liana, Xudong Liua, Qinghua Fana, Yanming Zhaoa,b,
a
Department of Physics, South China University of Technology, Guangzhou 510641, China
State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510641, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Ytterbium hexaboride (YbB6)
Nanowire
Magnetoresistance
Electronic transport
Semiconductor-insulator transition (SIT)
In this paper, we report the study of transport and magnetic properties of ytterbium hexaboride (YbB6)
nanowires grown by a low trigger-temperature (200–240 °C) solid state method. The temperature dependence
of resistivity shows that the YbB6 nanowire undergoes a semiconductor-insulator transition (SIT) below 20 K
with an activation energy ΔE of 1 meV. The value of ρ at 2 K reaches 49 times the value of ρ at 300 K (ρ2 K/ρ300 K
= 49). The observed non-saturating magnetoresistance (MR) has a linear relationship with B2. The anomalous
electronic transport in the YbB6 nanowire can be explained by the mixed valence of Yb ions due to the boron
deficiency supporting by the X-ray photoelectron spectroscopy (XPS) and paramagnetic magnetization.
1. Introduction
our knowledge, no results on the preparation and transport properties of
YbB6 nanowires are reported yet.
Recently, topological insulators (TIs) attracted much attention
because of their nontrivial properties of coexisting insulating bulk and
topologically protected conducting surface [1]. SmB6 is considered as the
first candidate material for a topological Kondo insulators (TKI) based
on many experimental measurements [2–5]. As a sister compound of
SmB6, YbB6 has the same CsCl-type crystal structure as SmB6, but shows
very different electronic structures due to the fact that Yb has a fully
filled 4f shell whereas Sm has a nearly half-filled 4f shell. In most RB6
compounds the R ions are trivalent except to it in SmB6 and YbB6 where
they are reported to show mixed valence. Recently, theorists predicted
that YbB6 is a moderately correlated TI [6]. However, measurements
results of angle-resolved photoemission spectroscopy (ARPES) on YbB6
suggest that its electronic state should originate from the hybridization
between Yb d-orbitals and B p-orbitals [7–9]. Very recent ARPES
experiments demonstrate the non-Kondo non-TI electronic structure
of YbB6 [10]. But the resistance-temperature (R-T) measurements of
YbB6 single crystals demonstrate four different curves from four groups
[3,9,11,12]. Thus, YbB6 becomes a controversial material in the field of
TIs. No matter whether YbB6 is a TI or not, it is still a very promising
material with potential new quantum phenomena, such as pressure-
induced quantum phase transitions in a YbB6 single crystal [12].
However, all the above measurements are carried on the bulk YbB6
single crystals and no measurements on YbB6 nanostructures are
reported. Due to the larger surface-to-bulk ratio, the measurements on
YbB6 nanostructures may present some different results, such as the
enhanced surface conduction in SmB6 films and nanowires [13,14]. To
Metal-insulator transition (MIT) or semiconductor-insulator tran-
sition (SIT) is a transformation from a highly charge conductive state to
another state where the charge conductivity is greatly suppressed when
decreasing the temperature [15]. MIT and SIT were found in many
materials, such as SmB6 [3], VO2 [16], PrRu4P12 [17], and MoS2 [18].
So far, no MIT and SIT are observed in YbB6 crystals, only a small
upturn is seen below 60 K in a YbB6 single crystal [12].
But the preparation of YbB6 crystals is still a challenge, especially YbB6
nanostructures. At present only few methods are reported to prepare YbB6
crystals, including the conventional Al-flux methods using pure Yb and B
elements or borothermal reduction in Yb2O3 at high temperatures of
1300–1500 °C [12]. Furthermore, the porous YbB6 ceramics are also
prepared from Yb2O3 and B4C powders by high-temperature sintering at
1750 °C [19]. All the above methods need high temperatures and no
nanostructures are obtained. Recently, Aprea el al. reported a metathesis
reaction to prepare YbB6 powders using hydrated YbCl3 and MgB2 at
650 °C under vacuum [20]. However, the polycrystalline YbB6 particles by
this method can’t be used for the transport measurements well. Thus, a
simple and effective method to prepare YbB6 single crystals needs to be
developed. To our knowledge, no results on the preparation of YbB6
nanowires are reported yet, although all other RB6 nanowires have been
successfully fabricated [21].
In this paper, we have successfully synthesized high-quality single
crystalline YbB6 nanowires at very low trigger-temperature of 200–
240 °C by a high pressure solid state (HPSS) method with a new
chemical reaction using Yb, H3BO3, Mg and I2 as raw materials. The
⁎
Corresponding author at: Department of Physics, South China University of Technology, Guangzhou 510641, China.
E-mail address: zhaoym@scut.edu.cn (Y. Zhao).
https://doi.org/10.1016/j.jssc.2018.03.029
Received 9 January 2018; Received in revised form 20 March 2018; Accepted 25 March 2018
Available online 27 March 2018
0022-4596/ © 2018 Elsevier Inc. All rights reserved.

W. Han et al.
Journal of Solid State Chemistry 262 (2018) 244–250
trigger-temperature of 200 °C is the lowest temperature of all the
synthetic methods for YbB6 crystals at present. We report the observa-
tion of semiconductor-insulator transition in an individual YbB6
nanowire. The value of ρ at 2 K reaches 49 times the value of ρ at
300 K (ρ2 K/ρ300 K = 49). The observed non-saturating magnetoresis-
tance (MR) has a linear relationship with B2. The anomalous electronic
transport in the YbB6 nanowire can be explained by the mixed valence
of Yb ions due to the boron deficiency supporting by the X-ray
photoelectron spectroscopy (XPS) and paramagnetic magnetization.
2. Experimental methods
2.1. Synthesis of YbB6 nanowires
The synthesis of YbB6 nanowires is based on the following chemical
reaction:
Yb(s) + 6H3BO3(s) + 10Mg(s) + I2(s) = YbB6(s) + 9MgO(s) + MgI2(s) +
9H2O(g)
In a typical synthesis, 0.3 g of Yb powders, 1.0 g of H3BO3, 1.0 g of
Mg and 1.9 g of I2 were mixed in a glove box (filled with Ar) and put
into a stainless steel autoclave of 30 mL capacity. The autoclave was
tightly sealed and then kept at 200–240 °C for 12 h in a muffle furnace.
After the reaction the temperature of the autoclave was naturally cooled
down to room temperature. The obtained powders were treated by hot
hydrochloric acid (80 °C) for 2 h to remove by-products. The suspen-
sion was filtered and then washed with distilled water for several times.
After drying at 70 °C for 3 h, the final black products were obtained.
Fig. 1. (a) The Rietveld refinement result of the YbB6 crystals; (b) The estimated value of
boron deficiency x in YbB6-x by the Vegard's law.
2.2. Characterization
The YbB6 nanowires were characterized and analyzed by X-ray
diffraction (XRD; TD-3500), Micro-Raman scattering spectroscopy
(Renishaw RM-2000), field emission scanning electron microscopy
(FE-SEM; Nova NanoSEM430), high resolution transmission electron
microscopy (HRTEM; FEI Tecnai G2 F30 at an accelerating voltage of
300 kV) installed with energy dispersive X-Ray spectroscopy (EDX;
Oxford), selected area electron diffraction (SAED), and X-ray photo-
electron spectroscopy (XPS; Kratos Axis Ultra DLD).
peaks indicate the high purity and crystallinity of the products. The
peaks can be indexed in the cubic crystal system (space group: Pm-3m)
with the peaks indexs as (100), (110), (111), (200), (210), (211), (220),
(300), (310) and (311). The summary of the fundamental crystal data
for YbB6 sample is listed in Table 1 and the refined unit cell of a = b = c
= 4.148(8) Å can be obtained. The boron deficiency x in YbB6-x can be
estimated by the Vegard's law, as shown in Fig. 1b. By fitting the cell
parameters of YbB6 (PDF-251343) and YbB5.862 (ICSD-79203) from
the database, we can obtain the boron deficiency of our YbB6 crystals is
about 0.203, which can be a reference for explaining the transport
result.
2.3. Device fabrication
YbB6 nanowires were mechanically separated in ethanol by 10 min
sonication and transferred onto a SiO2 coated Si substrate. The
nanowire device was prepared by the focused ion beam (FIB) techni-
que. The FEI Nanolab 600i SEM/FIB dual beam system was used to
fabricate the Pt-electrodes onto an individual YbB6 nanowire.
Fig. 2a presents the micro-Raman spectrum of the YbB6 nanos-
tructures acquired at room temperature with 633 nm excitation in
ambient atmosphere. Due to the Pm-3m symmetry of YbB6, the lattice
vibration modes can be obtained: Γ = A1g + Eg + T2g, which are the
Raman active phonons [22]. Three observed peaks at around 758,
1119, and 1249 cm−1 are the Raman active phonons with the repre-
sentations of T2g, Eg, and Ag, respectively. The previous X-ray photo-
electron spectroscopy (XPS) data suggest that the valence of Yb is
2.4. Transport and magnetic measurements
Temperature-dependent resistance of YbB6 nanowires was carried
out in Quantum Design PPMS-9 instrument. The resistance was
measured using a standard four-terminal device to eliminate the
contact resistance. The DC magnetic susceptibility and magnetization
up to 7 T were measured in the range of 2–300 K with a Quantum
Design superconducting quantum interference device (SQUID) mag-
netometer.
Table 1
Crystallographic data of YbB6 at room temperature obtained by XRD Rietveld refine-
ment.
Formula
YbB6
Crystal system
Space group
Cubic
Pm-3m
Unit cell dimensions
a = 4.148(8) Å, α = 90°
b = 4.148(8) Å, β = 90°
c = 4.148(8) Å, γ = 90°
1
3. Results and discussion
The cell parameters of YbB6 sample are obtained by the single-
phase model Rietveld refinement using the GSAS program. Fig. 1a
shows the Rietveld refinement result of the obtained YbB6, which
agrees well with the standard YbB6 diffraction pattern from the Powder
Diffraction File (PDF, NO. 25–1343), and the sharp and well-defined
Z
Volume
2θ range refined
Rp
Rwp
71.411(4) Å3
10–80°
8.21%
11.06%
245

W. Han et al.
Journal of Solid State Chemistry 262 (2018) 244–250
Fig. 2. (a) Raman spectrum of YbB6 nanostructures; (b) Full XPS spectrum of YbB6 nanostructures; (c) XPS spectrum for Yb 4d and B1s; (d) XPS spectrum for Yb 4p.
around 2.2, but ARPES measurements and magnetic susceptibility
results show that it is divalent in YbB6 [11,23]. Here, the YbB6
nanostructures have been further characterized by XPS to identify
the valence state of Yb ion in YbB6, as shown in Fig. 2b-d. Fig. 2b shows
the full XPS spectrum of YbB6 nanostructures. The O signals and F
Auger peak are from the residual oxygen and impurities, while the Mg
and I Auger peak come from the residual magnesium compound and
iodine compound. The B 1 s binding energy at 186.7 eV in Fig. 2c is
lower than that of other rare-earth hexaborides (188 eV in LaB6,
187.5 eV in NdB6) [24,25] due to the different coordination environ-
ments. The Yb 4d3/2 binding energy at 191.5 eV is consistent with the
reported value (191.2 eV) [26], and the weak Yb 4d5/2 peak is also
observed at 180.9 eV. The peaks at 346.3 and 397.7 eV in Fig. 2d are
attributed to Yb2+ 4p3/2 and Yb2+ 4p1/2 levels. Moreover, a trivalent
Yb3+ 4p3/2 peak (350.6 eV) is observed in Fig. 2d, which indicates that
the trivalent Yb3+ component exists in YbB6 nanostructures. The
mixed-valence Yb ions are also found in other Yb-compounds, such
as mixed-valence Yb4As3 and Yb4Sb3 [23]. The trivalent Yb3+ compo-
nent may originate from the surface oxidation of Yb2+ or boron
deficiency.
The morphology and structural of YbB6 were observed by scanning
electron microscopy (SEM) and transmission electron microscopy
(TEM). Fig. 3a is a typical scanning electron microscopy (SEM) image
of the YbB6 nanowires synthesized at 240 °C. The dense nanowires with
straight or curly shapes are there and, the diameters are ranged from
30 nm to more than 200 nm and the length of about 2–10 µm. Due to
the very low diffusion rates of atoms in the solid-state reactants by this
substrate-free method, the percentage of YbB6 nanowires in the total
product obtained at 240 °C is only 17%, as estimated from the SEM
images. As shown in Fig. 3b of low-magnification TEM image, the
diameter and length of a typical single YbB6 nanowire is about 120 nm
and more than 7 µm respectively. The nanowire with smooth surface is
straight and its tip is terminated with a flat surface (marked by purple
arrow). Fig. 3c is the high-resolution transmission electron microscopy
(HRTEM) lattice image along the tip of the nanowire. According to the
two d-spacing (0.413 nm and 0.413 nm), the corresponding (001) and
(010) planes are confirmed. So the growth direction of the nanowire is
determined to be the [001] direction. Selected area electron diffraction
(SAED) was also used to determine the lattice structure of the
nanowire. As shown in Fig. 3b inset, the SAED pattern of the nanowire
confirms the (001) and (010) planes, showing that this nanowire is
grown in the [001] crystallographic direction of the cubic symmetry
with lattice constant a = 0.413 nm. The diffraction pattern appears
identical when we moved the SAED aperture along the entire nanowire,
indicating that the YbB6 nanowire is single crystalline. Energy dis-
persive X-Ray spectroscopy (EDX) spectrum in Fig. 3d reveals the
presence of Yb and B elements in the YbB6 nanowire. The Cu signal
comes from the supporting Cu grid. Therefore, single-crystalline YbB6
nanowires grown along [001] have been successfully prepared by this
solid state method.
The general synthetic strategies of 1D RB6 nanostructures are
metal-catalyzed and self-catalyzed chemical vapor deposition (CVD)
[27–31]. However, although Yb has the second lowest melting point
(824 °C) in rare-earth metals, YbB6 nanowires have not been prepared
by the CVD methods. We speculate that, in order to form YbB6 crystal,
Yb ion and B ion require a larger binding energy due to the smallest
cation radii of Yb2+ in rare-earth ions. The general route to obtain
metal boride is known as self-propagating high-temperature synthesis
(SHS), uses the fact that fine elemental powders can be ignited to
produce a combustion-like reaction triggered by a very low tempera-
ture, if the heat of formation of the product is sufficiently exothermic
[32–36]. Here, in the HPSS method, I2 was used as the catalyst that
246

W. Han et al.
Journal of Solid State Chemistry 262 (2018) 244–250
Fig. 3. Morphology and structural characterization of YbB6 nanowires. (a) SEM image of YbB6 nanowires; (b) TEM image and SAED pattern (inset) of a single YbB6 nanowire; (c)
HRTEM image; (d) EDX spectrum.
can reduce the binding energy for Yb ion and B ion [19–21,37]. The
proposed reaction mechanism [Eqs. (1)–(4)] for the formation of YbB6
is as follows.
According to the estimation of the Gibbs free energy in Ref [40], the
overall reaction [Eq. (1)] is thermodynamically spontaneous (ΔrG ~
− 1900 kJ mol−1) and highly exothermic (ΔrH ~ − 2000 kJ mol−1). Due
to the highly exothermic reaction, the inner temperature is higher than
240 °C, and Mg will reduce the H3BO3 to B atoms [Eq. (2)].
In order to investigate the electronic transport properties of a single
YbB6 nanowire, four-probe device was fabricated by the focused ion
beam (FIB) technique. Fig. 4a is the SEM image of the YbB6 nanowire
device with four Pt electrodes, where the nanowire has a width of
210 nm and a length of 13.8 µm. The length of two inner Pt electrodes
is 3.1 µm. The temperature dependence resistivity ρ(T) (T = 2–300 K)
of the YbB6 nanowire is shown in Fig. 4b. The resistivity of the YbB6
nanowire at 300 K is about 2.9 mΩ cm. ρ(T) reveals
a negative
temperature dependence (dρ/dT < 0) like typical semiconductors
between room temperature and about 20 K, however, ρ(T) increases
Yb + 6H3BO3 + 10Mg + I2 → YbB6 + 9MgO + MgI2 + 9H2O
2H3BO3 + 3Mg → 2B + 3MgO + 3H2O
(1)
(2)
exponentially as the temperature is lowered below 20 K (ρ
~
140.8 mΩ cm at 2 K). We have found semiconductor-insulator
a
transition (SIT) in YbB6 nanowire at around 20 K (= TSI). The
resistivity normalized to room temperature (ρ/ρ300 K) vs temperature
for YbB6 nanowire is displayed in Fig. 4c. The value of ρ at 2 K reaches
49 times the value of ρ at 300 K (ρ2 K/ρ300 K = 49). Fig. 4d shows the
plot of Lnρ as a function of inverse temperature 1/T. The data can be fit
only over the limited temperature range of 6–34 K to an activation
conduction form ρ = ρ0 exp(ΔE/kBT), where ΔE is the activation energy
and kB is Boltzmann's constant. ΔE/kB is 11.7 K derived from a best fit.
A linear fit of the Arrhenius plot gives us an activation energy ΔE of
1 meV. The ground state of the YbB6 nanowire could be insulating with
this small energy gap.
At second step, Mg and Yb react with I2 to form MgI2 and YbI2 [Eqs.
(3) and (4)]. MgI2 can provide heat and promote the crystallization of
YbB6 [38,39]. While YbI2, as a powerful and versatile reducing or
coupling agent in organic synthesis like SmI2, is a vital intermediate for
the formation of YbB6 [41].
Mg + I2 → MgI2
Yb + I2 → YbI2
(3)
(4)
At last, The fresh B atoms and active YbI2 can react to produce YbB6
crystal nucleation [Eq. (5)].
This SIT transport behavior in YbB6 nanowire is different from the
metal-like transport behaviours in bulk YbB6 single crystals [9], but it
is very like the pressure-induced quantum phase transitions in YbB6
single crystals [12] and the semiconducting behavior with noticeably
increasing of resistivity below 20 K in YbB5.96 single crystals [40]. We
speculate that the anomalous electronic transport in YbB6 nanowire
YbI2 + 6B→YbB6 + I2
(5)
Due to its lowest surface energy, the crystals will grow preferentially in
the (001) plane. The fast transport of Yb and B species in the high-pressure
and high temperature environment, due to the I2 and H2O vapors (about
45 atm), can promote the growth of single crystalline YbB6 nanowires.
247

W. Han et al.
Journal of Solid State Chemistry 262 (2018) 244–250
Fig. 4. (a) SEM image of the YbB6 nanowire device with four Pt electrodes; (b) Temperature dependence resistivity ρ(T) of the YbB6 nanowire from 2 to 300 K; (c) The normalized
resistivity ρ/ρ300 K vs temperature; (d) Lnρ as a function of inverse temperature 1/T.
can be explained by the valence change of Yb because of slightly boron-
rich and boron-deficient in YbB6 crystals. Also due to the much larger
surface-to-bulk ratio for the nanowire (about three orders of magnitude
higher than bulk crystals), the transport of electrons is affected by the
quantum size effect when cooling down. The valence mixing between
the Yb2+ and Yb3+ configurations (supporting by XPS results in Fig. 1),
according to the equilibrium Yb2+→Yb3++4f, governs the mode of
electronic conduction. At low temperatures, due to the interaction
between 4f and 5d electrons the YbB6 nanowire opens an activation
energy gap and undergoes a transition from a semiconductor to an
insulator.
Fig. 5a shows the perpendicular magnetic field induced
Magnetoresistance (MR) for the YbB6 nanowire measured at T = 2,
10, 30, and 100 K with B = 0–7 T. The magnetic field is perpendicular
to the (001) plane of the nanowire (B ⊥ I). The MR ratios are estimated
using MR(%) = 100 × [R(B)−R(0)]/R(0). Here, R(B) and R(0) are
resistances with the applied field and without it, respectively. An
overall positive MR is observed at 2 K and 10 K, but no obvious MR
is observed at 30 K and 100 K. At 7 T, the MR increases from 2.1% at
10 K to 20.6% at 2 K. At 2 K and 10 K, the MR exhibits a non-saturating
quadratic dependence. As shown in Fig. 5b, all of the MR data at
various temperatures has a linear relationship with B2, which indicates
that MR is proportional to (µB)2 (µ is electron mobility).
transport. At 2 K, due to the large resistance of 98.5 kΩ for the YbB6
nanowire, the hole-dominant transport is reasonable [43,44]. Despite
the coexistence of electron and hole at 10 K, the electron-dominant
transport is mainly due to the much higher mobility of electron than
that of hole.
We can not detect the magnetic signal of a single nanowire by the
SQUID magnetometer because the nanowire is too small to be
measured. So we measured the magnetic properties of the YbB6
powders
including
nanowires
and
other
nanostructures.
Magnetization measurements were performed on YbB6 nanostructures
(Fig. 6). The inset of Fig. 6 shows the temperature dependence of
magnetic susceptibility χ(T) measured at an applied magnetic field of
2 kOe in the temperature range from 2 to 300 K. The χ(T) curve shows
that paramagnetic susceptibility increases with decreasing tempera-
ture, and has a sharp rise below 25 K. The paramagnetic susceptibility
may be caused by slightly boron-rich and boron-deficient because YbB6
crystals with Yb2+ ions are diamagnetic due to the filled 4f14 shell of
Yb2+ ions. The magnetization curve for YbB6 at 2 K yielded a saturation
value at 7 T of 0.132 μB / YbB6, much less than the Yb3+ free-ion value
of 4.5 μB / Yb3+ (μB is Bohr magneton) [45]. The concentration N of
Yb3+ ions can be estimated by the function of M = NgJμBBJ(x), where
the g is Lande factor and J is quantum number. BJ(x) is the Brillouin
function expressed as:
Kohler's law states that MR ratio is a universal function of B/R (0)
[42]:
⎛
⎞
⎛
⎞
2J + 1
2J
2J + 1
2J
1
2J
x
2J
B (x) =
coth
x −
coth
⎜
⎟
⎠
⎜
⎟
⎠
J
⎝
⎝
⎛
⎞
B
R(0)
MR = F
⎜
⎝
⎟
⎠
Fitting of the M(H) data by Brillouin function (red line in Fig. 6)
gives the g value of about 10.01. Assuming the lowest level of the
ground Yb3+–2F7/2 multiplet split by the cubo-octahedral crystal
electric field to be Γ6 or Γ7 doublet (J = 1/2), the concentration of
the Yb3+ ions may be estimated as N = M/(gJ) = 2.63% for YbB6
nanostructures [28,40]. Every Yb3+ ion appears due to one boron
vacancy, so the YbB6 crystals can be described by the chemical
formulas YbB5.82. This value agrees well with the fitting result of
YbB5.797 from the XRD data in Fig. 1. The mix valence in B-deficient
YbB6 nanowire leads to a transition from a semiconductor to an
insulator.
The MR ratio is proportional to B2 for the YbB6 nanowire, and could
2
∝
be expressed as ΔR(B)/R(0) [B/R(0)] . The observed nonsaturating
MR may be due to the two-carrier transport. To reveal the two-carrier
transport in the YbB6 nanowire, the Kohler's plots are presented in
Fig. 5c. In case of single type of carrier dominant transport, the
Kohler's plots at different temperatures should overlap with each other.
However, as shown in Fig. 5c, at 2 K and 10 K the curves separate from
each other due to the gradual increase of electron concentration with
increasing temperature. Because of the semiconductor-insulator tran-
sition, we propose that at 10 K the transport is the electron-dominant
248

W. Han et al.
Journal of Solid State Chemistry 262 (2018) 244–250
Fig. 5. (a) Magnetoresistance (MR) of the YbB6 nanowire with the B ⊥ (100) plane at various temperatures from 2 to 100 K; (b) MR as a function of B2; (c) The Kohler's plot of the MR
curves.
4. Conclusion
In conclusion, we have developed an HPSS method to prepare
single crystalline YbB6 nanowires at low trigger-temperatures of 200–
240 °C. The temperature dependence of resistivity shows that the YbB6
nanowire undergoes a semiconductor-insulator transition (SIT) below
20 K with an activation energy ΔE of 1 meV. The observed non-
saturating magnetoresistance (MR) has a linear relationship with B2.
The anomalous electronic transport in the YbB6 nanowire can be
explained by the mixed valence of Yb ions due to the boron deficiency
supporting by the X-ray photoelectron spectroscopy (XPS) and para-
magnetic magnetization. Moreover, these YbB6 nanowires with large
surface-to-volume ratios are still striking materials with potentially
novel quantum phenomena.
Fig. 6. Magnetization curve for YbB6 nanostructures at 2 K. Inset is the magnetic
Acknowledgments
susceptibility versus temperature χ(T) for YbB6 nanostructures measured at 2 kOe. (For
interpretation of the references to color in this figure, the reader is referred to the web
version of this article.)
The authors gratefully acknowledge the support of the projects from
the National Natural Science Foundation of China (Nos. 51372089 and
51672086).
249

W. Han et al.
Journal of Solid State Chemistry 262 (2018) 244–250
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